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 INTERNET-DRAFT                                             Clifford Neuman
                                                                 John Kohl
                                                             Theodore Ts'o
                                                               Ken Raeburn
                                                                    Tom Yu
                                                         November 20, 2001
                                                      Expires 20 May, 2002

The Kerberos Network Authentication Service (V5)
draft-ietf-cat-kerberos-revisions-10


STATUS OF THIS MEMO

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026. Internet-Drafts are working documents
of the Internet Engineering Task Force (IETF), its areas, and its working
groups. Note that other groups may also distribute working documents as
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Internet-Drafts are draft documents valid for a maximum of six months and
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The distribution of this memo is unlimited. It is filed as
draft-ietf-cat-kerberos-revisions-10.txt, and expires May 20, 2002. Please
send comments to: ietf-krb-wg@anl.gov

ABSTRACT

This document provides an overview and specification of Version 5 of the
Kerberos protocol, and updates RFC1510 to clarify aspects of the protocol
and its intended use that require more detailed or clearer explanation than
was provided in RFC1510. This document is intended to provide a detailed
description of the protocol, suitable for implementation, together with
descriptions of the appropriate use of protocol messages and fields within
those messages.

This document is not intended to describe Kerberos to the end user, system
administrator, or application developer. Higher level papers describing
Version 5 of the Kerberos system [NT94] and documenting version 4 [SNS88],
are available elsewhere.

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OVERVIEW

This INTERNET-DRAFT describes the concepts and model upon which the Kerberos
network authentication system is based. It also specifies Version 5 of the
Kerberos protocol.

The motivations, goals, assumptions, and rationale behind most design
decisions are treated cursorily; they are more fully described in a paper
available in IEEE communications [NT94] and earlier in the Kerberos portion
of the Athena Technical Plan [MNSS87]. The protocols have been a proposed
standard and are being considered for advancement for draft standard through
the IETF standard process. Comments are encouraged on the presentation, but
only minor refinements to the protocol as implemented or extensions that fit
within current protocol framework will be considered at this time.

Requests for addition to an electronic mailing list for discussion of
Kerberos, kerberos@MIT.EDU, may be addressed to kerberos-request@MIT.EDU.
This mailing list is gatewayed onto the Usenet as the group
comp.protocols.kerberos. Requests for further information, including
documents and code availability, may be sent to info-kerberos@MIT.EDU.

BACKGROUND

The Kerberos model is based in part on Needham and Schroeder's trusted
third-party authentication protocol [NS78] and on modifications suggested by
Denning and Sacco [DS81]. The original design and implementation of Kerberos
Versions 1 through 4 was the work of two former Project Athena staff
members, Steve Miller of Digital Equipment Corporation and Clifford Neuman
(now at the Information Sciences Institute of the University of Southern
California), along with Jerome Saltzer, Technical Director of Project
Athena, and Jeffrey Schiller, MIT Campus Network Manager. Many other members
of Project Athena have also contributed to the work on Kerberos.

Version 5 of the Kerberos protocol (described in this document) has evolved
from Version 4 based on new requirements and desires for features not
available in Version 4. The design of Version 5 of the Kerberos protocol was
led by Clifford Neuman and John Kohl with much input from the community. The
development of the MIT reference implementation was led at MIT by John Kohl
and Theodore T'so, with help and contributed code from many others. Since
RFC1510 was issued, extensions and revisions to the protocol have been
proposed by many individuals. Some of these proposals are reflected in this
document. Where such changes involved significant effort, the document cites
the contribution of the proposer.

Reference implementations of both version 4 and version 5 of Kerberos are
publicly available and commercial implementations have been developed and
are widely used. Details on the differences between Kerberos Versions 4 and
5 can be found in [KNT92].


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1. Introduction

Kerberos provides a means of verifying the identities of principals, (e.g. a
workstation user or a network server) on an open (unprotected) network. This
is accomplished without relying on assertions by the host operating system,
without basing trust on host addresses, without requiring physical security
of all the hosts on the network, and under the assumption that packets
traveling along the network can be read, modified, and inserted at
will[1.1]. Kerberos performs authentication under these conditions as a
trusted third-party authentication service by using conventional (shared
secret key [1.2]) cryptography. Kerberos extensions (outside the scope of
this document) can provide for the use of public key cryptography during
certain phases of the authentication protocol [@RFCE: if PKINIT advances
concurrently include reference to the RFC here]. Such extensions support
Kerberos authentication for users registered with public key certification
authorities and provide certain benefits of public key cryptography in
situations where they are needed.

The basic Kerberos authentication process proceeds as follows: A client
sends a request to the authentication server (AS) requesting "credentials"
for a given server. The AS responds with these credentials, encrypted in the
client's key. The credentials consist of a "ticket" for the server and a
temporary encryption key (often called a "session key"). The client
transmits the ticket (which contains the client's identity and a copy of the
session key, all encrypted in the server's key) to the server. The session
key (now shared by the client and server) is used to authenticate the
client, and may optionally be used to authenticate the server. It may also
be used to encrypt further communication between the two parties or to
exchange a separate sub-session key to be used to encrypt further
communication.

Implementation of the basic protocol consists of one or more authentication
servers running on physically secure hosts. The authentication servers
maintain a database of principals (i.e., users and servers) and their secret
keys. Code libraries provide encryption and implement the Kerberos protocol.
In order to add authentication to its transactions, a typical network
application adds one or two calls to the Kerberos library directly or
through the Generic Security Services Application Programming Interface,
GSSAPI, described in separate document [ref to GSSAPI RFC]. These calls
result in the transmission of the necessary messages to achieve
authentication.

The Kerberos protocol consists of several sub-protocols (or exchanges).
There are two basic methods by which a client can ask a Kerberos server for
credentials. In the first approach, the client sends a cleartext request for
a ticket for the desired server to the AS. The reply is sent encrypted in
the client's secret key. Usually this request is for a ticket-granting
ticket (TGT) which can later be used with the ticket-granting server (TGS).
In the second method, the client sends a request to the TGS. The client uses
the TGT to authenticate itself to the TGS in the same manner as if it were
contacting any other application server that requires Kerberos
authentication. The reply is encrypted in the session key from the TGT.
Though the protocol specification describes the AS and the TGS as separate
servers, they are implemented in practice as different protocol entry points
within a single Kerberos server.

Once obtained, credentials may be used to verify the identity of the
principals in a transaction, to ensure the integrity of messages exchanged
between them, or to preserve privacy of the messages. The application is
free to choose whatever protection may be necessary.

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To verify the identities of the principals in a transaction, the client
transmits the ticket to the application server. Since the ticket is sent "in
the clear" (parts of it are encrypted, but this encryption doesn't thwart
replay) and might be intercepted and reused by an attacker, additional
information is sent to prove that the message originated with the principal
to whom the ticket was issued. This information (called the authenticator)
is encrypted in the session key, and includes a timestamp. The timestamp
proves that the message was recently generated and is not a replay.
Encrypting the authenticator in the session key proves that it was generated
by a party possessing the session key. Since no one except the requesting
principal and the server know the session key (it is never sent over the
network in the clear) this guarantees the identity of the client.

The integrity of the messages exchanged between principals can also be
guaranteed using the session key (passed in the ticket and contained in the
credentials). This approach provides detection of both replay attacks and
message stream modification attacks. It is accomplished by generating and
transmitting a collision-proof checksum (elsewhere called a hash or digest
function) of the client's message, keyed with the session key. Privacy and
integrity of the messages exchanged between principals can be secured by
encrypting the data to be passed using the session key contained in the
ticket or the sub-session key found in the authenticator.

The authentication exchanges mentioned above require read-only access to the
Kerberos database. Sometimes, however, the entries in the database must be
modified, such as when adding new principals or changing a principal's key.
This is done using a protocol between a client and a third Kerberos server,
the Kerberos Administration Server (KADM). There is also a protocol for
maintaining multiple copies of the Kerberos database. Neither of these
protocols are described in this document.

1.1. Cross-realm operation

The Kerberos protocol is designed to operate across organizational
boundaries. A client in one organization can be authenticated to a server in
another. Each organization wishing to run a Kerberos server establishes its
own "realm". The name of the realm in which a client is registered is part
of the client's name, and can be used by the end-service to decide whether
to honor a request.

By establishing "inter-realm" keys, the administrators of two realms can
allow a client authenticated in the local realm to prove its identity to
servers in other realms[1.3]. The exchange of inter-realm keys (a separate
key may be used for each direction) registers the ticket-granting service of
each realm as a principal in the other realm. A client is then able to
obtain a ticket-granting ticket for the remote realm's ticket-granting
service from its local realm. When that ticket-granting ticket is used, the
remote ticket-granting service uses the inter-realm key (which usually
differs from its own normal TGS key) to decrypt the ticket-granting ticket,
and is thus certain that it was issued by the client's own TGS. Tickets
issued by the remote ticket-granting service will indicate to the
end-service that the client was authenticated from another realm.

A realm is said to communicate with another realm if the two realms share an
inter-realm key, or if the local realm shares an inter-realm key with an
intermediate realm that communicates with the remote realm. An
authentication path is the sequence of intermediate realms that are
transited in communicating from one realm to another.


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Realms are typically organized hierarchically. Each realm shares a key with
its parent and a different key with each child. If an inter-realm key is not
directly shared by two realms, the hierarchical organization allows an
authentication path to be easily constructed. If a hierarchical organization
is not used, it may be necessary to consult a database in order to construct
an authentication path between realms.

Although realms are typically hierarchical, intermediate realms may be
bypassed to achieve cross-realm authentication through alternate
authentication paths (these might be established to make communication
between two realms more efficient). It is important for the end-service to
know which realms were transited when deciding how much faith to place in
the authentication process. To facilitate this decision, a field in each
ticket contains the names of the realms that were involved in authenticating
the client.

The application server is ultimately responsible for accepting or rejecting
authentication and should check the transited field. The application server
may choose to rely on the KDC for the application server's realm to check
the transited field. The application server's KDC will set the
TRANSITED-POLICY-CHECKED flag in this case. The KDC's for intermediate
realms may also check the transited field as they issue
ticket-granting-tickets for other realms, but they are encouraged not to do
so. A client may request that the KDC's not check the transited field by
setting the DISABLE-TRANSITED-CHECK flag. KDC's are encouraged but not
required to honor this flag.

1.2. Choosing a principal with which to communicate

The Kerberos protocol provides the means for verifying (subject to the
assumptions in 1.4) that the entity with which one communicates is the same
entity that was registered with the KDC using the claimed identity
(principal name). It is still necessary to determine whether that identity
corresponds to the entity with which one intends to communicate.

When appropriate data has been exchanged in advance, this determination may
be performed syntactically by the application based on the application
protocol specification, information provided by the user, and configuration
files. For example, the server principal name (including realm) for a telnet
server might be derived from the user specified host name (from the telnet
command line), the "host/" prefix specified in the application protocol
specification, and a mapping to a Kerberos realm derived syntactically from
the domain part of the specified hostname and information from the local
Kerberos realms database.

One can also rely on trusted third parties to make this determination, but
only when the data obtained from the third party is suitably integrity
protected wile resident on the third party server and when transmitted.
Thus, for example, one should not rely on an unprotected domain name system
record to map a host alias to the primary name of a server, accepting the
primary name as the party one intends to contact, since an attacker can
modify the mapping and impersonate the party with which one intended to
communicate.


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1.3. Authorization

As an authentication service, Kerberos provides a means of verifying the
identity of principals on a network. Authentication is usually useful
primarily as a first step in the process of authorization, determining
whether a client may use a service, which objects the client is allowed to
access, and the type of access allowed for each. Kerberos does not, by
itself, provide authorization. Possession of a client ticket for a service
provides only for authentication of the client to that service, and in the
absence of a separate authorization procedure, it should not be considered
by an application as authorizing the use of that service.

Such separate authorization methods may be implemented as application
specific access control functions and may utilize files on the application
server, or on separately issued authorization credentials such as those
based on proxies [Neu93], or on other authorization services. Separately
authenticated authorization credentials may be embedded in a tickets
authorization data when encapsulated by the kdc-issued authorization data
element.

Applications should not accept the mere issuance of a service ticket by the
Kerberos server (even by a modified Kerberos server) as granting authority
to use the service, since such applications may become vulnerable to the
bypass of this authorization check in an environment if they interoperate
with other KDCs or where other options for application authentication (e.g.
the PKTAPP proposal) are provided.

1.4. Environmental assumptions

Kerberos imposes a few assumptions on the environment in which it can
properly function:

   * "Denial of service" attacks are not solved with Kerberos. There are
     places in the protocols where an intruder can prevent an application
     from participating in the proper authentication steps. Detection and
     solution of such attacks (some of which can appear to be not-uncommon
     "normal" failure modes for the system) is usually best left to the
     human administrators and users.
   * Principals must keep their secret keys secret. If an intruder somehow
     steals a principal's key, it will be able to masquerade as that
     principal or impersonate any server to the legitimate principal.
   * "Password guessing" attacks are not solved by Kerberos. If a user
     chooses a poor password, it is possible for an attacker to successfully
     mount an offline dictionary attack by repeatedly attempting to decrypt,
     with successive entries from a dictionary, messages obtained which are
     encrypted under a key derived from the user's password.
   * Each host on the network must have a clock which is "loosely
     synchronized" to the time of the other hosts; this synchronization is
     used to reduce the bookkeeping needs of application servers when they
     do replay detection. The degree of "looseness" can be configured on a
     per-server basis, but is typically on the order of 5 minutes. If the
     clocks are synchronized over the network, the clock synchronization
     protocol must itself be secured from network attackers.
   * Principal identifiers are not recycled on a short-term basis. A typical
     mode of access control will use access control lists (ACLs) to grant
     permissions to particular principals. If a stale ACL entry remains for
     a deleted principal and the principal identifier is reused, the new
     principal will inherit rights specified in the stale ACL entry. By not
     re-using principal identifiers, the danger of inadvertent access is
     removed.


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1.5. Glossary of terms

Below is a list of terms used throughout this document.

Authentication
     Verifying the claimed identity of a principal.
Authentication header
     A record containing a Ticket and an Authenticator to be presented to a
     server as part of the authentication process.
Authentication path
     A sequence of intermediate realms transited in the authentication
     process when communicating from one realm to another.
Authenticator
     A record containing information that can be shown to have been recently
     generated using the session key known only by the client and server.
Authorization
     The process of determining whether a client may use a service, which
     objects the client is allowed to access, and the type of access allowed
     for each.
Capability
     A token that grants the bearer permission to access an object or
     service. In Kerberos, this might be a ticket whose use is restricted by
     the contents of the authorization data field, but which lists no
     network addresses, together with the session key necessary to use the
     ticket.
Ciphertext
     The output of an encryption function. Encryption transforms plaintext
     into ciphertext.
Client
     A process that makes use of a network service on behalf of a user. Note
     that in some cases a Server may itself be a client of some other server
     (e.g. a print server may be a client of a file server).
Credentials
     A ticket plus the secret session key necessary to successfully use that
     ticket in an authentication exchange.
KDC
     Key Distribution Center, a network service that supplies tickets and
     temporary session keys; or an instance of that service or the host on
     which it runs. The KDC services both initial ticket and ticket-granting
     ticket requests. The initial ticket portion is sometimes referred to as
     the Authentication Server (or service). The ticket-granting ticket
     portion is sometimes referred to as the ticket-granting server (or
     service).
Kerberos
     Aside from the 3-headed dog guarding Hades, the name given to Project
     Athena's authentication service, the protocol used by that service, or
     the code used to implement the authentication service.
Plaintext
     The input to an encryption function or the output of a decryption
     function. Decryption transforms ciphertext into plaintext.
Principal
     A named client or server entity that participates in a network
     communication, with one name that is considered canonical.
Principal identifier
     The canonical name used to uniquely identify each different principal.
Seal
     To encipher a record containing several fields in such a way that the
     fields cannot be individually replaced without either knowledge of the
     encryption key or leaving evidence of tampering.

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Secret key
     An encryption key shared by a principal and the KDC, distributed
     outside the bounds of the system, with a long lifetime. In the case of
     a human user's principal, the secret key may be derived from a
     password.
Server
     A particular Principal which provides a resource to network clients.
     The server is sometimes referred to as the Application Server.
Service
     A resource provided to network clients; often provided by more than one
     server (for example, remote file service).
Session key
     A temporary encryption key used between two principals, with a lifetime
     limited to the duration of a single login "session".
Sub-session key
     A temporary encryption key used between two principals, selected and
     exchanged by the principals using the session key, and with a lifetime
     limited to the duration of a single association.
Ticket
     A record that helps a client authenticate itself to a server; it
     contains the client's identity, a session key, a timestamp, and other
     information, all sealed using the server's secret key. It only serves
     to authenticate a client when presented along with a fresh
     Authenticator.

2. Ticket flag uses and requests

Each Kerberos ticket contains a set of flags which are used to indicate
attributes of that ticket. Most flags may be requested by a client when the
ticket is obtained; some are automatically turned on and off by a Kerberos
server as required. The following sections explain what the various flags
mean, and gives examples of reasons to use such a flag. With the excepttion
of the ANONYMOUS and INVALID flags clients may ignore ticket flags that are
not recognized.

2.1. Initial, pre-authenticated, and hardware authenticated tickets

The INITIAL flag indicates that a ticket was issued using the AS protocol
and not issued based on a ticket-granting ticket. Application servers that
want to require the demonstrated knowledge of a client's secret key (e.g. a
password-changing program) can insist that this flag be set in any tickets
they accept, and thus be assured that the client's key was recently
presented to the application client.

The PRE-AUTHENT and HW-AUTHENT flags provide additional information about
the initial authentication, regardless of whether the current ticket was
issued directly (in which case INITIAL will also be set) or issued on the
basis of a ticket-granting ticket (in which case the INITIAL flag is clear,
but the PRE-AUTHENT and HW-AUTHENT flags are carried forward from the
ticket-granting ticket).

2.2. Invalid tickets

The INVALID flag indicates that a ticket is invalid. Application servers
must reject tickets which have this flag set. A postdated ticket will
usually be issued in this form. Invalid tickets must be validated by the KDC
before use, by presenting them to the KDC in a TGS request with the VALIDATE
option specified. The KDC will only validate tickets after their starttime
has passed. The validation is required so that postdated tickets which have
been stolen before their starttime can be rendered permanently invalid
(through a hot-list mechanism) (see section 3.3.3.1).


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2.3. Renewable tickets

Applications may desire to hold tickets which can be valid for long periods
of time. However, this can expose their credentials to potential theft for
equally long periods, and those stolen credentials would be valid until the
expiration time of the ticket(s). Simply using short-lived tickets and
obtaining new ones periodically would require the client to have long-term
access to its secret key, an even greater risk. Renewable tickets can be
used to mitigate the consequences of theft. Renewable tickets have two
"expiration times": the first is when the current instance of the ticket
expires, and the second is the latest permissible value for an individual
expiration time. An application client must periodically (i.e. before it
expires) present a renewable ticket to the KDC, with the RENEW option set in
the KDC request. The KDC will issue a new ticket with a new session key and
a later expiration time. All other fields of the ticket are left unmodified
by the renewal process. When the latest permissible expiration time arrives,
the ticket expires permanently. At each renewal, the KDC may consult a
hot-list to determine if the ticket had been reported stolen since its last
renewal; it will refuse to renew such stolen tickets, and thus the usable
lifetime of stolen tickets is reduced.

The RENEWABLE flag in a ticket is normally only interpreted by the
ticket-granting service (discussed below in section 3.3). It can usually be
ignored by application servers. However, some particularly careful
application servers may wish to disallow renewable tickets.

If a renewable ticket is not renewed by its expiration time, the KDC will
not renew the ticket. The RENEWABLE flag is reset by default, but a client
may request it be set by setting the RENEWABLE option in the KRB_AS_REQ
message. If it is set, then the renew-till field in the ticket contains the
time after which the ticket may not be renewed.

2.4. Postdated tickets

Applications may occasionally need to obtain tickets for use much later,
e.g. a batch submission system would need tickets to be valid at the time
the batch job is serviced. However, it is dangerous to hold valid tickets in
a batch queue, since they will be on-line longer and more prone to theft.
Postdated tickets provide a way to obtain these tickets from the KDC at job
submission time, but to leave them "dormant" until they are activated and
validated by a further request of the KDC. If a ticket theft were reported
in the interim, the KDC would refuse to validate the ticket, and the thief
would be foiled.

The MAY-POSTDATE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. This flag
must be set in a ticket-granting ticket in order to issue a postdated ticket
based on the presented ticket. It is reset by default; it may be requested
by a client by setting the ALLOW-POSTDATE option in the KRB_AS_REQ message.
This flag does not allow a client to obtain a postdated ticket-granting
ticket; postdated ticket-granting tickets can only by obtained by requesting
the postdating in the KRB_AS_REQ message. The life (endtime-starttime) of a
postdated ticket will be the remaining life of the ticket-granting ticket at
the time of the request, unless the RENEWABLE option is also set, in which
case it can be the full life (endtime-starttime) of the ticket-granting
ticket. The KDC may limit how far in the future a ticket may be postdated.


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The POSTDATED flag indicates that a ticket has been postdated. The
application server can check the authtime field in the ticket to see when
the original authentication occurred. Some services may choose to reject
postdated tickets, or they may only accept them within a certain period
after the original authentication. When the KDC issues a POSTDATED ticket,
it will also be marked as INVALID, so that the application client must
present the ticket to the KDC to be validated before use.

2.5. Proxiable and proxy tickets

At times it may be necessary for a principal to allow a service to perform
an operation on its behalf. The service must be able to take on the identity
of the client, but only for a particular purpose. A principal can allow a
service to take on the principal's identity for a particular purpose by
granting it a proxy.

The process of granting a proxy using the proxy and proxiable flags is used
to provide credentials for use with specific services. Though conceptually
also a proxy, user's wishing to delegate their identity for ANY purpose must
use the ticket forwarding mechanism described in the next section to forward
a ticket granting ticket.

The PROXIABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. When set,
this flag tells the ticket-granting server that it is OK to issue a new
ticket (but not a ticket-granting ticket) with a different network address
based on this ticket. This flag is set if requested by the client on initial
authentication. By default, the client will request that it be set when
requesting a ticket granting ticket, and reset when requesting any other
ticket.

This flag allows a client to pass a proxy to a server to perform a remote
request on its behalf, e.g. a print service client can give the print server
a proxy to access the client's files on a particular file server in order to
satisfy a print request.

In order to complicate the use of stolen credentials, Kerberos tickets are
usually valid from only those network addresses specifically included in the
ticket[2.1]. When granting a proxy, the client must specify the new network
address from which the proxy is to be used, or indicate that the proxy is to
be issued for use from any address.

The PROXY flag is set in a ticket by the TGS when it issues a proxy ticket.
Application servers may check this flag and at their option they may require
additional authentication from the agent presenting the proxy in order to
provide an audit trail.

2.6. Forwardable tickets

Authentication forwarding is an instance of a proxy where the service
granted is complete use of the client's identity. An example where it might
be used is when a user logs in to a remote system and wants authentication
to work from that system as if the login were local.

The FORWARDABLE flag in a ticket is normally only interpreted by the
ticket-granting service. It can be ignored by application servers. The
FORWARDABLE flag has an interpretation similar to that of the PROXIABLE
flag, except ticket-granting tickets may also be issued with different
network addresses. This flag is reset by default, but users may request that
it be set by setting the FORWARDABLE option in the AS request when they
request their initial ticket-granting ticket.


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This flag allows for authentication forwarding without requiring the user to
enter a password again. If the flag is not set, then authentication
forwarding is not permitted, but the same result can still be achieved if
the user engages in the AS exchange specifying the requested network
addresses and supplies a password.

The FORWARDED flag is set by the TGS when a client presents a ticket with
the FORWARDABLE flag set and requests a forwarded ticket by specifying the
FORWARDED KDC option and supplying a set of addresses for the new ticket. It
is also set in all tickets issued based on tickets with the FORWARDED flag
set. Application servers may choose to process FORWARDED tickets differently
than non-FORWARDED tickets.

2.7 Transited Policy Checking

In Kerberos, the application server is ultimately responsible for accepting
or rejecting authentication and should check that only suitably trusted
KDC's are relied upon to authenticate a principal. The transited field in
the ticket identifies which KDC's were involved in the authentication
process and an application server would normally check this field. While the
end server ultimately decides whether authentication is valid, the KDC for
the end server's realm may apply a realm specific policy for validating the
transited field and accepting credentials for cross-realm authentication.
When the KDC applies such checks and accepts such cross-realm authentication
it will set the TRANSITED-POLICY-CHECKED flag in the service tickets it
issues based on the cross-realm TGT. A client may request that the KDC's not
check the transited field by setting the DISABLE-TRANSITED-CHECK flag. KDC's
are encouraged but not required to honor this flag.

2.8 Anonymous Tickets

When policy allows, a KDC may issue anonymous tickets for the purpose of
enabling encrypted communication between a client and server without
identifying the client to the server. Such anonymous tickets are issued with
a generic principal name configured on the KDC (e.g. "anonymous@") and will
have the ANONYMOUS flag set. A server accepting such a ticket may assume
that subsequent requests using the same ticket and session key originate
from the same user. Requests with the same username but different tickets
are likely to originate from different users. Users request anonymous ticket
by setting the REQUEST-ANONYMOUS option in an AS or TGS request.

2.9. Other KDC options

There are three additional options which may be set in a client's request of
the KDC.

2.9.1 Renewable-OK

The RENEWABLE-OK option indicates that the client will accept a renewable
ticket if a ticket with the requested life cannot otherwise be provided. If
a ticket with the requested life cannot be provided, then the KDC may issue
a renewable ticket with a renew-till equal to the the requested endtime. The
value of the renew-till field may still be adjusted by site-determined
limits or limits imposed by the individual principal or server.


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2.9.2 ENC-TKT-IN-SKEY

In its basic form the Kerberos protocol supports authentication in a client
server setting and is not well suited to authentication in a peer-to-peer
environment because the long term key of the user does not remain on the
workstation after initial login. Authentication of such peers may be
supported by Kerberos in its user-to-user variant. The ENC-TKT-IN-SKEY
option supports user-to-user authentication by allowing the KDC to issue a
service ticket encrypted using the session key from another ticket granting
ticket issued to another user. The ENC-TKT-IN-SKEY option is honored only by
the ticket-granting service. It indicates that the ticket to be issued for
the end server is to be encrypted in the session key from the additional
second ticket-granting ticket provided with the request. See section 3.3.3
for specific details.

3. Message Exchanges

The following sections describe the interactions between network clients and
servers and the messages involved in those exchanges.

3.1. The Authentication Service Exchange

                          Summary
      Message direction       Message type    Section
      1. Client to Kerberos   KRB_AS_REQ      5.4.1
      2. Kerberos to client   KRB_AS_REP or   5.4.2
                              KRB_ERROR       5.9.1

The Authentication Service (AS) Exchange between the client and the Kerberos
Authentication Server is initiated by a client when it wishes to obtain
authentication credentials for a given server but currently holds no
credentials. In its basic form, the client's secret key is used for
encryption and decryption. This exchange is typically used at the initiation
of a login session to obtain credentials for a Ticket-Granting Server which
will subsequently be used to obtain credentials for other servers (see
section 3.3) without requiring further use of the client's secret key. This
exchange is also used to request credentials for services which must not be
mediated through the Ticket-Granting Service, but rather require a
principal's secret key, such as the password-changing service[3.1]. This
exchange does not by itself provide any assurance of the the identity of the
user[3.2].

The exchange consists of two messages: KRB_AS_REQ from the client to
Kerberos, and KRB_AS_REP or KRB_ERROR in reply. The formats for these
messages are described in sections 5.4.1, 5.4.2, and 5.9.1.

In the request, the client sends (in cleartext) its own identity and the
identity of the server for which it is requesting credentials. The response,
KRB_AS_REP, contains a ticket for the client to present to the server, and a
session key that will be shared by the client and the server. The session
key and additional information are encrypted in the client's secret key. The
KRB_AS_REP message contains information which can be used to detect replays,
and to associate it with the message to which it replies.

Without pre-authentication, the authentication server does not know whether
the client is actually the principal named in the request. It simply sends a
reply without knowing or caring whether they are the same. This is
acceptable because nobody but the principal whose identity was given in the
request will be able to use the reply. Its critical information is encrypted
in that principal's key. The initial request supports an optional field that
can be used to pass additional information that might be needed for the
initial exchange. This field may be used for pre-authentication as described
in section 3.1.1.


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Various errors can occur; these are indicated by an error response
(KRB_ERROR) instead of the KRB_AS_REP response. The error message is not
encrypted. The KRB_ERROR message contains information which can be used to
associate it with the message to which it replies. If suitable
preauthentication has occurred, an optional checksum may be included in the
KRB_ERROR message to prevent fabrication or modification of the KRB_ERROR
message. When a checksum is not present, the lack of integrity protection
precludes the ability to detect replays, fabrications, or modifications of
the message, and the client must not depend on information in the KRB_ERROR
message for security critical operations.

3.1.1. Generation of KRB_AS_REQ message

The client may specify a number of options in the initial request. Among
these options are whether pre-authentication is to be performed; whether the
requested ticket is to be renewable, proxiable, or forwardable; whether it
should be postdated or allow postdating of derivative tickets; whether the
client requests an anonymous ticket; and whether a renewable ticket will be
accepted in lieu of a non-renewable ticket if the requested ticket
expiration date cannot be satisfied by a non-renewable ticket (due to
configuration constraints; see section 4). See section A.1 for pseudocode.

The client prepares the KRB_AS_REQ message and sends it to the KDC.

3.1.2. Receipt of KRB_AS_REQ message

If all goes well, processing the KRB_AS_REQ message will result in the
creation of a ticket for the client to present to the server. The format for
the ticket is described in section 5.3.1. The contents of the ticket are
determined as follows.

3.1.3. Generation of KRB_AS_REP message

The authentication server looks up the client and server principals named in
the KRB_AS_REQ in its database, extracting their respective keys. If the
requested client principal named in the request is not known because it
doesn't exist in the KDC's principal database, then an error message with a
KDC_ERR_C_PRINCIPAL_UNKNOWN is returned.

If required, the server pre-authenticates the request, and if the
pre-authentication check fails, an error message with the code
KDC_ERR_PREAUTH_FAILED is returned. If pre-authentication is required, but
was not present in the request, an error message with the code
KDC_ERR_PREAUTH_REQUIRED is returned and the PA-ETYPE-INFO
pre-authentication field will be included in the KRB-ERROR message. If the
server cannot accommodate an encryption type requested by the client, an
error message with code KDC_ERR_ETYPE_NOSUPP is returned. Otherwise the KDC
generates a 'random' session key[3.3].

When responding to an AS request, if there are multiple encryption keys
registered for a client in the Kerberos database (or if the key registered
supports multiple encryption types; e.g. DES3-CBC-SHA1 and
DES3-CBC-SHA1-KD), then the etype field from the AS request is used by the
KDC to select the encryption method to be used to protect the encrypted part
of the KRB_AS_REP message which is sent to the client. If there is more than
one supported strong encryption type in the etype list, the first valid
etype for which an encryption key is available is used. The encryption
method used to protect the encrypted part of the KRB_TGS_REP message is the
keytype of the session key found in the ticket granting ticket presented in
the KRB_TGS_REQ.


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If the user's key was generated using an alternate string to key function
than that used by the selected encryption type, information needed by the
string to key function will be returned to the client in the padata field of
the KRB_AS_REP message using the PA-PW-SALT, PA-AFS3-SALT, or similar
pre-authentication typed values. This does not affect the encryption
performed by the KDC since the key stored in the principal database already
has the string to key transformation applied.

When the etype field is present in a KDC request, whether an AS or TGS
request, the KDC will attempt to assign the type of the random session key
from the list of methods in the etype field. The KDC will select the
appropriate type using the list of methods provided together with
information from the Kerberos database indicating acceptable encryption
methods for the application server. The KDC will not issue tickets with a
weak session key encryption type.

If the requested start time is absent, indicates a time in the past, or is
within the window of acceptable clock skew for the KDC and the POSTDATE
option has not been specified, then the start time of the ticket is set to
the authentication server's current time. If it indicates a time in the
future beyond the acceptable clock skew, but the POSTDATED option has not
been specified then the error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise
the requested start time is checked against the policy of the local realm
(the administrator might decide to prohibit certain types or ranges of
postdated tickets), and if acceptable, the ticket's start time is set as
requested and the INVALID flag is set in the new ticket. The postdated
ticket must be validated before use by presenting it to the KDC after the
start time has been reached.

The expiration time of the ticket will be set to the earlier of the
requested endtime and a time determined by local policy, possibly determined
using realm or principal specific factors. For example, the expiration time
may be set to the minimum of the following:

   * The expiration time (endtime) requested in the KRB_AS_REQ message.
   * The ticket's start time plus the maximum allowable lifetime associated
     with the client principal from the authentication server's database
     (see section 4).
   * The ticket's start time plus the maximum allowable lifetime associated
     with the server principal.
   * The ticket's start time plus the maximum lifetime set by the policy of
     the local realm.

If the requested expiration time minus the start time (as determined above)
is less than a site-determined minimum lifetime, an error message with code
KDC_ERR_NEVER_VALID is returned. If the requested expiration time for the
ticket exceeds what was determined as above, and if the 'RENEWABLE-OK'
option was requested, then the 'RENEWABLE' flag is set in the new ticket,
and the renew-till value is set as if the 'RENEWABLE' option were requested
(the field and option names are described fully in section 5.4.1).

If the RENEWABLE option has been requested or if the RENEWABLE-OK option has
been set and a renewable ticket is to be issued, then the renew-till field
is set to the minimum of:

   * Its requested value.
   * The start time of the ticket plus the minimum of the two maximum
     renewable lifetimes associated with the principals' database entries.
   * The start time of the ticket plus the maximum renewable lifetime set by
     the policy of the local realm.


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The flags field of the new ticket will have the following options set if
they have been requested and if the policy of the local realm allows:
FORWARDABLE, MAY-POSTDATE, POSTDATED, PROXIABLE, RENEWABLE, ANONYMOUS. If
the new ticket is post-dated (the start time is in the future), its INVALID
flag will also be set.

If all of the above succeed, the server will encrypt ciphertext part of the
ticket using the encryption key extracted from the server principal's record
in the Kerberos database using the encryption type associated with the
server principal's key (this choice is NOT affected by the etype field in
the request). It then formats a KRB_AS_REP message (see section 5.4.2),
copying the addresses in the request into the caddr of the response, placing
any required pre-authentication data into the padata of the response, and
encrypts the ciphertext part in the client's key using an acceptable
encryption method requested in the etype field of the request, and sends the
message to the client. See section A.2 for pseudocode.

3.1.4. Generation of KRB_ERROR message

Several errors can occur, and the Authentication Server responds by
returning an error message, KRB_ERROR, to the client, with the error-code,
e-text, and optional e-cksum fields set to appropriate values. The error
message contents and details are described in Section 5.9.1.

3.1.5. Receipt of KRB_AS_REP message

If the reply message type is KRB_AS_REP, then the client verifies that the
cname and crealm fields in the cleartext portion of the reply match what it
requested. If any padata fields are present, they may be used to derive the
proper secret key to decrypt the message. The client decrypts the encrypted
part of the response using its secret key, verifies that the nonce in the
encrypted part matches the nonce it supplied in its request (to detect
replays). It also verifies that the sname and srealm in the response match
those in the request (or are otherwise expected values), and that the host
address field is also correct. It then stores the ticket, session key, start
and expiration times, and other information for later use. The
key-expiration field from the encrypted part of the response may be checked
to notify the user of impending key expiration (the client program could
then suggest remedial action, such as a password change). See section A.3
for pseudocode.

Proper decryption of the KRB_AS_REP message is not sufficient for the host
to verify the identity of the user; the user and an attacker could cooperate
to generate a KRB_AS_REP format message which decrypts properly but is not
from the proper KDC. If the host wishes to verify the identity of the user,
it must require the user to present application credentials which can be
verified using a securely-stored secret key for the host. If those
credentials can be verified, then the identity of the user can be assured.

3.1.6. Receipt of KRB_ERROR message

If the reply message type is KRB_ERROR, then the client interprets it as an
error and performs whatever application-specific tasks are necessary to
recover.


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3.2. The Client/Server Authentication Exchange

                             Summary
Message direction                         Message type    Section
Client to Application server              KRB_AP_REQ      5.5.1
[optional] Application server to client   KRB_AP_REP or   5.5.2
                                          KRB_ERROR       5.9.1

The client/server authentication (CS) exchange is used by network
applications to authenticate the client to the server and vice versa. The
client must have already acquired credentials for the server using the AS or
TGS exchange.

3.2.1. The KRB_AP_REQ message

The KRB_AP_REQ contains authentication information which should be part of
the first message in an authenticated transaction. It contains a ticket, an
authenticator, and some additional bookkeeping information (see section
5.5.1 for the exact format). The ticket by itself is insufficient to
authenticate a client, since tickets are passed across the network in
cleartext[3.4], so the authenticator is used to prevent invalid replay of
tickets by proving to the server that the client knows the session key of
the ticket and thus is entitled to use the ticket. The KRB_AP_REQ message is
referred to elsewhere as the 'authentication header.'

3.2.2. Generation of a KRB_AP_REQ message

When a client wishes to initiate authentication to a server, it obtains
(either through a credentials cache, the AS exchange, or the TGS exchange) a
ticket and session key for the desired service. The client may re-use any
tickets it holds until they expire. To use a ticket the client constructs a
new Authenticator from the the system time, its name, and optionally an
application specific checksum, an initial sequence number to be used in
KRB_SAFE or KRB_PRIV messages, and/or a session subkey to be used in
negotiations for a session key unique to this particular session.
Authenticators may not be re-used and will be rejected if replayed to a
server[3.5]. If a sequence number is to be included, it should be randomly
chosen so that even after many messages have been exchanged it is not likely
to collide with other sequence numbers in use.

The client may indicate a requirement of mutual authentication or the use of
a session-key based ticket by setting the appropriate flag(s) in the
ap-options field of the message.

The Authenticator is encrypted in the session key and combined with the
ticket to form the KRB_AP_REQ message which is then sent to the end server
along with any additional application-specific information. See section A.9
for pseudocode.

3.2.3. Receipt of KRB_AP_REQ message

Authentication is based on the server's current time of day (clocks must be
loosely synchronized), the authenticator, and the ticket. Several errors are
possible. If an error occurs, the server is expected to reply to the client
with a KRB_ERROR message. This message may be encapsulated in the
application protocol if its 'raw' form is not acceptable to the protocol.
The format of error messages is described in section 5.9.1.


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The algorithm for verifying authentication information is as follows. If the
message type is not KRB_AP_REQ, the server returns the KRB_AP_ERR_MSG_TYPE
error. If the key version indicated by the Ticket in the KRB_AP_REQ is not
one the server can use (e.g., it indicates an old key, and the server no
longer possesses a copy of the old key), the KRB_AP_ERR_BADKEYVER error is
returned. If the USE-SESSION-KEY flag is set in the ap-options field, it
indicates to the server that the ticket is encrypted in the session key from
the server's ticket-granting ticket rather than its secret key [3.6].

Since it is possible for the server to be registered in multiple realms,
with different keys in each, the srealm field in the unencrypted portion of
the ticket in the KRB_AP_REQ is used to specify which secret key the server
should use to decrypt that ticket. The KRB_AP_ERR_NOKEY error code is
returned if the server doesn't have the proper key to decipher the ticket.

The ticket is decrypted using the version of the server's key specified by
the ticket. If the decryption routines detect a modification of the ticket
(each encryption system must provide safeguards to detect modified
ciphertext; see section 6), the KRB_AP_ERR_BAD_INTEGRITY error is returned
(chances are good that different keys were used to encrypt and decrypt).

The authenticator is decrypted using the session key extracted from the
decrypted ticket. If decryption shows it to have been modified, the
KRB_AP_ERR_BAD_INTEGRITY error is returned. The name and realm of the client
from the ticket are compared against the same fields in the authenticator.
If they don't match, the KRB_AP_ERR_BADMATCH error is returned (they might
not match, for example, if the wrong session key was used to encrypt the
authenticator). The addresses in the ticket (if any) are then searched for
an address matching the operating-system reported address of the client. If
no match is found or the server insists on ticket addresses but none are
present in the ticket, the KRB_AP_ERR_BADADDR error is returned. If the
local (server) time and the client time in the authenticator differ by more
than the allowable clock skew (e.g., 5 minutes), the KRB_AP_ERR_SKEW error
is returned.

Unless the application server provides its own suitable means to protect
against replay (for example, a challenge-response sequence initiated by the
server after authentication, or use of a server-generated encryption
subkey), the server must utilize a replay cache to remember any
authenticator presented within the allowable clock skew. Careful analysis of
the application protocol and implementation is recommended before
eliminating this cache. The replay cache will store the server name, along
with the client name, time and microsecond fields from the recently-seen
authenticators and if a matching tuple is found, the KRB_AP_ERR_REPEAT error
is returned [3.7]. If a server loses track of authenticators presented
within the allowable clock skew, it must reject all requests until the clock
skew interval has passed, providing assurance that any lost or re-played
authenticators will fall outside the allowable clock skew and can no longer
be successfully replayed[3.8].

If a sequence number is provided in the authenticator, the server saves it
for later use in processing KRB_SAFE and/or KRB_PRIV messages. If a subkey
is present, the server either saves it for later use or uses it to help
generate its own choice for a subkey to be returned in a KRB_AP_REP message.

If multiple servers (for example, different services on one machine, or a
single service implemented on multiple machines) share a service principal
(a practice we do not recommend in general, but acknowledge will be used in
some cases), they should also share this replay cache, or the application
protocol should be designed so as to eliminate the need for it. Note that

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this applies to all of the services, if any of the application protocols
does not have replay protection built in; an authenticator used with such a
service could later be replayed to a different service with the same service
principal but no replay protection, if the former doesn't record the
authenticator information in the common replay cache.

The server computes the age of the ticket: local (server) time minus the
start time inside the Ticket. If the start time is later than the current
time by more than the allowable clock skew or if the INVALID flag is set in
the ticket, the KRB_AP_ERR_TKT_NYV error is returned. Otherwise, if the
current time is later than end time by more than the allowable clock skew,
the KRB_AP_ERR_TKT_EXPIRED error is returned.

If all these checks succeed without an error, the server is assured that the
client possesses the credentials of the principal named in the ticket and
thus, the client has been authenticated to the server. See section A.10 for
pseudocode.

Passing these checks provides only authentication of the named principal; it
does not imply authorization to use the named service. Applications must
make a separate authorization decisions based upon the authenticated name of
the user, the requested operation, local access control information such as
that contained in a .k5login or .k5users file, and possibly a separate
distributed authorization service.

3.2.4. Generation of a KRB_AP_REP message

Typically, a client's request will include both the authentication
information and its initial request in the same message, and the server need
not explicitly reply to the KRB_AP_REQ. However, if mutual authentication
(not only authenticating the client to the server, but also the server to
the client) is being performed, the KRB_AP_REQ message will have
MUTUAL-REQUIRED set in its ap-options field, and a KRB_AP_REP message is
required in response. As with the error message, this message may be
encapsulated in the application protocol if its "raw" form is not acceptable
to the application's protocol. The timestamp and microsecond field used in
the reply must be the client's timestamp and microsecond field (as provided
in the authenticator)[3.9]. If a sequence number is to be included, it
should be randomly chosen as described above for the authenticator. A subkey
may be included if the server desires to negotiate a different subkey. The
KRB_AP_REP message is encrypted in the session key extracted from the
ticket. See section A.11 for pseudocode.

3.2.5. Receipt of KRB_AP_REP message

If a KRB_AP_REP message is returned, the client uses the session key from
the credentials obtained for the server[3.10] to decrypt the message, and
verifies that the timestamp and microsecond fields match those in the
Authenticator it sent to the server. If they match, then the client is
assured that the server is genuine. The sequence number and subkey (if
present) are retained for later use. See section A.12 for pseudocode.

3.2.6. Using the encryption key

After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and server
share an encryption key which can be used by the application. In some cases,
the use of this session key will be implicit in the protocol; in others the
method of use must be chosen from several alternatives. The 'true session
key' to be used for KRB_PRIV, KRB_SAFE, or other application-specific uses
may be chosen by the application based on the session key from the ticket

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and subkeys in the KRB_AP_REP message and the authenticator[3.11]. To
mitigate the effect of failures in random number generation on the client it
is strongly encouraged that any key derived by an application for subsequent
use include the full key entropy derived from the KDC generated session key
carried in the ticket. We leave the protocol negotiations of how to use the
key (e.g. selecting an encryption or checksum type) to the application
programmer; the Kerberos protocol does not constrain the implementation
options, but an example of how this might be done follows.

One way that an application may choose to negotiate a key to be used for
subsequent integrity and privacy protection is for the client to propose a
key in the subkey field of the authenticator. The server can then choose a
key using the proposed key from the client as input, returning the new
subkey in the subkey field of the application reply. This key could then be
used for subsequent communication.

To make this example more concrete, if the communication patterns of an
application dictates the use of encryption modes of operation incompatible
with the encryption system used for the authenticator, then a key compatible
with the required encryption system may be generated by either the client,
the server, or collaboratively by both and exchanged using the subkey field.
This generation might involve the use of a random number as a pre-key,
initially generated by either party, which could then be encrypted using the
session key from the ticket, and the result exchanged and used for
subsequent encryption. By encrypting the pre-key with the session key from
the ticket, randomness from the KDC generated key is assured of being
present in the negotiated key. Application developers must be careful
however, to use a means of introducing this entropy that does not allow an
attacker to learn the session key from the ticket if it learns the key
generated and used for subsequent communication. The reader should note that
this is only an example, and that an analysis of the particular cryptosystem
to be used, must be made before deciding how to generate values for the
subkey fields, and the key to be used for subsequent communication.

With both the one-way and mutual authentication exchanges, the peers should
take care not to send sensitive information to each other without proper
assurances. In particular, applications that require privacy or integrity
should use the KRB_AP_REP response from the server to client to assure both
client and server of their peer's identity. If an application protocol
requires privacy of its messages, it can use the KRB_PRIV message (section
3.5). The KRB_SAFE message (section 3.4) can be used to assure integrity.

3.3. The Ticket-Granting Service (TGS) Exchange

                          Summary
      Message direction       Message type     Section
      1. Client to Kerberos   KRB_TGS_REQ      5.4.1
      2. Kerberos to client   KRB_TGS_REP or   5.4.2
                              KRB_ERROR        5.9.1

The TGS exchange between a client and the Kerberos Ticket-Granting Server is
initiated by a client when it wishes to obtain authentication credentials
for a given server (which might be registered in a remote realm), when it
wishes to renew or validate an existing ticket, or when it wishes to obtain
a proxy ticket. In the first case, the client must already have acquired a
ticket for the Ticket-Granting Service using the AS exchange (the
ticket-granting ticket is usually obtained when a client initially
authenticates to the system, such as when a user logs in). The message

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format for the TGS exchange is almost identical to that for the AS exchange.
The primary difference is that encryption and decryption in the TGS exchange
does not take place under the client's key. Instead, the session key from
the ticket-granting ticket or renewable ticket, or sub-session key from an
Authenticator is used. As is the case for all application servers, expired
tickets are not accepted by the TGS, so once a renewable or ticket-granting
ticket expires, the client must use a separate exchange to obtain valid
tickets.

The TGS exchange consists of two messages: A request (KRB_TGS_REQ) from the
client to the Kerberos Ticket-Granting Server, and a reply (KRB_TGS_REP or
KRB_ERROR). The KRB_TGS_REQ message includes information authenticating the
client plus a request for credentials. The authentication information
consists of the authentication header (KRB_AP_REQ) which includes the
client's previously obtained ticket-granting, renewable, or invalid ticket.
In the ticket-granting ticket and proxy cases, the request may include one
or more of: a list of network addresses, a collection of typed authorization
data to be sealed in the ticket for authorization use by the application
server, or additional tickets (the use of which are described later). The
TGS reply (KRB_TGS_REP) contains the requested credentials, encrypted in the
session key from the ticket-granting ticket or renewable ticket, or if
present, in the sub-session key from the Authenticator (part of the
authentication header). The KRB_ERROR message contains an error code and
text explaining what went wrong. The KRB_ERROR message is not encrypted. The
KRB_TGS_REP message contains information which can be used to detect
replays, and to associate it with the message to which it replies. The
KRB_ERROR message also contains information which can be used to associate
it with the message to which it replies, but except when an optional
checksum is included in the KRB_ERROR message, it is not possible to detect
replays or fabrications of such messages.

3.3.1. Generation of KRB_TGS_REQ message

Before sending a request to the ticket-granting service, the client must
determine in which realm the application server is believed to be
registered[3.12]. If the client knows the service principal name and realm
and it does not already possess a ticket-granting ticket for the appropriate
realm, then one must be obtained. This is first attempted by requesting a
ticket-granting ticket for the destination realm from a Kerberos server for
which the client possesses a ticket-granting ticket (using the KRB_TGS_REQ
message recursively). The Kerberos server may return a TGT for the desired
realm in which case one can proceed. Alternatively, the Kerberos server may
return a TGT for a realm which is 'closer' to the desired realm (further
along the standard hierarchical path between the client's realm and the
requested realm server's realm).

Once the client obtains a ticket-granting ticket for the appropriate realm,
it determines which Kerberos servers serve that realm, and contacts one. The
list might be obtained through a configuration file or network service or it
may be generated from the name of the realm; as long as the secret keys
exchanged by realms are kept secret, only denial of service results from
using a false Kerberos server.

As in the AS exchange, the client may specify a number of options in the
KRB_TGS_REQ message. The client prepares the KRB_TGS_REQ message, providing
an authentication header as an element of the padata field, and including
the same fields as used in the KRB_AS_REQ message along with several
optional fields: the enc-authorization-data field for application server use
and additional tickets required by some options.


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In preparing the authentication header, the client can select a sub-session
key under which the response from the Kerberos server will be
encrypted[3.13]. If the sub-session key is not specified, the session key
from the ticket-granting ticket will be used. If the enc-authorization-data
is present, it must be encrypted in the sub-session key, if present, from
the authenticator portion of the authentication header, or if not present,
using the session key from the ticket-granting ticket.

Once prepared, the message is sent to a Kerberos server for the destination
realm. See section A.5 for pseudocode.

3.3.2. Receipt of KRB_TGS_REQ message

The KRB_TGS_REQ message is processed in a manner similar to the KRB_AS_REQ
message, but there are many additional checks to be performed. First, the
Kerberos server must determine which server the accompanying ticket is for
and it must select the appropriate key to decrypt it. For a normal
KRB_TGS_REQ message, it will be for the ticket granting service, and the
TGS's key will be used. If the TGT was issued by another realm, then the
appropriate inter-realm key must be used. If the accompanying ticket is not
a ticket granting ticket for the current realm, but is for an application
server in the current realm, the RENEW, VALIDATE, or PROXY options are
specified in the request, and the server for which a ticket is requested is
the server named in the accompanying ticket, then the KDC will decrypt the
ticket in the authentication header using the key of the server for which it
was issued. If no ticket can be found in the padata field, the
KDC_ERR_PADATA_TYPE_NOSUPP error is returned.

Once the accompanying ticket has been decrypted, the user-supplied checksum
in the Authenticator must be verified against the contents of the request,
and the message rejected if the checksums do not match (with an error code
of KRB_AP_ERR_MODIFIED) or if the checksum is not keyed or not
collision-proof (with an error code of KRB_AP_ERR_INAPP_CKSUM). If the
checksum type is not supported, the KDC_ERR_SUMTYPE_NOSUPP error is
returned. If the authorization-data are present, they are decrypted using
the sub-session key from the Authenticator.

If any of the decryptions indicate failed integrity checks, the
KRB_AP_ERR_BAD_INTEGRITY error is returned.

3.3.3. Generation of KRB_TGS_REP message

The KRB_TGS_REP message shares its format with the KRB_AS_REP (KRB_KDC_REP),
but with its type field set to KRB_TGS_REP. The detailed specification is in
section 5.4.2.

The response will include a ticket for the requested server or for a ticket
granting server of an intermediate KDC to be contacted to obtain the
requested ticket. The Kerberos database is queried to retrieve the record
for the appropriate server (including the key with which the ticket will be
encrypted). If the request is for a ticket granting ticket for a remote
realm, and if no key is shared with the requested realm, then the Kerberos
server will select the realm 'closest' to the requested realm with which it
does share a key, and use that realm instead. If the requested server cannot
be found in the TGS database, then a TGT for another trusted realm may be
returned instead of a ticket for the service. This TGT is a referral
mechanism to cause the client to retry the request to the realm of the TGT.
These are the only cases where the response for the KDC will be for a
different server than that requested by the client.


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By default, the address field, the client's name and realm, the list of
transited realms, the time of initial authentication, the expiration time,
and the authorization data of the newly-issued ticket will be copied from
the ticket-granting ticket (TGT) or renewable ticket. If the transited field
needs to be updated, but the transited type is not supported, the
KDC_ERR_TRTYPE_NOSUPP error is returned.

If the request specifies an endtime, then the endtime of the new ticket is
set to the minimum of (a) that request, (b) the endtime from the TGT, and
(c) the starttime of the TGT plus the minimum of the maximum life for the
application server and the maximum life for the local realm (the maximum
life for the requesting principal was already applied when the TGT was
issued). If the new ticket is to be a renewal, then the endtime above is
replaced by the minimum of (a) the value of the renew_till field of the
ticket and (b) the starttime for the new ticket plus the life
(endtime-starttime) of the old ticket.

If the FORWARDED option has been requested, then the resulting ticket will
contain the addresses specified by the client. This option will only be
honored if the FORWARDABLE flag is set in the TGT. The PROXY option is
similar; the resulting ticket will contain the addresses specified by the
client. It will be honored only if the PROXIABLE flag in the TGT is set. The
PROXY option will not be honored on requests for additional ticket-granting
tickets.

If the requested start time is absent, indicates a time in the past, or is
within the window of acceptable clock skew for the KDC and the POSTDATE
option has not been specified, then the start time of the ticket is set to
the authentication server's current time. If it indicates a time in the
future beyond the acceptable clock skew, but the POSTDATED option has not
been specified or the MAY-POSTDATE flag is not set in the TGT, then the
error KDC_ERR_CANNOT_POSTDATE is returned. Otherwise, if the ticket-granting
ticket has the MAY-POSTDATE flag set, then the resulting ticket will be
postdated and the requested starttime is checked against the policy of the
local realm. If acceptable, the ticket's start time is set as requested, and
the INVALID flag is set. The postdated ticket must be validated before use
by presenting it to the KDC after the starttime has been reached. However,
in no case may the starttime, endtime, or renew-till time of a newly-issued
postdated ticket extend beyond the renew-till time of the ticket-granting
ticket.

If the ENC-TKT-IN-SKEY option has been specified and an additional ticket
has been included in the request, the KDC will decrypt the additional ticket
using the key for the server to which the additional ticket was issued and
verify that it is a ticket-granting ticket. If the name of the requested
server is missing from the request, the name of the client in the additional
ticket will be used. Otherwise the name of the requested server will be
compared to the name of the client in the additional ticket and if
different, the request will be rejected. If the request succeeds, the
session key from the additional ticket will be used to encrypt the new
ticket that is issued instead of using the key of the server for which the
new ticket will be used.


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If the name of the server in the ticket that is presented to the KDC as part
of the authentication header is not that of the ticket-granting server
itself, the server is registered in the realm of the KDC, and the RENEW
option is requested, then the KDC will verify that the RENEWABLE flag is set
in the ticket, that the INVALID flag is not set in the ticket, and that the
renew_till time is still in the future. If the VALIDATE option is requested,
the KDC will check that the starttime has passed and the INVALID flag is
set. If the PROXY option is requested, then the KDC will check that the
PROXIABLE flag is set in the ticket. If the tests succeed, and the ticket
passes the hotlist check described in the next section, the KDC will issue
the appropriate new ticket.

The ciphertext part of the response in the KRB_TGS_REP message is encrypted
in the sub-session key from the Authenticator, if present, or the session
key key from the ticket-granting ticket. It is not encrypted using the
client's secret key. Furthermore, the client's key's expiration date and the
key version number fields are left out since these values are stored along
with the client's database record, and that record is not needed to satisfy
a request based on a ticket-granting ticket. See section A.6 for pseudocode.

3.3.3.1. Checking for revoked tickets

Whenever a request is made to the ticket-granting server, the presented
ticket(s) is(are) checked against a hot-list of tickets which have been
canceled. This hot-list might be implemented by storing a range of issue
timestamps for 'suspect tickets'; if a presented ticket had an authtime in
that range, it would be rejected. In this way, a stolen ticket-granting
ticket or renewable ticket cannot be used to gain additional tickets
(renewals or otherwise) once the theft has been reported to the KDC for the
realm in which the server resides. Any normal ticket obtained before it was
reported stolen will still be valid (because they require no interaction
with the KDC), but only until their normal expiration time. If TGT's have
been issued for cross-realm authentication, use of the cross-realm TGT will
not be affected unless the hot-list is propagated to the KDC's for the
realms for which such cross-realm tickets were issued.

3.3.3.2. Encoding the transited field

If the identity of the server in the TGT that is presented to the KDC as
part of the authentication header is that of the ticket-granting service,
but the TGT was issued from another realm, the KDC will look up the
inter-realm key shared with that realm and use that key to decrypt the
ticket. If the ticket is valid, then the KDC will honor the request, subject
to the constraints outlined above in the section describing the AS exchange.
The realm part of the client's identity will be taken from the
ticket-granting ticket. The name of the realm that issued the
ticket-granting ticket will be added to the transited field of the ticket to
be issued. This is accomplished by reading the transited field from the
ticket-granting ticket (which is treated as an unordered set of realm
names), adding the new realm to the set, then constructing and writing out
its encoded (shorthand) form (this may involve a rearrangement of the
existing encoding).

Note that the ticket-granting service does not add the name of its own
realm. Instead, its responsibility is to add the name of the previous realm.
This prevents a malicious Kerberos server from intentionally leaving out its
own name (it could, however, omit other realms' names).


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The names of neither the local realm nor the principal's realm are to be
included in the transited field. They appear elsewhere in the ticket and
both are known to have taken part in authenticating the principal. Since the
endpoints are not included, both local and single-hop inter-realm
authentication result in a transited field that is empty.

Because the name of each realm transited is added to this field, it might
potentially be very long. To decrease the length of this field, its contents
are encoded. The initially supported encoding is optimized for the normal
case of inter-realm communication: a hierarchical arrangement of realms
using either domain or X.500 style realm names. This encoding (called
DOMAIN-X500-COMPRESS) is now described.

Realm names in the transited field are separated by a ",". The ",", "\",
trailing "."s, and leading spaces (" ") are special characters, and if they
are part of a realm name, they must be quoted in the transited field by
preceding them with a "\".

A realm name ending with a "." is interpreted as being prepended to the
previous realm. For example, we can encode traversal of EDU, MIT.EDU,
ATHENA.MIT.EDU, WASHINGTON.EDU, and CS.WASHINGTON.EDU as:

     "EDU,MIT.,ATHENA.,WASHINGTON.EDU,CS.".

Note that if ATHENA.MIT.EDU, or CS.WASHINGTON.EDU were end-points, that they
would not be included in this field, and we would have:

     "EDU,MIT.,WASHINGTON.EDU"

A realm name beginning with a "/" is interpreted as being appended to the
previous realm[18]. If it is to stand by itself, then it should be preceded
by a space (" "). For example, we can encode traversal of /COM/HP/APOLLO,
/COM/HP, /COM, and /COM/DEC as:

     "/COM,/HP,/APOLLO, /COM/DEC".

Like the example above, if /COM/HP/APOLLO and /COM/DEC are endpoints, they
they would not be included in this field, and we would have:

     "/COM,/HP"

A null subfield preceding or following a "," indicates that all realms
between the previous realm and the next realm have been traversed[19]. Thus,
"," means that all realms along the path between the client and the server
have been traversed. ",EDU, /COM," means that that all realms from the
client's realm up to EDU (in a domain style hierarchy) have been traversed,
and that everything from /COM down to the server's realm in an X.500 style
has also been traversed. This could occur if the EDU realm in one hierarchy
shares an inter-realm key directly with the /COM realm in another hierarchy.

3.3.4. Receipt of KRB_TGS_REP message

When the KRB_TGS_REP is received by the client, it is processed in the same
manner as the KRB_AS_REP processing described above. The primary difference
is that the ciphertext part of the response must be decrypted using the
session key from the ticket-granting ticket rather than the client's secret
key. The server name returned in the reply is the true principal name of the
service. See section A.7 for pseudocode.


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3.4. The KRB_SAFE Exchange

The KRB_SAFE message may be used by clients requiring the ability to detect
modifications of messages they exchange. It achieves this by including a
keyed collision-proof checksum of the user data and some control
information. The checksum is keyed with an encryption key (usually the last
key negotiated via subkeys, or the session key if no negotiation has
occurred).

3.4.1. Generation of a KRB_SAFE message

When an application wishes to send a KRB_SAFE message, it collects its data
and the appropriate control information and computes a checksum over them.
The checksum algorithm should be a keyed one-way hash function (such as the
RSA- MD5-DES checksum algorithm specified in section 6.4.5, or the DES MAC),
generated using the sub-session key if present, or the session key.
Different algorithms may be selected by changing the checksum type in the
message. Unkeyed or non-collision-proof checksums are not suitable for this
use.

The control information for the KRB_SAFE message includes both a timestamp
and a sequence number. The designer of an application using the KRB_SAFE
message must choose at least one of the two mechanisms. This choice should
be based on the needs of the application protocol.

Sequence numbers are useful when all messages sent will be received by one's
peer. Connection state is presently required to maintain the session key, so
maintaining the next sequence number should not present an additional
problem.

If the application protocol is expected to tolerate lost messages without
them being resent, the use of the timestamp is the appropriate replay
detection mechanism. Using timestamps is also the appropriate mechanism for
multi-cast protocols where all of one's peers share a common sub-session
key, but some messages will be sent to a subset of one's peers.

After computing the checksum, the client then transmits the information and
checksum to the recipient in the message format specified in section 5.6.1.

3.4.2. Receipt of KRB_SAFE message

When an application receives a KRB_SAFE message, it verifies it as follows.
If any error occurs, an error code is reported for use by the application.

The message is first checked by verifying that the protocol version and type
fields match the current version and KRB_SAFE, respectively. A mismatch
generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
application verifies that the checksum used is a collision-proof keyed
checksum, and if it is not, a KRB_AP_ERR_INAPP_CKSUM error is generated. If
the sender's address was included in the control information, the recipient
verifies that the operating system's report of the sender's address matches
the sender's address in the message, and (if a recipient address is
specified or the recipient requires an address) that one of the recipient's
addresses appears as the recipient's address in the message. A failed match
for either case generates a KRB_AP_ERR_BADADDR error. Then the timestamp and
usec and/or the sequence number fields are checked. If timestamp and usec
are expected and not present, or they are present but not current, the
KRB_AP_ERR_SKEW error is generated. If the server name, along with the

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client name, time and microsecond fields from the Authenticator match any
recently-seen (sent or received[20] ) such tuples, the KRB_AP_ERR_REPEAT
error is generated. If an incorrect sequence number is included, or a
sequence number is expected but not present, the KRB_AP_ERR_BADORDER error
is generated. If neither a time-stamp and usec or a sequence number is
present, a KRB_AP_ERR_MODIFIED error is generated. Finally, the checksum is
computed over the data and control information, and if it doesn't match the
received checksum, a KRB_AP_ERR_MODIFIED error is generated.

If all the checks succeed, the application is assured that the message was
generated by its peer and was not modified in transit.

3.5. The KRB_PRIV Exchange

The KRB_PRIV message may be used by clients requiring confidentiality and
the ability to detect modifications of exchanged messages. It achieves this
by encrypting the messages and adding control information.

3.5.1. Generation of a KRB_PRIV message

When an application wishes to send a KRB_PRIV message, it collects its data
and the appropriate control information (specified in section 5.7.1) and
encrypts them under an encryption key (usually the last key negotiated via
subkeys, or the session key if no negotiation has occurred). As part of the
control information, the client must choose to use either a timestamp or a
sequence number (or both); see the discussion in section 3.4.1 for
guidelines on which to use. After the user data and control information are
encrypted, the client transmits the ciphertext and some 'envelope'
information to the recipient.

3.5.2. Receipt of KRB_PRIV message

When an application receives a KRB_PRIV message, it verifies it as follows.
If any error occurs, an error code is reported for use by the application.

The message is first checked by verifying that the protocol version and type
fields match the current version and KRB_PRIV, respectively. A mismatch
generates a KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The
application then decrypts the ciphertext and processes the resultant
plaintext. If decryption shows the data to have been modified, a
KRB_AP_ERR_BAD_INTEGRITY error is generated. If the sender's address was
included in the control information, the recipient verifies that the
operating system's report of the sender's address matches the sender's
address in the message, and (if a recipient address is specified or the
recipient requires an address) that one of the recipient's addresses appears
as the recipient's address in the message. A failed match for either case
generates a KRB_AP_ERR_BADADDR error. Then the timestamp and usec and/or the
sequence number fields are checked. If timestamp and usec are expected and
not present, or they are present but not current, the KRB_AP_ERR_SKEW error
is generated. If the server name, along with the client name, time and
microsecond fields from the Authenticator match any recently-seen such
tuples, the KRB_AP_ERR_REPEAT error is generated. If an incorrect sequence
number is included, or a sequence number is expected but not present, the
KRB_AP_ERR_BADORDER error is generated. If neither a time-stamp and usec or
a sequence number is present, a KRB_AP_ERR_MODIFIED error is generated.

If all the checks succeed, the application can assume the message was
generated by its peer, and was securely transmitted (without intruders able
to see the unencrypted contents).


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3.6. The KRB_CRED Exchange

The KRB_CRED message may be used by clients requiring the ability to send
Kerberos credentials from one host to another. It achieves this by sending
the tickets together with encrypted data containing the session keys and
other information associated with the tickets.

3.6.1. Generation of a KRB_CRED message

When an application wishes to send a KRB_CRED message it first (using the
KRB_TGS exchange) obtains credentials to be sent to the remote host. It then
constructs a KRB_CRED message using the ticket or tickets so obtained,
placing the session key needed to use each ticket in the key field of the
corresponding KrbCredInfo sequence of the encrypted part of the the KRB_CRED
message.

Other information associated with each ticket and obtained during the
KRB_TGS exchange is also placed in the corresponding KrbCredInfo sequence in
the encrypted part of the KRB_CRED message. The current time and, if
specifically required by the application the nonce, s-address, and r-address
fields, are placed in the encrypted part of the KRB_CRED message which is
then encrypted under an encryption key previously exchanged in the KRB_AP
exchange (usually the last key negotiated via subkeys, or the session key if
no negotiation has occurred).

3.6.2. Receipt of KRB_CRED message

When an application receives a KRB_CRED message, it verifies it. If any
error occurs, an error code is reported for use by the application. The
message is verified by checking that the protocol version and type fields
match the current version and KRB_CRED, respectively. A mismatch generates a
KRB_AP_ERR_BADVERSION or KRB_AP_ERR_MSG_TYPE error. The application then
decrypts the ciphertext and processes the resultant plaintext. If decryption
shows the data to have been modified, a KRB_AP_ERR_BAD_INTEGRITY error is
generated.

If present or required, the recipient verifies that the operating system's
report of the sender's address matches the sender's address in the message,
and that one of the recipient's addresses appears as the recipient's address
in the message. A failed match for either case generates a
KRB_AP_ERR_BADADDR error. The timestamp and usec fields (and the nonce field
if required) are checked next. If the timestamp and usec are not present, or
they are present but not current, the KRB_AP_ERR_SKEW error is generated.

If all the checks succeed, the application stores each of the new tickets in
its ticket cache together with the session key and other information in the
corresponding KrbCredInfo sequence from the encrypted part of the KRB_CRED
message.

4. The Kerberos Database

The Kerberos server must have access to a database containing the principal
identifiers and secret keys of any principals to be authenticated[4.1] using
such secret keys. The keying material in the database must be protected so
that they are only accessible to the Kerberos server and administrative
functions specifically authorized to access such material. Specific
implementations may handle the storage of keying material separate from the
Kerberos database (e.g. in hardware) or by encrypting the keying material
before placing it in the Kerberos database. Some implementations might
provide a means for using long term secret keys, but not for retrieving them
from the Kerberos database.


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4.1. Database contents

A database entry will typically contain the following fields, though in some
instances a KDC may obtain these values through other means:

Field                Value

name                 Principal's identifier
key                  Principal's secret key
p_kvno               Principal's key version
max_life             Maximum lifetime for Tickets
max_renewable_life   Maximum total lifetime for renewable Tickets

The name field is an encoding of the principal's identifier. The key field
contains an encryption key. This key is the principal's secret key. (The key
can be encrypted before storage under a Kerberos "master key" to protect it
in case the database is compromised but the master key is not. In that case,
an extra field must be added to indicate the master key version used, see
below.) The p_kvno field is the key version number of the principal's secret
key. The max_life field contains the maximum allowable lifetime (endtime -
starttime) for any Ticket issued for this principal. The max_renewable_life
field contains the maximum allowable total lifetime for any renewable Ticket
issued for this principal. (See section 3.1 for a description of how these
lifetimes are used in determining the lifetime of a given Ticket.)

A server may provide KDC service to several realms, as long as the database
representation provides a mechanism to distinguish between principal records
with identifiers which differ only in the realm name.

When an application server's key changes, if the change is routine (i.e. not
the result of disclosure of the old key), the old key should be retained by
the server until all tickets that had been issued using that key have
expired. Because of this, it is possible for several keys to be active for a
single principal. Ciphertext encrypted in a principal's key is always tagged
with the version of the key that was used for encryption, to help the
recipient find the proper key for decryption.

When more than one key is active for a particular principal, the principal
will have more than one record in the Kerberos database. The keys and key
version numbers will differ between the records (the rest of the fields may
or may not be the same). Whenever Kerberos issues a ticket, or responds to a
request for initial authentication, the most recent key (known by the
Kerberos server) will be used for encryption. This is the key with the
highest key version number.

4.2. Additional fields

Project Athena's KDC implementation uses additional fields in its database:

Field        Value

K_kvno       Kerberos' key version
expiration   Expiration date for entry
attributes   Bit field of attributes
mod_date     Timestamp of last modification
mod_name     Modifying principal's identifier

The K_kvno field indicates the key version of the Kerberos master key under
which the principal's secret key is encrypted.


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After an entry's expiration date has passed, the KDC will return an error to
any client attempting to gain tickets as or for the principal. (A database
may want to maintain two expiration dates: one for the principal, and one
for the principal's current key. This allows password aging to work
independently of the principal's expiration date. However, due to the
limited space in the responses, the KDC combines the key expiration and
principal expiration date into a single value called 'key_exp', which is
used as a hint to the user to take administrative action.)

The attributes field is a bitfield used to govern the operations involving
the principal. This field might be useful in conjunction with user
registration procedures, for site-specific policy implementations (Project
Athena currently uses it for their user registration process controlled by
the system-wide database service, Moira [LGDSR87]), to identify whether a
principal can play the role of a client or server or both, to note whether a
server is appropriately trusted to receive credentials delegated by a
client, or to identify the 'string to key' conversion algorithm used for a
principal's key[4.2]. Other bits are used to indicate that certain ticket
options should not be allowed in tickets encrypted under a principal's key
(one bit each): Disallow issuing postdated tickets, disallow issuing
forwardable tickets, disallow issuing tickets based on TGT authentication,
disallow issuing renewable tickets, disallow issuing proxiable tickets, and
disallow issuing tickets for which the principal is the server.

The mod_date field contains the time of last modification of the entry, and
the mod_name field contains the name of the principal which last modified
the entry.

4.3. Frequently Changing Fields

Some KDC implementations may wish to maintain the last time that a request
was made by a particular principal. Information that might be maintained
includes the time of the last request, the time of the last request for a
ticket-granting ticket, the time of the last use of a ticket-granting
ticket, or other times. This information can then be returned to the user in
the last-req field (see section 5.2).

Other frequently changing information that can be maintained is the latest
expiration time for any tickets that have been issued using each key. This
field would be used to indicate how long old keys must remain valid to allow
the continued use of outstanding tickets.

4.4. Site Constants

The KDC implementation should have the following configurable constants or
options, to allow an administrator to make and enforce policy decisions:

   * The minimum supported lifetime (used to determine whether the
     KDC_ERR_NEVER_VALID error should be returned). This constant should
     reflect reasonable expectations of round-trip time to the KDC,
     encryption/decryption time, and processing time by the client and
     target server, and it should allow for a minimum 'useful' lifetime.
   * The maximum allowable total (renewable) lifetime of a ticket
     (renew_till - starttime).
   * The maximum allowable lifetime of a ticket (endtime - starttime).
   * Whether to allow the issue of tickets with empty address fields
     (including the ability to specify that such tickets may only be issued
     if the request specifies some authorization_data).
   * Whether proxiable, forwardable, renewable or post-datable tickets are
     to be issued.


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5. Message Specifications

NOTE: We are continuing to work on changes to message format extensibility
as discussed at the London meeting. We believe the general form discussed in
London will continue to be a useful strategy for pursuing this goal. We
expect to have additional information by the Salt Lake City meeting. TODO:
TypedData needs to be looked at carefully, particularly with regard to
TD-APP-DEFINED-ERROR, etc. Some significant changes from 1510 to here have
been written up; more proofreading is needed. - tlyu

The Kerberos protocol is defined here in terms of Abstract Syntax Notation
One (ASN.1), which provides a syntax for specifying both the abstract layout
of protocol messages as well as their encodings. Implementors not utilizing
an existing ASN.1 compiler or support library are cautioned to thoroughly
understand the actual ASN.1 specification to ensure correct implementation
behavior, as there is more complexity in the notation than is immediately
obvious, and some tutorials and guides to ASN.1 are misleading or erroneous.

Note that in several places, there have been changes here from RFC 1510 that
change the abstract types. This is in part to address widespread assumptions
that various implementations have made, in some cases unintentionally
violating the ASN.1 standard in various ways. These will be clearly flagged
when they occur. The changes to the abstract types can cause incompatible
encodings to be emitted when certain encoding rules, e.g. the Packed
Encoding Rules (PER) are used. This should not be relevant for Kerberos,
since Kerberos explicitly specifies the use of the Distinguished Encoding
Rules (DER). This might be an issue for protocols wishing to use Kerberos
types with other encoding rules. (This practice is not recommended.) With
very few exceptions (most notably the usages of BIT STRING), the encodings
emitted by the DER, which are the only encodings permitted by this document
and by RFC 1510, remain identical.

The type definitions in this section assume an ASN.1 module definition of
the following form:

Kerberos5 {
    iso (1), org(3), dod(6), internet(1), security(5), kerberosV5(2)
} DEFINITIONS ::= BEGIN

-- rest of definitions here

END

This specifies an explicit non-automatic tagging for the ASN.1 type
definitions.

Note that in some other publications [RFC1510] [RFC1964], the "dod" portion
of the object identifier is erroneously specified as having the value "5".

Note that elsewhere in this document, nomenclature for various message types
is inconsistent, but seems to largely follow C language conventions,
including use of underscore (_) characters and all-caps spelling of names
intended to be numeric constants. Also, in some places, identifiers
(especially ones refering to constants) are written in all-caps in order to
distinguish them from surrounding explanatory text.

The ASN.1 notation does not permit underscores in identifiers, so in actual
ASN.1 definitions, underscores are replaced with hyphens (-). Additionally,
structure member names and defined values in ASN.1 must begin with a
lowercase letter, while type names must begin with an uppercase letter.


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5.1. Specific Compatibility Notes on ASN.1

For compatibility purposes, implementors should heed the following specific
notes regarding the use of ASN.1 in Kerberos. These notes do not describe a
non-standard usage of ASN.1, but rather some historical quirks and
non-compliance of various implementations, as well as historical
ambiguities, which, while being valid ASN.1, can lead to confusion during
implementation.

5.1.1. ASN.1 Distinguished Encoding Rules

The encoding of Kerberos protocol messages shall obey the Distinguished
Encoding Rules (DER) of ASN.1 as described in X.690 (1997). Some
implementations (believed to be primarly ones derived from DCE 1.1 and
earlier) are known to use the more general Basic Encoding Rules (BER); in
particular, these implementations send indefinite encodings of lengths.
Implementations may accept such encodings in the interests of backwards
compatibility, though implementors are warned that decoding fully-general
BER is fraught with peril.

5.1.2. Optional Fields in ASN.1 Sequences

Some implementations behave as if certain default values are equivalent to
omission of an optional value. Implementations should handle this case
gracefully. For example, the seq-number field in an Authenticator is
optional, but some implementations use an internal value of zero to indicate
that the field is to be omitted upon encoding. [While it is possible to use
the DEFAULT qualifier for the ASN.1 notation of a SEQUENCE member in order
to mandate this behavior, the result would be that the member would be
mandatory to omit if the value intended is that specified by the DEFAULT
keyword. This limits the possible semantics of the protocol.]

5.1.3. Zero-length SEQUENCE Types

There are places in the protocol where a message contains a SEQUENCE OF type
as an optional member, or a SEQUENCE type where all members are optional.
This can result in an encoding that contains an zero-length SEQUENCE or
SEQUENCE OF encoding. In general, implementations should not send
zero-length SEQUENCE OF or SEQUENCE encodings that are marked OPTIONAL, but
should accept them. [XXX there may be cases where an empty SEQUENCE type has
useful semantics, though]

5.1.4. Unrecognized Tag Numbers

Future revisions to this protocol may include new message types with
different APPLICATION class tag numbers. Such revisions should protect older
implementations by only sending the message types to parties that are known
to understand them, e.g. by means of a flag bit set by the receiver in a
preceding request. In the interest of robust error handling, implementations
should gracefully handle receiving a message with an unrecognized tag
anyway, and return an error message if appropriate.

5.1.5. Tag Numbers Greater Than 30

A naive implementation of a DER ASN.1 decoder may experience problems with
ASN.1 tag numbers greater than 30, due such tag numbers being encoded using
more than one byte. Future revisions of this protocol may utilize tag
numbers greater than 30, and implementations should be prepared to
gracefully return an error, if appropriate, if they do not recognize the
tag.


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5.2. Basic Kerberos Types

This section defines a number of basic types that are potentially used in
multiple Kerberos protocol messages.

5.2.1. KerberosString

[XXX The following paragraphs may need some editing, or maybe they want to
live in a footnote]

The original specification of the Kerberos protocol in RFC 1510 uses
GeneralString in numerous places for human-readable string data. Historical
implementations of Kerberos cannot utilize the full power of GeneralString.
This ASN.1 type requires the use of designation and invocation escape
sequences as specified in ISO 2022 to switch character sets, and the default
character set that is designated for G0 is basically US ASCII, which mostly
works. In practice, many implementations end up treating GeneralStrings as
if they were strings of whatever character set the implementation defaults
to, without regard for correct usage of character set designation escape
sequences.

Also, DER prohibits the invocation of character sets into any but the G0 and
C0 sets, which seems to outright prohibit the encoding of characters with
the high bit set. Unfortunately, this seems to have the side effect of
prohibiting the transmission of Latin-1 characters or any other characters
that belong to a 96-character set, since it is prohibited to invoke them
into G0. Some inconclusive discussion has taken place within the ASN.1
community on this subject. For now, we must assume that the ASN.1
specification of GeneralString as currently published is fundamentally
flawed in several ways.

One method of resolving these myriad difficulties is to constrain the use of
GeneralString to only include IA5String, which is essentially the US-ASCII.
US-ASCII control characters should in general not be used in KerberosString,
except for cases such as newlines in lengthy error messages.

The new (since RFC 1510) type KerberosString, defined below, is a CHOICE
containing a GeneralString that is constrained to only contain characters in
IA5String (which are US-ASCII). Note that the ASN.1 standard does not permit
the use of escape sequences to change the character sets while encoding an
IA5String.

KerberosString ::= CHOICE {
    general GeneralString (IA5String),
    ...
}

This CHOICE is extensible, so that when an interoperable solution for
internationalization is chosen, it will be easier to specify the changed
types. In the future, changes to this protocol that allow for extensions to
this CHOICE will be specified so that the transmitting party has some way of
knowing whether the receiving party can accept the chosen alternative of the
CHOICE.

Implementations may choose to accept GeneralString values that contain
characters other than those permitted by IA5String, but they should be aware
that character set designation codes will likely be absent, and that the
encoding should probably be treated as locale-specific in almost every way.
Implementations may also choose to emit GeneralString values that are beyond
those permitted by IA5String, but should be aware that doing so is
extraordinarily risky from an interoperability perspective.


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Some existing implementations use GeneralString to encode unescaped
locale-specific characters. This is in violation of the ASN.1 standard. Most
of these implementations encode US-ASCII in the left-hand half, so as long
the implementation transmits only US-ASCII, the ASN.1 standard is not
violated in this regard. As soon as such an implementation encodes unescaped
locale-specific characters with the high bit set, it violates the ASN.1
standard.

Other implementations have been known to use GeneralString to contain a
UTF-8 encoding. This also violates the ASN.1 standard, since UTF-8 is a
different encoding, not a 94 or 96 character "G" set as defined by ISO 2022.
It is believed that these implementations do not even use the ISO 2022
escape sequence to change the character encoding. Even if implementations
were to announce the change of encoding by using that escape sequence, the
ASN.1 standard prohibits the use of any escape sequences other than those
used to designate/invoke "G" or "C" sets allowed by GeneralString.

Future revisions to this protocol will almost certainly allow for a more
interoperable representation of principal names, probably including
UTF8String.

Note that both applying a new constraint to a previously unconstrained type
and replacing a type with a CHOICE containing that type constitute creations
of new ASN.1 types. In the case here, the change here does not result in a
changed encoding under DER. Also, note that various text in the ASN.1
standard actually suggests the strategy of replacing a type with a CHOICE
containing that type for certain deprecated types, even though this creates
a new type.

5.2.2. Realm and PrincipalName

Realm ::=           KerberosString
PrincipalName ::=   SEQUENCE {
                    name-type[0]     Int32,
                    name-string[1]   SEQUENCE OF KerberosString
}

Kerberos realm names are encoded as KerberosStrings. Realms shall not
contain a character with the code 0 (the ASCII NUL). Most realms will
usually consist of several components separated by periods (.), in the style
of Internet Domain Names, or separated by slashes (/) in the style of X.500
names. Acceptable forms for realm names are specified in section 7. A
PrincipalName is a typed sequence of components consisting of the following
sub-fields:

name-type
     This field specifies the type of name that follows. Pre-defined values
     for this field are specified in section 7.2. The name-type should be
     treated as a hint. Ignoring the name type, no two names can be the same
     (i.e. at least one of the components, or the realm, must be different).
     This constraint may be eliminated in the future.
name-string
     This field encodes a sequence of components that form a name, each
     component encoded as a KerberosString. Taken together, a PrincipalName
     and a Realm form a principal identifier. Most PrincipalNames will have
     only a few components (typically one or two).


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5.2.3. KerberosTime

KerberosTime ::=   GeneralizedTime
                   -- with no fractional seconds

The timestamps used in Kerberos are encoded as GeneralizedTimes. A
KerberosTime value shall not include any fractional portions of the seconds.
As required by the DER, it further shall not include any separators, and it
shall specify the UTC time zone (Z). Example: The only valid format for UTC
time 6 minutes, 27 seconds after 9 pm on 6 November 1985 is 19851106210627Z.

5.2.4. Constrained Integer types

Some integer members of types should be constrained to values representable
in 32 bits, for compatibility with reasonable implementation limits.

Int32 ::= INTEGER (-2147483648..2147483647)
          -- signed values representable in 32 bits

UInt32 :: = INTEGER (0..4294967295)
             -- unsigned 32 bit values

Microseconds ::= INTEGER (0..99999)
                 -- microseconds

While this results in changes to the abstract types from the RFC 1510
version, the encoding in DER should be unaltered. Historical implementations
were typically limited to 32-bit integer values anyway, and assigned numbers
should fall in the space of integer values representable in 32 bits in order
to promote interoperability anyway.

There are some members of messages types that are still defined as
unconstrained INTEGER types, but many of these have a (non-ASN.1) constraint
applied in the descriptive text. There are specific cases where more
discussion needs to occur regarding possible constraints, such as for the
nonce fields in various messages.

5.2.5. HostAddress and HostAddresses

HostAddress ::=     SEQUENCE  {
                    addr-type[0]             Int32,
                    address[1]               OCTET STRING
}

HostAddresses ::=   SEQUENCE OF HostAddress

The host address encodings consists of two fields:

addr-type
     This field specifies the type of address that follows. Pre-defined
     values for this field are specified in section 8.1.
address
     This field encodes a single address of type addr-type.

The two forms differ slightly. HostAddress contains exactly one address;
HostAddresses contains a sequence of possibly many addresses.


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5.2.6. AuthorizationData

AuthorizationData ::=   SEQUENCE OF SEQUENCE {
                        ad-type[0]               Int32,
                        ad-data[1]               OCTET STRING
}

ad-data
     This field contains authorization data to be interpreted according to
     the value of the corresponding ad-type field.
ad-type
     This field specifies the format for the ad-data subfield. All negative
     values are reserved for local use. Non-negative values are reserved for
     registered use.

Each sequence of type and data is referred to as an authorization element.
Elements may be application specific, however, there is a common set of
recursive elements that should be understood by all implementations. These
elements contain other elements embedded within them, and the interpretation
of the encapsulating element determines which of the embedded elements must
be interpreted, and which may be ignored. Definitions for these common
elements may be found in Appendix B.

5.2.7. PA-DATA

Historically, PA-DATA have been known as "pre-authentication data", meaning
that they were used to augment the initial authentication with the KDC.
Since that time, they have also been used as a typed hole with which to
extend protocol exchanges with the KDC.

PA-DATA ::=        SEQUENCE {
                   padata-type[1]     Int32,
                   padata-value[2]    OCTET STRING
                                      -- might be encoded AP-REQ
}

padata-type
     indicates the way that the padata-value element is to be interpreted.
     Negative values of padata-type are reserved for unregistered use;
     non-negative values are used for a registered interpretation of the
     element type.
padata-value
     Usually contains the DER encoding of another type; the padata-type
     field identifies which type is encoded here.

 padata-type          name              contents of padata-value

 1            pa-tgs-req            DER encoding of AP-REQ

 2            pa-enc-timestamp      DER encoding of PA-ENC-TIMESTAMP

 3            pa-pw-salt            salt (not ASN.1 encoded)

 10           pa-etype-info         DER encoding of PA-ETYPE-INFO

 20           pa-use-specified-kvno DER encoding of INTEGER

[XXX -- the following paragraph needs discussion, as does the general
concept of authenticating the cleartext pieces of the protocol]


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This field may also contain information needed by certain extensions to the
Kerberos protocol. For example, it might be used to initially verify the
identity of a client before any response is returned. When this field is
used to authenticate or pre-authenticate a request, it should contain a
keyed checksum over the KDC-REQ-BODY to bind the pre-authentication data to
rest of the request. The KDC, as a matter of policy, may decide whether to
honor a KDC-REQ which includes any pre-authentication data that does not
contain the checksum field.

It may also be used by the client to specify the version of a key that is
being used for accompanying preauthentication, and/or which should be used
to encrypt the reply from the KDC. [XXX the following paragraph should apply
perhaps to PA-DATA in general]

The padata field can also contain information needed to help the KDC or the
client select the key needed for generating or decrypting the response. This
form of the padata is useful for supporting the use of certain token cards
with Kerberos. The details of such extensions are specified in separate
documents. See [Pat92] for additional uses of this field.

5.2.7.1. PA-TGS-REQ

In the case of requests for additional tickets (KRB_TGS_REQ), padata-value
will contain an encoded AP-REQ. The checksum in the authenticator (which
must be collision-proof) is to be computed over the KDC-REQ-BODY encoding.

5.2.7.2. Encrypted Timestamp Pre-authentication

There are pre-authentication types that may be used to pre-authenticate a
client by means of an encrypted timestamp. The original PA-ENC-TIMESTAMP
does not contain a checksum of the KDC-REQ-BODY, while the PA-ENC-TIMESTAMP2
does.

PA-ENC-TIMESTAMP ::= EncryptedData -- encrypted PA-ENC-TS-ENC

PA-ENC-TS-ENC   ::= SEQUENCE {
       patimestamp[0]               KerberosTime, -- client's time
       pausec[1]                    Microseconds OPTIONAL
}

-- XXX maybe remove ENC-TIMESTAMP2 for now?

PA-ENC-TIMESTAMP2 ::= EncryptedData -- encrypted PA-ENC-TS2-ENC

PA-ENC-TS2-ENC   ::= SEQUENCE {
       patimestamp[0]               KerberosTime, -- client's time
       pausec[1]                    Microseconds OPTIONAL,
       pachecksum[2]                Checksum OPTIONAL
                                    -- keyed checksum of KDC-REQ-BODY
}

Patimestamp contains the client's time, and pausec contains the
microseconds, which may be omitted if a client will not generate more than
one request per second. The ciphertext (padata-value) consists of the
PA-ENC-TS-ENC or PA-ENC-TS2-ENC encoding, encrypted using the client's
secret key.

This preauthentication type was not present in RFC 1510, but many
implementations support it.


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5.2.7.3. PA-PW-SALT

The padata-value for this preauthentication type contains the salt for the
string-to-key to be used by the client to obtain the key for decrypting the
encrypted part of an AS-REP message. Unfortunately, for historical reasons,
the character set to be used is unspecified and probably locale-specific.

This preauthentication type was not present in RFC 1510, but many
implementations support it. It is necessary in any case where the salt for
the string-to-key algorithm is not the default.

In the trivial example, a zero-length salt string is very commonplace for
realms that have converted their principal databases from Kerberos 4.

5.2.7.4. PA-ETYPE-INFO

The ETYPE-INFO preauthentication type is sent by the KDC in a KRB-ERROR
indicating a requirement for additional preauthentication. It is usually
used to notify a client of which key to use for the encryption of an
encrypted timestamp for the purposes of sending a PA-ENC-TIMESTAMP
preauthentication value.

ETYPE-INFO-ENTRY ::= SEQUENCE {
        etype[0]        INTEGER,
        salt[1]         OCTET STRING OPTIONAL
}

ETYPE-INFO ::= SEQUENCE OF ETYPE-INFO-ENTRY

The salt, like that of PA-PW-SALT, is also completely unspecified with
respect to character set and is probably locale-specific.

[XXX -- not clear whether ETYPE-INFO or PW-SALT should take precedence if
they conflict]

This preauthentication type was not present in RFC 1510, but many
implementations that support encrypted timestamps for preauthentication need
to support ETYPE-INFO as well.

5.2.7.5. PA-USE-SPECIFIED-KVNO

The KDC should only accept and abide by the value of the use-specified-kvno
preauthentication data field when the specified key is still valid and until
use of a new key is confirmed. This situation is likely to occur primarily
during the period during which an updated key is propagating to other KDC's
in a realm.

5.2.8. KerberosFlags

For several message types, a specific constrained bit string type,
KerberosFlags, is used.

KerberosFlags ::= BIT STRING (SIZE (32..MAX))

Compatibility note: the following paragraphs describe a change from the
RFC1510 description of bit strings that would result in incompatility in the
case of an implementation that strictly conformed to ASN.1 DER and RFC1510.


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ASN.1 bit strings have multiple uses. The simplest use of a bit string is to
contain a vector of bits, with no particular meaning attached to individual
bits. This vector of bits is not necessarily a multiple of eight bits long.
The use in Kerberos of a bit string as a compact boolean vector wherein each
element has a distinct meaning poses some problems. The natural notation for
a compact boolean vector is the ASN.1 "NamedBit" notation, and the DER
require that encodings of a bit string using "NamedBit" notation exclude any
trailing zero bits. This truncation is easy to neglect, especially given C
language implementations that may naturally choose to store boolean vectors
as 32 bit integers.

For example, if the notation for KDCOptions were to include the "NamedBit"
notation, as in RFC 1510, and a KDCOptions value to be encoded had only the
"forwardable" (bit number one) bit set, the DER encoding must only include
two bits: the first reserved bit ("reserved", bit number zero, value zero)
and the one-valued bit (bit number one) for "forwardable".

Most existing implementations of Kerberos unconditionally send 32 bits on
the wire when encoding bit strings used as boolean vectors. This behavior
violates the ASN.1 syntax used for flag values in RFC 1510, but occurs on
such a widely installed base that the protocol description is being modified
to accomodate it.

Consequently, this document removes the "NamedBit" notations for individual
bits, relegating them to comments. The size constraint on the KerberosFlags
type requires that at least 32 bits be encoded at all times, though a
lenient implementation may choose to accept fewer than 32 bits and to treat
the missing bits as set to zero.

Currently, no uses of KerberosFlags specify more than 32 bits worth of
flags, although future revisions of this document may do so. When more than
32 bits are to be transmitted in a KerberosFlags value, future revisions to
this document will likely specify that the smallest number of bits needed to
encode the highest-numbered one-valued bit should be sent. This is somewhat
similar to the DER encoding of a bit string that is declared with the
"NamedBit" notation.

5.2.9. Cryptosystem-related Types

Many Kerberos protocol messages contain an EncryptedData as a container for
arbitrary encrypted data, which is often the encrypted encoding of another
data type. Fields within EncryptedData assist the recipient in selecting a
key with which to decrypt the enclosed data.

EncryptedData ::=   SEQUENCE {
                    etype[0]     Int32, -- EncryptionType
                    kvno[1]      INTEGER OPTIONAL,
                    cipher[2]    OCTET STRING -- ciphertext
}

etype
     This field identifies which encryption algorithm was used to encipher
     the cipher. Detailed specifications for selected encryption types
     appear in section 6.
kvno
     This field contains the version number of the key under which data is
     encrypted. It is only present in messages encrypted under long lasting
     keys, such as principals' secret keys.
cipher
     This field contains the enciphered text, encoded as an OCTET STRING.


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The EncryptionKey type is the means by which cryptographic keys used for
encryption are transfered.

EncryptionKey ::=   SEQUENCE {
                    keytype[0]    Int32, -- actually encryption type
                    keyvalue[1]   OCTET STRING
}

keytype
     This field specifies the encryption type of the encryption key that
     follows in the keyvalue field. While its name is "keytype", it actually
     specifies an encryption type. Previously, multiple cryptosystems that
     performed encryption differently but were capable of using keys with
     the same characteristics were permitted to share an assigned number to
     designate the type of key; this usage is now deprecated.
keyvalue
     This field contains the key itself, encoded as an octet string.

     All negative values for the encryption key type are reserved for local
     use. All non-negative values are reserved for officially assigned type
     fields and interpretations.

Messages containing cleartext data to be authenticated will usually do so by
using a member of type Checksum. Most instances of Checksum use a keyed
hash, though exceptions will be noted.

Checksum ::=   SEQUENCE {
               cksumtype[0]   Int32,
               checksum[1]    OCTET STRING
}

cksumtype
     This field indicates the algorithm used to generate the accompanying
     checksum.
checksum
     This field contains the checksum itself, encoded as an octet string.

     Detailed specification of selected checksum types appear in section 6.
     Negative values for the checksum type are reserved for local use. All
     non-negative values are reserved for officially assigned type fields
     and interpretations.

5.3. Tickets and Authenticators

This section describes the format and encryption parameters for tickets and
authenticators. When a ticket or authenticator is included in a protocol
message it is treated as an opaque object.


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5.3.1. Tickets

A ticket is a record that helps a client authenticate to a service. A Ticket
contains the following information:

Ticket ::=       [APPLICATION 1] SEQUENCE {
                  tkt-vno[0]                   INTEGER,
                  realm[1]                     Realm,
                  sname[2]                     PrincipalName,
                  enc-part[3]                  EncryptedData --EncTicketPart
}

-- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
                  flags[0]                     TicketFlags,
                  key[1]                       EncryptionKey,
                  crealm[2]                    Realm,
                  cname[3]                     PrincipalName,
                  transited[4]                 TransitedEncoding,
                  authtime[5]                  KerberosTime,
                  starttime[6]                 KerberosTime OPTIONAL,
                  endtime[7]                   KerberosTime,
                  renew-till[8]                KerberosTime OPTIONAL,
                  caddr[9]                     HostAddresses OPTIONAL,
                  authorization-data[10]       AuthorizationData OPTIONAL
}

-- encoded Transited field
TransitedEncoding ::=   SEQUENCE {
                        tr-type[0]             Int32, -- must be registered
                        contents[1]            OCTET STRING
}

TicketFlags ::= KerberosFlags
                  -- reserved(0),
                  -- forwardable(1),
                  -- forwarded(2),
                  -- proxiable(3),
                  -- proxy(4),
                  -- may-postdate(5),
                  -- postdated(6),
                  -- invalid(7),
                  -- renewable(8),
                  -- initial(9),
                  -- pre-authent(10),
                  -- hw-authent(11),
                  -- transited-policy-checked(12),
                  -- ok-as-delegate(13)
                  -- anonymous(14)


The encoding of EncTicketPart is encrypted in the key shared by Kerberos and
the end server (the server's secret key). See section 6 for the format of
the ciphertext.

tkt-vno
     This field specifies the version number for the ticket format. This
     document describes version number 5.

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realm
     This field specifies the realm that issued a ticket. It also serves to
     identify the realm part of the server's principal identifier. Since a
     Kerberos server can only issue tickets for servers within its realm,
     the two will always be identical.
sname
     This field specifies all components of the name part of the server's
     identity, including those parts that identify a specific instance of a
     service.
enc-part
     This field holds the encrypted encoding of the EncTicketPart sequence.
flags
     This field indicates which of various options were used or requested
     when the ticket was issued. It is a bit-field, where the selected
     options are indicated by the bit being set (1), and the unselected
     options and reserved fields being reset (0). [XXX X.690 ref and notes
     on pitfalls?] The meanings of the flags are:
      Bit(s)           Name                        Description

      0       reserved               Reserved for future expansion of this
                                     field.

                                     The FORWARDABLE flag is normally only
                                     interpreted by the TGS, and can be
                                     ignored by end servers. When set, this
      1       forwardable            flag tells the ticket-granting server
                                     that it is OK to issue a new
                                     ticket-granting ticket with a
                                     different network address based on the
                                     presented ticket.

                                     When set, this flag indicates that the
                                     ticket has either been forwarded or
      2       forwarded              was issued based on authentication
                                     involving a forwarded ticket-granting
                                     ticket.

                                     The PROXIABLE flag is normally only
                                     interpreted by the TGS, and can be
                                     ignored by end servers. The PROXIABLE
                                     flag has an interpretation identical
      3       proxiable              to that of the FORWARDABLE flag,
                                     except that the PROXIABLE flag tells
                                     the ticket-granting server that only
                                     non-ticket-granting tickets may be
                                     issued with different network
                                     addresses.

      4       proxy                  When set, this flag indicates that a
                                     ticket is a proxy.

                                     The MAY-POSTDATE flag is normally only
                                     interpreted by the TGS, and can be
      5       may-postdate           ignored by end servers. This flag
                                     tells the ticket-granting server that
                                     a post-dated ticket may be issued
                                     based on this ticket-granting ticket.

                                     This flag indicates that this ticket
                                     has been postdated. The end-service

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      6       postdated              can check the authtime field to see
                                     when the original authentication
                                     occurred.

                                     This flag indicates that a ticket is
                                     invalid, and it must be validated by
      7       invalid                the KDC before use. Application
                                     servers must reject tickets which have
                                     this flag set.

                                     The RENEWABLE flag is normally only
                                     interpreted by the TGS, and can
                                     usually be ignored by end servers
      8       renewable              (some particularly careful servers may
                                     wish to disallow renewable tickets). A
                                     renewable ticket can be used to obtain
                                     a replacement ticket that expires at a
                                     later date.

                                     This flag indicates that this ticket
      9       initial                was issued using the AS protocol, and
                                     not issued based on a ticket-granting
                                     ticket.

                                     This flag indicates that during
                                     initial authentication, the client was
                                     authenticated by the KDC before a
      10      pre-authent            ticket was issued. The strength of the
                                     preauthentication method is not
                                     indicated, but is acceptable to the
                                     KDC.

                                     This flag indicates that the protocol
                                     employed for initial authentication
                                     required the use of hardware expected
      11      hw-authent             to be possessed solely by the named
                                     client. The hardware authentication
                                     method is selected by the KDC and the
                                     strength of the method is not
                                     indicated.

                                     This flag indicates that the KDC for
                                     the realm has checked the transited
                                     field against a realm defined policy
                                     for trusted certifiers. If this flag
                                     is reset (0), then the application
                                     server must check the transited field
                                     itself, and if unable to do so it must
                                     reject the authentication. If the flag

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

      12      transited-             is set (1) then the application server
              policy-checked
                                     may skip its own validation of the
                                     transited field, relying on the
                                     validation performed by the KDC. At
                                     its option the application server may
                                     still apply its own validation based
                                     on a separate policy for acceptance.

                                     This flag is new since RFC 1510.

                                     This flag indicates that the server
                                     (not the client) specified in the
                                     ticket has been determined by policy
                                     of the realm to be a suitable
                                     recipient of delegation. A client can
                                     use the presence of this flag to help
                                     it make a decision whether to delegate
                                     credentials (either grant a proxy or a
                                     forwarded ticket granting ticket) to
      13      ok-as-delegate         this server. The client is free to
                                     ignore the value of this flag. When
                                     setting this flag, an administrator
                                     should consider the Security and
                                     placement of the server on which the
                                     service will run, as well as whether
                                     the service requires the use of
                                     delegated credentials.

                                     This flag is new since RFC 1510.

                                     This flag indicates that the principal
                                     named in the ticket is a generic
                                     principal for the realm and does not
                                     identify the individual using the
                                     ticket. The purpose of the ticket is
                                     only to securely distribute a session
                                     key, and not to identify the user.
      14      anonymous              Subsequent requests using the same
                                     ticket and session may be considered
                                     as originating from the same user, but
                                     requests with the same username but a
                                     different ticket are likely to
                                     originate from different users.

                                     This flag is new since RFC 1510.

      15-31   reserved               Reserved for future use.
key
     This field exists in the ticket and the KDC response and is used to
     pass the session key from Kerberos to the application server and the
     client. The field's encoding is described in section 6.2.
crealm
     This field contains the name of the realm in which the client is
     registered and in which initial authentication took place.
cname
     This field contains the name part of the client's principal identifier.
transited
     This field lists the names of the Kerberos realms that took part in
     authenticating the user to whom this ticket was issued. It does not

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     specify the order in which the realms were transited. See section
     3.3.3.2 for details on how this field encodes the traversed realms.
     When the names of CA's are to be embedded in the transited field (as
     specified for some extensions to the protocol), the X.500 names of the
     CA's should be mapped into items in the transited field using the
     mapping defined by RFC2253.
authtime
     This field indicates the time of initial authentication for the named
     principal. It is the time of issue for the original ticket on which
     this ticket is based. It is included in the ticket to provide
     additional information to the end service, and to provide the necessary
     information for implementation of a `hot list' service at the KDC. An
     end service that is particularly paranoid could refuse to accept
     tickets for which the initial authentication occurred "too far" in the
     past. This field is also returned as part of the response from the KDC.
     When returned as part of the response to initial authentication
     (KRB_AS_REP), this is the current time on the Kerberos server[24].
starttime
     This field in the ticket specifies the time after which the ticket is
     valid. Together with endtime, this field specifies the life of the
     ticket. If it is absent from the ticket, its value should be treated as
     that of the authtime field.
endtime
     This field contains the time after which the ticket will not be honored
     (its expiration time). Note that individual services may place their
     own limits on the life of a ticket and may reject tickets which have
     not yet expired. As such, this is really an upper bound on the
     expiration time for the ticket.
renew-till
     This field is only present in tickets that have the RENEWABLE flag set
     in the flags field. It indicates the maximum endtime that may be
     included in a renewal. It can be thought of as the absolute expiration
     time for the ticket, including all renewals.
caddr
     This field in a ticket contains zero (if omitted) or more (if present)
     host addresses. These are the addresses from which the ticket can be
     used. If there are no addresses, the ticket can be used from any
     location. The decision by the KDC to issue or by the end server to
     accept zero-address tickets is a policy decision and is left to the
     Kerberos and end-service administrators; they may refuse to issue or
     accept such tickets. The suggested and default policy, however, is that
     such tickets will only be issued or accepted when additional
     information that can be used to restrict the use of the ticket is
     included in the authorization_data field. Such a ticket is a
     capability.

     Network addresses are included in the ticket to make it harder for an
     attacker to use stolen credentials. Because the session key is not sent
     over the network in cleartext, credentials can't be stolen simply by
     listening to the network; an attacker has to gain access to the session
     key (perhaps through operating system security breaches or a careless
     user's unattended session) to make use of stolen tickets.

     It is important to note that the network address from which a
     connection is received cannot be reliably determined. Even if it could
     be, an attacker who has compromised the client's workstation could use
     the credentials from there. Including the network addresses only makes
     it more difficult, not impossible, for an attacker to walk off with
     stolen credentials and then use them from a "safe" location.

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

authorization-data
     The authorization-data field is used to pass authorization data from
     the principal on whose behalf a ticket was issued to the application
     service. If no authorization data is included, this field will be left
     out. Experience has shown that the name of this field is confusing, and
     that a better name for this field would be restrictions. Unfortunately,
     it is not possible to change the name of this field at this time.

     This field contains restrictions on any authority obtained on the basis
     of authentication using the ticket. It is possible for any principal in
     posession of credentials to add entries to the authorization data field
     since these entries further restrict what can be done with the ticket.
     Such additions can be made by specifying the additional entries when a
     new ticket is obtained during the TGS exchange, or they may be added
     during chained delegation using the authorization data field of the
     authenticator.

     Because entries may be added to this field by the holder of
     credentials, except when an entry is separately authenticated by
     encapsulation in the kdc-issued element, it is not allowable for the
     presence of an entry in the authorization data field of a ticket to
     amplify the privileges one would obtain from using a ticket.

     The data in this field may be specific to the end service; the field
     will contain the names of service specific objects, and the rights to
     those objects. The format for this field is described in section 5.2.
     Although Kerberos is not concerned with the format of the contents of
     the sub-fields, it does carry type information (ad-type).

     By using the authorization_data field, a principal is able to issue a
     proxy that is valid for a specific purpose. For example, a client
     wishing to print a file can obtain a file server proxy to be passed to
     the print server. By specifying the name of the file in the
     authorization_data field, the file server knows that the print server
     can only use the client's rights when accessing the particular file to
     be printed.

     A separate service providing authorization or certifying group
     membership may be built using the authorization-data field. In this
     case, the entity granting authorization (not the authorized entity),
     may obtain a ticket in its own name (e.g. the ticket is issued in the
     name of a privilege server), and this entity adds restrictions on its
     own authority and delegates the restricted authority through a proxy to
     the client. The client would then present this authorization credential
     to the application server separately from the authentication exchange.
     Alternatively, such authorization credentials may be embedded in the
     ticket authenticating the authorized entity, when the authorization is
     separately authenticated using the kdc-issued authorization data
     element (see B.4).

     Similarly, if one specifies the authorization-data field of a proxy and
     leaves the host addresses blank, the resulting ticket and session key
     can be treated as a capability. See [Neu93] for some suggested uses of
     this field.

     The authorization-data field is optional and does not have to be
     included in a ticket.


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5.3.2. Authenticators

An authenticator is a record sent with a ticket to a server to certify the
client's knowledge of the encryption key in the ticket, to help the server
detect replays, and to help choose a "true session key" to use with the
particular session. The encoding is encrypted in the ticket's session key
shared by the client and the server:

-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE  {
                  authenticator-vno[0]          INTEGER,
                  crealm[1]                     Realm,
                  cname[2]                      PrincipalName,
                  cksum[3]                      Checksum OPTIONAL,
                  cusec[4]                      Microseconds,
                  ctime[5]                      KerberosTime,
                  subkey[6]                     EncryptionKey OPTIONAL,
                  seq-number[7]                 UInt32 OPTIONAL,
                  authorization-data[8]         AuthorizationData OPTIONAL
}


authenticator-vno
     This field specifies the version number for the format of the
     authenticator. This document specifies version 5.
crealm and cname
     These fields are the same as those described for the ticket in section
     5.3.1.
cksum
     This field contains a checksum of the the application data that
     accompanies the KRB_AP_REQ.
cusec
     This field contains the microsecond part of the client's timestamp. Its
     value (before encryption) ranges from 0 to 999999. It often appears
     along with ctime. The two fields are used together to specify a
     reasonably accurate timestamp.
ctime
     This field contains the current time on the client's host.
subkey
     This field contains the client's choice for an encryption key which is
     to be used to protect this specific application session. Unless an
     application specifies otherwise, if this field is left out the session
     key from the ticket will be used.
seq-number
     This optional field includes the initial sequence number to be used by
     the KRB_PRIV or KRB_SAFE messages when sequence numbers are used to
     detect replays (It may also be used by application specific messages).
     When included in the authenticator this field specifies the initial
     sequence number for messages from the client to the server. When
     included in the AP-REP message, the initial sequence number is that for
     messages from the server to the client. When used in KRB_PRIV or
     KRB_SAFE messages, it is incremented by one after each message is sent.
     Sequence numbers fall in the range of 0 through 2^32 - 1 and wrap to
     zero following the value 2^32 - 1.

     For sequence numbers to adequately support the detection of replays
     they should be non-repeating, even across connection boundaries. The
     initial sequence number should be random and uniformly distributed
     across the full space of possible sequence numbers, so that it cannot
     be guessed by an attacker and so that it and the successive sequence
     numbers do not repeat other sequences.

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authorization-data
     This field is the same as described for the ticket in section 5.3.1. It
     is optional and will only appear when additional restrictions are to be
     placed on the use of a ticket, beyond those carried in the ticket
     itself.

5.4. Specifications for the AS and TGS exchanges

This section specifies the format of the messages used in the exchange
between the client and the Kerberos server. The format of possible error
messages appears in section 5.9.1.

5.4.1. KRB_KDC_REQ definition

The KRB_KDC_REQ message has no type of its own. Instead, its type is one of
KRB_AS_REQ or KRB_TGS_REQ depending on whether the request is for an initial
ticket or an additional ticket. In either case, the message is sent from the
client to the Authentication Server to request credentials for a service.

The message fields are:

AS-REQ ::=         [APPLICATION 10] KDC-REQ
TGS-REQ ::=        [APPLICATION 12] KDC-REQ

KDC-REQ ::=        SEQUENCE {
                   pvno[1]            INTEGER,
                   msg-type[2]        INTEGER,
                   padata[3]          SEQUENCE OF PA-DATA OPTIONAL,
                   req-body[4]        KDC-REQ-BODY
}

KDC-REQ-BODY ::=   SEQUENCE {
                    kdc-options[0]         KDCOptions,
                    cname[1]               PrincipalName OPTIONAL,
                                           -- Used only in AS-REQ
                    realm[2]               Realm, -- Server's realm
                                           -- Also client's in AS-REQ
                    sname[3]               PrincipalName OPTIONAL,
                    from[4]                KerberosTime OPTIONAL,
                    till[5]                KerberosTime,
                    rtime[6]               KerberosTime OPTIONAL,
                    nonce[7]               INTEGER,
                    etype[8]               SEQUENCE OF Int32,
                                           -- EncryptionType,
                                           -- in preference order
                    addresses[9]           HostAddresses OPTIONAL,
                enc-authorization-data[10] EncryptedData OPTIONAL,
                                           -- Encrypted AuthorizationData
                                           -- encoding
                    additional-tickets[11] SEQUENCE OF Ticket OPTIONAL
}


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KDCOptions ::=   KerberosFlags
                  -- reserved(0),
                  -- forwardable(1),
                  -- forwarded(2),
                  -- proxiable(3),
                  -- proxy(4),
                  -- allow-postdate(5),
                  -- postdated(6),
                  -- unused7(7),
                  -- renewable(8),
                  -- unused9(9),
                  -- unused10(10),
                  -- unused11(11),
                  -- unused12(12),
                  -- unused13(13),
                  -- requestanonymous(14),
                  -- canonicalize(15),
                  -- disable-transited-check(26),
                  -- renewable-ok(27),
                  -- enc-tkt-in-skey(28),
                  -- renew(30),
                  -- validate(31)

The fields in this message are:

pvno
     This field is included in each message, and specifies the protocol
     version number. This document specifies protocol version 5.
msg-type
     This field indicates the type of a protocol message. It will almost
     always be the same as the application identifier associated with a
     message. It is included to make the identifier more readily accessible
     to the application. For the KDC-REQ message, this type will be
     KRB_AS_REQ or KRB_TGS_REQ.
padata
     Contains pre-authentication data. Requests for additional tickets
     (KRB_TGS_REQ) must contain a padata of PA-TGS-REQ.

     The padata (pre-authentication data) field contains a sequence of
     authentication information which may be needed before credentials can
     be issued or decrypted. In most requests for initial authentication
     (KRB_AS_REQ) and most replies (KDC-REP), the padata field will be left
     out.
req-body
     This field is a placeholder delimiting the extent of the remaining
     fields. If a checksum is to be calculated over the request, it is
     calculated over an encoding of the KDC-REQ-BODY sequence which is
     enclosed within the req-body field.
kdc-options
     This field appears in the KRB_AS_REQ and KRB_TGS_REQ requests to the
     KDC and indicates the flags that the client wants set on the tickets as
     well as other information that is to modify the behavior of the KDC.
     Where appropriate, the name of an option may be the same as the flag
     that is set by that option. Although in most case, the bit in the
     options field will be the same as that in the flags field, this is not
     guaranteed, so it is not acceptable to simply copy the options field to
     the flags field. There are various checks that must be made before
     honoring an option anyway.


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     The kdc_options field is a bit-field, where the selected options are
     indicated by the bit being set (1), and the unselected options and
     reserved fields being reset (0). The encoding of the bits is specified
     in section 5.2. The options are described in more detail above in
     section 2. The meanings of the options are:
      Bits             Name                        Description

      0      RESERVED                  Reserved for future expansion of
                                       this field.

                                       The FORWARDABLE option indicates
                                       that the ticket to be issued is to
                                       have its forwardable flag set. It
      1      FORWARDABLE               may only be set on the initial
                                       request, or in a subsequent request
                                       if the ticket-granting ticket on
                                       which it is based is also
                                       forwardable.

                                       The FORWARDED option is only
                                       specified in a request to the
                                       ticket-granting server and will only
                                       be honored if the ticket-granting
                                       ticket in the request has its
      2      FORWARDED                 FORWARDABLE bit set. This option
                                       indicates that this is a request for
                                       forwarding. The address(es) of the
                                       host from which the resulting ticket
                                       is to be valid are included in the
                                       addresses field of the request.

                                       The PROXIABLE option indicates that
                                       the ticket to be issued is to have
                                       its proxiable flag set. It may only
      3      PROXIABLE                 be set on the initial request, or in
                                       a subsequent request if the
                                       ticket-granting ticket on which it
                                       is based is also proxiable.

                                       The PROXY option indicates that this
                                       is a request for a proxy. This
                                       option will only be honored if the
                                       ticket-granting ticket in the
      4      PROXY                     request has its PROXIABLE bit set.
                                       The address(es) of the host from
                                       which the resulting ticket is to be
                                       valid are included in the addresses
                                       field of the request.

                                       The ALLOW-POSTDATE option indicates
                                       that the ticket to be issued is to
                                       have its MAY-POSTDATE flag set. It
      5      ALLOW-POSTDATE            may only be set on the initial
                                       request, or in a subsequent request
                                       if the ticket-granting ticket on
                                       which it is based also has its
                                       MAY-POSTDATE flag set.

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                                       The POSTDATED option indicates that
                                       this is a request for a postdated
                                       ticket. This option will only be
                                       honored if the ticket-granting
                                       ticket on which it is based has its
      6      POSTDATED                 MAY-POSTDATE flag set. The resulting
                                       ticket will also have its INVALID
                                       flag set, and that flag may be reset
                                       by a subsequent request to the KDC
                                       after the starttime in the ticket
                                       has been reached.

      7      UNUSED                    This option is presently unused.

                                       The RENEWABLE option indicates that
                                       the ticket to be issued is to have
                                       its RENEWABLE flag set. It may only
                                       be set on the initial request, or
                                       when the ticket-granting ticket on
      8      RENEWABLE                 which the request is based is also
                                       renewable. If this option is
                                       requested, then the rtime field in
                                       the request contains the desired
                                       absolute expiration time for the
                                       ticket.

      9      RESERVED                  Reserved for PK-Cross

      10-13  UNUSED                    These options are presently unused.

                                       The REQUEST-ANONYMOUS option
                                       indicates that the ticket to be
                                       issued is not to identify the user
                                       to which it was issued. Instead, the
                                       principal identifier is to be
                                       generic, as specified by the policy
                                       of the realm (e.g. usually
                                       anonymous@realm). The purpose of the
      14     REQUEST-ANONYMOUS         ticket is only to securely
                                       distribute a session key, and not to
                                       identify the user. The ANONYMOUS
                                       flag on the ticket to be returned
                                       should be set. If the local realms
                                       policy does not permit anonymous
                                       credentials, the request is to be
                                       rejected.

                                       This flag is new since RFC 1510

                                       The CANONICALIZE option indicates
                                       that the client will accept the
                                       return of a true server name instead
                                       of the name specified in the
                                       request. In addition the client will
                                       be able to process any TGT referrals
                                       that will direct the client to
                                       another realm to locate the
      15     CANONICALIZE              requested server. If a KDC does not
                                       support name- canonicalization, the
                                       option is ignored and the
                                       appropriate
                                       KDC_ERR_C_PRINCIPAL_UNKNOWN or
                                       KDC_ERR_S_PRINCIPAL_UNKNOWN error is
                                       returned. [JBrezak]

                                       This flag is new since RFC 1510

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f


      16-25  RESERVED                  Reserved for future use.

                                       By default the KDC will check the
                                       transited field of a
                                       ticket-granting-ticket against the
                                       policy of the local realm before it
                                       will issue derivative tickets based
                                       on the ticket granting ticket. If
                                       this flag is set in the request,
                                       checking of the transited field is
                                       disabled. Tickets issued without the
      26     DISABLE-TRANSITED-CHECK   performance of this check will be
                                       noted by the reset (0) value of the
                                       TRANSITED-POLICY-CHECKED flag,
                                       indicating to the application server
                                       that the tranisted field must be
                                       checked locally. KDC's are
                                       encouraged but not required to honor
                                       the DISABLE-TRANSITED-CHECK option.

                                       This flag is new since RFC 1510

                                       The RENEWABLE-OK option indicates
                                       that a renewable ticket will be
                                       acceptable if a ticket with the
                                       requested life cannot otherwise be
                                       provided. If a ticket with the
                                       requested life cannot be provided,
      27     RENEWABLE-OK              then a renewable ticket may be
                                       issued with a renew-till equal to
                                       the the requested endtime. The value
                                       of the renew-till field may still be
                                       limited by local limits, or limits
                                       selected by the individual principal
                                       or server.

                                       This option is used only by the
                                       ticket-granting service. The
                                       ENC-TKT-IN-SKEY option indicates
      28     ENC-TKT-IN-SKEY           that the ticket for the end server
                                       is to be encrypted in the session
                                       key from the additional
                                       ticket-granting ticket provided.

      29     RESERVED                  Reserved for future use.

                                       This option is used only by the
                                       ticket-granting service. The RENEW
                                       option indicates that the present
                                       request is for a renewal. The ticket
                                       provided is encrypted in the secret
                                       key for the server on which it is
      30     RENEW                     valid. This option will only be
                                       honored if the ticket to be renewed
                                       has its RENEWABLE flag set and if
                                       the time in its renew-till field has
                                       not passed. The ticket to be renewed
                                       is passed in the padata field as
                                       part of the authentication header.

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                                       This option is used only by the
                                       ticket-granting service. The
                                       VALIDATE option indicates that the
                                       request is to validate a postdated
                                       ticket. It will only be honored if
                                       the ticket presented is postdated,
                                       presently has its INVALID flag set,
      31     VALIDATE                  and would be otherwise usable at
                                       this time. A ticket cannot be
                                       validated before its starttime. The
                                       ticket presented for validation is
                                       encrypted in the key of the server
                                       for which it is valid and is passed
                                       in the padata field as part of the
                                       authentication header.
cname and sname
     These fields are the same as those described for the ticket in section
     5.3.1. sname may only be absent when the ENC-TKT-IN-SKEY option is
     specified. If absent, the name of the server is taken from the name of
     the client in the ticket passed as additional-tickets.
enc-authorization-data
     The enc-authorization-data, if present (and it can only be present in
     the TGS_REQ form), is an encoding of the desired authorization-data
     encrypted under the sub-session key if present in the Authenticator, or
     alternatively from the session key in the ticket-granting ticket, both
     from the padata field in the KRB_AP_REQ.
realm
     This field specifies the realm part of the server's principal
     identifier. In the AS exchange, this is also the realm part of the
     client's principal identifier. If the CANONICALIZE option is set, the
     realm is used as a hint to the KDC for its database lookup.
from
     This field is included in the KRB_AS_REQ and KRB_TGS_REQ ticket
     requests when the requested ticket is to be postdated. It specifies the
     desired start time for the requested ticket. If this field is omitted
     then the KDC should use the current time instead.
till
     This field contains the expiration date requested by the client in a
     ticket request. [XXX This was optional in kerberos-revisions, but
     required in 1510. we should make it required and specify semantics for
     19700101000000Z] It is optional and if omitted the requested ticket is
     to have the maximum endtime permitted according to KDC policy for the
     parties to the authentication exchange as limited by expiration date of
     the ticket granting ticket or other preauthentication credentials.
rtime
     This field is the requested renew-till time sent from a client to the
     KDC in a ticket request. It is optional.
nonce
     This field is part of the KDC request and response. It it intended to
     hold a random number generated by the client. If the same number is
     included in the encrypted response from the KDC, it provides evidence
     that the response is fresh and has not been replayed by an attacker.
     Nonces must never be re-used. Ideally, it should be generated randomly,
     but if the correct time is known, it may suffice[25].
etype
     This field specifies the desired encryption algorithm to be used in the
     response.

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addresses
     This field is included in the initial request for tickets, and
     optionally included in requests for additional tickets from the
     ticket-granting server. It specifies the addresses from which the
     requested ticket is to be valid. Normally it includes the addresses for
     the client's host. If a proxy is requested, this field will contain
     other addresses. The contents of this field are usually copied by the
     KDC into the caddr field of the resulting ticket.
additional-tickets
     Additional tickets may be optionally included in a request to the
     ticket-granting server. If the ENC-TKT-IN-SKEY option has been
     specified, then the session key from the additional ticket will be used
     in place of the server's key to encrypt the new ticket. When the
     ENC-TKT-IN-SKEY option is used for user-to-user authentication, this
     addional ticket may be a TGT issued by the local realm or an
     inter-realm TGT issued for the current KDC's realm by a remote KDC. If
     more than one option which requires additional tickets has been
     specified, then the additional tickets are used in the order specified
     by the ordering of the options bits (see kdc-options, above).

The application tag number will be either ten (10) or twelve (12) depending
on whether the request is for an initial ticket (AS-REQ) or for an
additional ticket (TGS-REQ).

The optional fields (addresses, authorization-data and additional-tickets)
are only included if necessary to perform the operation specified in the
kdc-options field.

It should be noted that in KRB_TGS_REQ, the protocol version number appears
twice and two different message types appear: the KRB_TGS_REQ message
contains these fields as does the authentication header (KRB_AP_REQ) that is
passed in the padata field.

5.4.2. KRB_KDC_REP definition

The KRB_KDC_REP message format is used for the reply from the KDC for either
an initial (AS) request or a subsequent (TGS) request. There is no message
type for KRB_KDC_REP. Instead, the type will be either KRB_AS_REP or
KRB_TGS_REP. The key used to encrypt the ciphertext part of the reply
depends on the message type. For KRB_AS_REP, the ciphertext is encrypted in
the client's secret key, and the client's key version number is included in
the key version number for the encrypted data. For KRB_TGS_REP, the
ciphertext is encrypted in the sub-session key from the Authenticator, or if
absent, the session key from the ticket-granting ticket used in the request.
In that case, no version number will be present in the EncryptedData
sequence.

The KRB_KDC_REP message contains the following fields:

AS-REP ::=    [APPLICATION 11] KDC-REP
TGS-REP ::=   [APPLICATION 13] KDC-REP

KDC-REP ::=   SEQUENCE {
              pvno[0]                    INTEGER,
              msg-type[1]                INTEGER,
              padata[2]                  SEQUENCE OF PA-DATA OPTIONAL,
              crealm[3]                  Realm,
              cname[4]                   PrincipalName,
              ticket[5]                  Ticket,
              enc-part[6]                EncryptedData
                                         -- EncASREpPart or EncTGSReoOart
}


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EncASRepPart ::=    [APPLICATION 25] EncKDCRepPart -- note [27]
EncTGSRepPart ::=   [APPLICATION 26] EncKDCRepPart

EncKDCRepPart ::=   SEQUENCE {
                    key[0]               EncryptionKey,
                    last-req[1]          LastReq,
                    nonce[2]             INTEGER,
                    key-expiration[3]    KerberosTime OPTIONAL,
                    flags[4]             TicketFlags,
                    authtime[5]          KerberosTime,
                    starttime[6]         KerberosTime OPTIONAL,
                    endtime[7]           KerberosTime,
                    renew-till[8]        KerberosTime OPTIONAL,
                    srealm[9]            Realm,
                    sname[10]            PrincipalName,
                    caddr[11]            HostAddresses OPTIONAL
}
LastReq ::=   SEQUENCE OF SEQUENCE {
               lr-type[0]               Int32,
               lr-value[1]              KerberosTime
}

pvno and msg-type
     These fields are described above in section 5.4.1. msg-type is either
     KRB_AS_REP or KRB_TGS_REP.
padata
     This field is described in detail in section 5.4.1. One possible use
     for this field is to encode an alternate "mix-in" string to be used
     with a string-to-key algorithm (such as is described in section 6.3.2).
     This ability is useful to ease transitions if a realm name needs to
     change (e.g. when a company is acquired); in such a case all existing
     password-derived entries in the KDC database would be flagged as
     needing a special mix-in string until the next password change.
crealm, cname, srealm and sname
     These fields are the same as those described for the ticket in section
     5.3.1.
ticket
     The newly-issued ticket, from section 5.3.1.
enc-part
     This field is a place holder for the ciphertext and related information
     that forms the encrypted part of a message. The description of the
     encrypted part of the message follows each appearance of this field.
     The encrypted part is encoded as described in section 6.1.
key
     This field is the same as described for the ticket in section 5.3.1.
last-req
     This field is returned by the KDC and specifies the time(s) of the last
     request by a principal. Depending on what information is available,
     this might be the last time that a request for a ticket-granting ticket
     was made, or the last time that a request based on a ticket-granting
     ticket was successful. It also might cover all servers for a realm, or
     just the particular server. Some implementations may display this
     information to the user to aid in discovering unauthorized use of one's
     identity. It is similar in spirit to the last login time displayed when
     logging into timesharing systems.
     lr-type
          This field indicates how the following lr-value field is to be
          interpreted. Negative values indicate that the information
          pertains only to the responding server. Non-negative values
          pertain to all servers for the realm.


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          If the lr-type field is zero (0), then no information is conveyed
          by the lr-value subfield. If the absolute value of the lr-type
          field is one (1), then the lr-value subfield is the time of last
          initial request for a TGT. If it is two (2), then the lr-value
          subfield is the time of last initial request. If it is three (3),
          then the lr-value subfield is the time of issue for the newest
          ticket-granting ticket used. If it is four (4), then the lr-value
          subfield is the time of the last renewal. If it is five (5), then
          the lr-value subfield is the time of last request (of any type).
          If it is (6), then the lr-value subfield is the time when the
          password will expire.
     lr-value
          This field contains the time of the last request. the time must be
          interpreted according to the contents of the accompanying lr-type
          subfield.
nonce
     This field is described above in section 5.4.1.
key-expiration
     The key-expiration field is part of the response from the KDC and
     specifies the time that the client's secret key is due to expire. The
     expiration might be the result of password aging or an account
     expiration. This field will usually be left out of the TGS reply since
     the response to the TGS request is encrypted in a session key and no
     client information need be retrieved from the KDC database. It is up to
     the application client (usually the login program) to take appropriate
     action (such as notifying the user) if the expiration time is imminent.
flags, authtime, starttime, endtime, renew-till and caddr
     These fields are duplicates of those found in the encrypted portion of
     the attached ticket (see section 5.3.1), provided so the client may
     verify they match the intended request and to assist in proper ticket
     caching. If the message is of type KRB_TGS_REP, the caddr field will
     only be filled in if the request was for a proxy or forwarded ticket,
     or if the user is substituting a subset of the addresses from the
     ticket granting ticket. If the client-requested addresses are not
     present or not used, then the addresses contained in the ticket will be
     the same as those included in the ticket-granting ticket.

5.5. Client/Server (CS) message specifications

This section specifies the format of the messages used for the
authentication of the client to the application server.

5.5.1. KRB_AP_REQ definition

The KRB_AP_REQ message contains the Kerberos protocol version number, the
message type KRB_AP_REQ, an options field to indicate any options in use,
and the ticket and authenticator themselves. The KRB_AP_REQ message is often
referred to as the 'authentication header'.

AP-REQ ::=      [APPLICATION 14] SEQUENCE {
                pvno[0]                       INTEGER,
                msg-type[1]                   INTEGER,
                ap-options[2]                 APOptions,
                ticket[3]                     Ticket,
                authenticator[4]              EncryptedData
                                              -- Authenticator from 5.3.2
}


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APOptions ::=   KerberosFlags
                  -- reserved(0),
                  -- use-session-key(1),
                  -- mutual-required(2)


pvno and msg-type
     These fields are described above in section 5.4.1. msg-type is
     KRB_AP_REQ.
ap-options
     This field appears in the application request (KRB_AP_REQ) and affects
     the way the request is processed. It is a bit-field, where the selected
     options are indicated by the bit being set (1), and the unselected
     options and reserved fields being reset (0). The encoding of the bits
     is specified in section 5.2. The meanings of the options are:
      Bit(s)       Name                        Description

      0       reserved        Reserved for future expansion of this field.

                              The USE-SESSION-KEY option indicates that the
                              ticket the client is presenting to a server
      1       use-session-key is encrypted in the session key from the
                              server's ticket-granting ticket. When this
                              option is not specified, the ticket is
                              encrypted in the server's secret key.

                              The MUTUAL-REQUIRED option tells the server
      2       mutual-required that the client requires mutual
                              authentication, and that it must respond with
                              a KRB_AP_REP message.

      3-31    reserved        Reserved for future use.
ticket
     This field is a ticket authenticating the client to the server.
authenticator
     This contains the authenticator, which includes the client's choice of
     a subkey. Its encoding is described in section 5.3.2.

5.5.2. KRB_AP_REP definition

The KRB_AP_REP message contains the Kerberos protocol version number, the
message type, and an encrypted time- stamp. The message is sent in in
response to an application request (KRB_AP_REQ) where the mutual
authentication option has been selected in the ap-options field.

AP-REP ::=         [APPLICATION 15] SEQUENCE {
                   pvno[0]                           INTEGER,
                   msg-type[1]                       INTEGER,
                   enc-part[2]                       EncryptedData
                                                     -- EncAPRepPart
}

EncAPRepPart ::=   [APPLICATION 27] SEQUENCE { -- note [29]
                   ctime[0]                          KerberosTime,
                   cusec[1]                          Microseconds,
                   subkey[2]                         EncryptionKey OPTIONAL,
                   seq-number[3]                     UInt32 OPTIONAL
}


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The encoded EncAPRepPart is encrypted in the shared session key of the
ticket. The optional subkey field can be used in an application-arranged
negotiation to choose a per association session key.

pvno and msg-type
     These fields are described above in section 5.4.1. msg-type is
     KRB_AP_REP.
enc-part
     This field is described above in section 5.4.2.
ctime
     This field contains the current time on the client's host.
cusec
     This field contains the microsecond part of the client's timestamp.
subkey
     This field contains an encryption key which is to be used to protect
     this specific application session. See section 3.2.6 for specifics on
     how this field is used to negotiate a key. Unless an application
     specifies otherwise, if this field is left out, the sub-session key
     from the authenticator, or if also left out, the session key from the
     ticket will be used.
seq-number
     This field is described above in section 5.3.2.

5.5.3. Error message reply

If an error occurs while processing the application request, the KRB_ERROR
message will be sent in response. See section 5.9.1 for the format of the
error message. The cname and crealm fields may be left out if the server
cannot determine their appropriate values from the corresponding KRB_AP_REQ
message. If the authenticator was decipherable, the ctime and cusec fields
will contain the values from it.

5.6. KRB_SAFE message specification

This section specifies the format of a message that can be used by either
side (client or server) of an application to send a tamper-proof message to
its peer. It presumes that a session key has previously been exchanged (for
example, by using the KRB_AP_REQ/KRB_AP_REP messages).

There are two KRB_SAFE messages; the KRB-SAFE message is the one specified
in RFC 1510. The KRB-SAFE2 message is new with this document, and shares a
number of fields with the old KRB-SAFE message.

5.6.1. KRB_SAFE definition

The KRB_SAFE message contains user data along with a collision-proof
checksum keyed with the last encryption key negotiated via subkeys, or the
session key if no negotiation has occurred. The message fields are:

KRB-SAFE ::=        [APPLICATION 20] SEQUENCE {
                    pvno[0]                       INTEGER,
                    msg-type[1]                   INTEGER,
                    safe-body[2]                  KRB-SAFE-BODY,
                    cksum[3]                      Checksum
}

KRB-SAFE-BODY ::=   SEQUENCE {
                    user-data[0]                  OCTET STRING,
                    timestamp[1]                  KerberosTime OPTIONAL,
                    usec[2]                       Microseconds OPTIONAL,
                    seq-number[3]                 UInt32 OPTIONAL,
                    s-address[4]                  HostAddress,
                    r-address[5]                  HostAddress OPTIONAL
}


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pvno and msg-type
     These fields are described above in section 5.4.1. msg-type is KRB_SAFE
     or KRB_SAFE2, respectively, for the KRB-SAFE and KRB-SAFE2 messages.
safe-body
     This field is a placeholder for the body of the KRB-SAFE message.
cksum
     This field contains the checksum of the application data. Checksum
     details are described in section 6.4.

     The checksum is computed over the encoding of the KRB-SAFE sequence.
     First, the cksum is set to a type zero, zero-length value and the
     checksum is computed over the encoding of the KRB-SAFE sequence, then
     the checksum is set to the result of that computation, and finally the
     KRB-SAFE sequence is encoded again. This method, while different than
     the one specified in RFC 1510, corresponds to existing practice.
user-data
     This field is part of the KRB_SAFE and KRB_PRIV messages and contain
     the application specific data that is being passed from the sender to
     the recipient.
timestamp
     This field is part of the KRB_SAFE and KRB_PRIV messages. Its contents
     are the current time as known by the sender of the message. By checking
     the timestamp, the recipient of the message is able to make sure that
     it was recently generated, and is not a replay.
usec
     This field is part of the KRB_SAFE and KRB_PRIV headers. It contains
     the microsecond part of the timestamp.
seq-number
     This field is described above in section 5.3.2.
s-address
     Sender's address.

     This field specifies the address in use by the sender of the message.
     It may be omitted if not required by the application protocol.
r-address
     This field specifies the address in use by the recipient of the
     message. It may be omitted for some uses (such as broadcast protocols),
     but the recipient may arbitrarily reject such messages. This field,
     along with s-address, can be used to help detect messages which have
     been incorrectly or maliciously delivered to the wrong recipient.

5.7. KRB_PRIV message specification

This section specifies the format of a message that can be used by either
side (client or server) of an application to securely and privately send a
message to its peer. It presumes that a session key has previously been
exchanged (for example, by using the KRB_AP_REQ/KRB_AP_REP messages).

5.7.1. KRB_PRIV definition

The KRB_PRIV message contains user data encrypted in the Session Key. The
message fields are:

KRB-PRIV ::=         [APPLICATION 21] SEQUENCE {
                     pvno[0]                           INTEGER,
                     msg-type[1]                       INTEGER,
                     enc-part[3]                       EncryptedData
                                                       -- EncKrbPrivPart
}


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EncKrbPrivPart ::=   [APPLICATION 28] SEQUENCE { --note [31]
                     user-data[0]        OCTET STRING,
                     timestamp[1]        KerberosTime OPTIONAL,
                     usec[2]             Microseconds OPTIONAL,
                     seq-number[3]       UInt32 OPTIONAL,
                     s-address[4]        HostAddress, -- sender's addr
                     r-address[5]        HostAddress OPTIONAL -- recip's addr
}

pvno and msg-type
     These fields are described above in section 5.4.1. msg-type is
     KRB_PRIV.
enc-part
     This field holds an encoding of the EncKrbPrivPart sequence encrypted
     under the session key[32]. This encrypted encoding is used for the
     enc-part field of the KRB-PRIV message. See section 6 for the format of
     the ciphertext.
user-data, timestamp, usec, s-address and r-address
     These fields are described above in section 5.6.1.
seq-number
     This field is described above in section 5.3.2.

5.8. KRB_CRED message specification

This section specifies the format of a message that can be used to send
Kerberos credentials from one principal to another. It is presented here to
encourage a common mechanism to be used by applications when forwarding
tickets or providing proxies to subordinate servers. It presumes that a
session key has already been exchanged perhaps by using the
KRB_AP_REQ/KRB_AP_REP messages.

5.8.1. KRB_CRED definition

The KRB_CRED message contains a sequence of tickets to be sent and
information needed to use the tickets, including the session key from each.
The information needed to use the tickets is encrypted under an encryption
key previously exchanged or transferred alongside the KRB_CRED message. The
message fields are:

KRB-CRED         ::= [APPLICATION 22]   SEQUENCE {
                 pvno[0]                INTEGER,
                 msg-type[1]            INTEGER, -- KRB_CRED
                 tickets[2]             SEQUENCE OF Ticket,
                 enc-part[3]            EncryptedData -- EncKrbCredPart
}

EncKrbCredPart   ::= [APPLICATION 29]   SEQUENCE {
                 ticket-info[0]         SEQUENCE OF KrbCredInfo,
                 nonce[1]               INTEGER OPTIONAL,
                 timestamp[2]           KerberosTime OPTIONAL,
                 usec[3]                Microseconds OPTIONAL,
                 s-address[4]           HostAddress OPTIONAL,
                 r-address[5]           HostAddress OPTIONAL
}


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KrbCredInfo      ::=                    SEQUENCE {
                 key[0]                 EncryptionKey,
                 prealm[1]              Realm OPTIONAL,
                 pname[2]               PrincipalName OPTIONAL,
                 flags[3]               TicketFlags OPTIONAL,
                 authtime[4]            KerberosTime OPTIONAL,
                 starttime[5]           KerberosTime OPTIONAL,
                 endtime[6]             KerberosTime OPTIONAL
                 renew-till[7]          KerberosTime OPTIONAL,
                 srealm[8]              Realm OPTIONAL,
                 sname[9]               PrincipalName OPTIONAL,
                 caddr[10]              HostAddresses OPTIONAL
}

pvno and msg-type
     These fields are described above in section 5.4.1. msg-type is
     KRB_CRED.
tickets
     These are the tickets obtained from the KDC specifically for use by the
     intended recipient. Successive tickets are paired with the
     corresponding KrbCredInfo sequence from the enc-part of the KRB-CRED
     message.
enc-part
     This field holds an encoding of the EncKrbCredPart sequence encrypted
     under the session key shared between the sender and the intended
     recipient. This encrypted encoding is used for the enc-part field of
     the KRB-CRED message. See section 6 for the format of the ciphertext.
nonce
     If practical, an application may require the inclusion of a nonce
     generated by the recipient of the message. If the same value is
     included as the nonce in the message, it provides evidence that the
     message is fresh and has not been replayed by an attacker. A nonce must
     never be re-used; it should be generated randomly by the recipient of
     the message and provided to the sender of the message in an application
     specific manner.
timestamp and usec
     These fields specify the time that the KRB-CRED message was generated.
     The time is used to provide assurance that the message is fresh.
s-address and r-address
     These fields are described above in section 5.6.1. They are used
     optionally to provide additional assurance of the integrity of the
     KRB-CRED message.
key
     This field exists in the corresponding ticket passed by the KRB-CRED
     message and is used to pass the session key from the sender to the
     intended recipient. The field's encoding is described in section 6.2.

The following fields are optional. If present, they can be associated with
the credentials in the remote ticket file. If left out, then it is assumed
that the recipient of the credentials already knows their value.

prealm and pname
     The name and realm of the delegated principal identity.
flags, authtime, starttime, endtime, renew-till, srealm, sname, and caddr
     These fields contain the values of the corresponding fields from the
     ticket found in the ticket field. Descriptions of the fields are
     identical to the descriptions in the KDC-REP message.


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5.9. Error message specification

This section specifies the format for the KRB_ERROR message. The fields
included in the message are intended to return as much information as
possible about an error. It is not expected that all the information
required by the fields will be available for all types of errors. If the
appropriate information is not available when the message is composed, the
corresponding field will be left out of the message.

Note that since the KRB_ERROR message is only optionally integrity
protected, it is quite possible for an intruder to synthesize or modify such
a message. In particular, this means that unless appropriate integrity
protection mechanisms have been applied to the KRB_ERROR message, the client
should not use any fields in this message for security-critical purposes,
such as setting a system clock or generating a fresh authenticator. The
message can be useful, however, for advising a user on the reason for some
failure.

5.9.1. KRB_ERROR definition

The KRB_ERROR message consists of the following fields:

KRB-ERROR ::=   [APPLICATION 30] SEQUENCE {
                pvno[0]                       INTEGER,
                msg-type[1]                   INTEGER,
                ctime[2]                      KerberosTime OPTIONAL,
                cusec[3]                      Microseconds OPTIONAL,
                stime[4]                      KerberosTime,
                susec[5]                      Microseconds,
                error-code[6]                 Int32,
                crealm[7]                     Realm OPTIONAL,
                cname[8]                      PrincipalName OPTIONAL,
                realm[9]                      Realm, -- Correct realm
                sname[10]                     PrincipalName, -- Correct name
                e-text[11]                    KerberosString OPTIONAL,
                e-data[12]                    OCTET STRING OPTIONAL
}


pvno and msg-type
     These fields are described above in section 5.4.1. msg-type is
     KRB_ERROR.
ctime
     This field is described above in section 5.4.1.
cusec
     This field is described above in section 5.5.2.
stime
     This field contains the current time on the server. It is of type
     KerberosTime.
susec
     This field contains the microsecond part of the server's timestamp. Its
     value ranges from 0 to 999999. It appears along with stime. The two
     fields are used in conjunction to specify a reasonably accurate
     timestamp.
error-code
     This field contains the error code returned by Kerberos or the server
     when a request fails. To interpret the value of this field see the list
     of error codes in section 8. Implementations are encouraged to provide
     for national language support in the display of error messages.
crealm, cname, srealm and sname
     These fields are described above in section 5.3.1.

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e-text
     This field contains additional text to help explain the error code
     associated with the failed request (for example, it might include a
     principal name which was unknown).
e-data
     This field contains additional data about the error for use by the
     application to help it recover from or handle the error. If present,
     this field will contain the encoding of a sequence of TypedData
     (TYPED-DATA below), unless the errorcode is KDC_ERR_PREAUTH_REQUIRED,
     in which case it will contain the encoding of a sequence of of padata
     fields (METHOD-DATA below), each corresponding to an acceptable
     pre-authentication method and optionally containing data for the
     method:

     TYPED-DATA   ::=   SEQUENCE of TypedData
     METHOD-DATA  ::=   SEQUENCE of PA-DATA

     TypedData ::=   SEQUENCE {
                         data-type[0]   Int32,
                         data-value[1]  OCTET STRING OPTIONAL
     }

     Note that the padata-type field in the PA-DATA structure and the
     data-type field in the TypedData structure share a common range of
     allocated values which are coordinated to avoid conflicts. One Kerberos
     error message, KDC_ERR_PREAUTH_REQUIRED, embeds elements of type
     PA-DATA, while all other error messages embed TypedData.

     While preauthentication methods of type PA-DATA should be encapsulated
     within a TypedData element of type TD-PADATA, for compatibility with
     old clients, the KDC should include PA-DATA types below 22 directly as
     method-data. All new implementations interpreting the METHOD-DATA field
     for the KDC_ERR_PREAUTH_REQUIRED message must accept a type of
     TD-PADATA, extract the typed data field and interpret the use any
     elements encapsulated in the TD-PADATA elements as if they were present
     in the METHOD-DATA sequence.

     Unless otherwise specified, unrecognized TypedData elements within the
     KRB-ERROR message MAY be ignored by implementations that do not support
     them. Note that all TypedData MAY be bound to the KRB-ERROR message via
     the checksum field.

     An application may use the TD-APP-DEFINED-ERROR typed data type for
     data carried in a Kerberos error message that is specific to the
     application. TD-APP-SPECIFIC must set the data-type value of TypedData
     to TD-APP-SPECIFIC and the data-value field to

        AppSpecificTypedData as follows:
            AppSpecificTypedData ::= SEQUENCE {
                    oid[0]           OPTIONAL OBJECT IDENTIFIER,
                                     -- identifies the application
                    data-value[1]    OCTET STRING
                                     -- application
                                     -- specific data
            }


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        o The TD-REQ-NONCE TypedData MAY be used to bind a KRB-ERROR to a
          KDC-REQ. The data-value is an INTEGER that is equivalent to the
          nonce in a KDC-REQ.

        o The TD-REQ-SEQ TypedData MAY be used for binding a KRB-ERROR to
          the sequence number from an authenticator. The data-value is an
          INTEGER, and it is identical to sequence number sent in the
          authenticator.

        o The data-value for TD-KRB-PRINCIPAL is the Kerberos-defined
          PrincipalName. The data-value for TD-KRB-REALM is the
          Kerberos-defined Realm. These TypedData types MAY be used to
          indicate principal and realm name when appropriate.

  ------------------------------------------------------------------------

5.10. Application Tag Numbers

The following table lists the application class tag numbers used by various
data types defined in this section.
 Tag Number(s)    Type Name    Comments

 0                             unused

 1              Ticket

 2              Authenticator

 3              EncTicketPart

 4-10                          unused

 10             AS-REQ

 11             AS-REP

 12             TGS-REQ

 13             TGS-REP

 14             AP-REQ

 15             AP-REP

 16             TGT-REQ

 17-19                         unused

 20             KRB-SAFE

 21             KRB-PRIV

 22             KRB-PRIV

 23-24                         unused

 25             EncASRepPart

 26             EncTGSRepPart


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 27             EncApRepPart

 28             EncKrbPrivPart

 29             EncKrbCredPart

 30             KRB-ERROR

6. Encryption and Checksum Specifications

     Work is still needed on this section.

        * Re-synchronize the key usage value list with any changes Tom
          makes to the message definitions. KRB-ERROR checksum, for
          example, and any new message types.
        * More talking with cryptographers about the "combine-keys"
          function in the simplified profile. I've been talking a
          little with Uri Blumenthal, but he hasn't had a lot of time
          for this.
        * Test vections need to go into an appendix. Submitted,
          appendix letter not yet assigned; I'm using "Z" in this text.
          Appendix letter also not yet assigned for deprecated checksum
          algorithms, assuming "Y".
        * More detailed list of differences from RFC 1510, to update
          the "Significant changes" appendix.
        * Are sections 6.2 and 6.3 what we want to recommend for
          external use in section 6.7, or just a subset?
        * Fix up reference to Bellovin paper.
        * Fix anything marked with "@@".

     See end notes for other issues.

     -- Ken 2001-11-20

The Kerberos protocols described in this document are designed to encrypt
blocks of arbitrary sizes, using stream encryption ciphers, or more
commonly, block encryption ciphers, such as the Data Encryption Standard
[DES77], and triple DES variants, in conjunction with block chaining and
checksum methods [DESM80]. Encryption is used to prove the identities of the
network entities participating in message exchanges. The Key Distribution
Center for each realm is trusted by all principals registered in that realm
to store a secret key in confidence. Proof of knowledge of this secret key
is used to verify the authenticity of a principal.

The KDC uses the principal's secret key (in the AS exchange) or a shared
session key (in the TGS exchange) to encrypt responses to ticket requests;
the ability to obtain the secret key or session key implies the knowledge of
the appropriate keys and the identity of the KDC. The ability of a principal
to decrypt the KDC response and present a Ticket and a properly formed
Authenticator (generated with the session key from the KDC response) to a
service verifies the identity of the principal; likewise the ability of the
service to extract the session key from the Ticket and prove its knowledge
thereof in a response verifies the identity of the service.

The Kerberos protocols generally assume that the encryption used is secure
from cryptanalysis; however, in some cases, the order of fields in the
encrypted portions of messages as defined in this section is chosen to
minimize the effects of poorly chosen keys under certain types of
cryptographic attacks. It is still important to choose good keys. If keys
are derived from user-typed passwords, those passwords need to be well
chosen to make brute force attacks more difficult. Poorly chosen keys still
make easy targets for intruders.


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The following sections specify the encryption and checksum mechanisms
currently defined for Kerberos, as well as a framework for defining future
mechanisms. The encodings, chaining, padding and other requirements for each
are described. Test vectors for several functions are given in appendix Z.

See also appendix Y for deprecated checksum algorithms defined in RFC 1510,
which may still be in use by some implementations, but are not part of this
version of the specification.

6.1. Concepts

Both encryption and checksum mechanism are defined in terms of profiles,
detailed in later sections. Each specifies a collection of operations and
attributes that must be defined for a mechanism. A Kerberos encryption or
checksum mechanism is not complete if it does not specify all of these
operations and attributes.

An encryption mechanism must provide for confidentiality and integrity of
the original plaintext. (Integrity checking may be achieved by incorporating
a checksum, if the encryption mode does not provide an integrity check
itself.) It must also provide non-malleability [Bellare98, Dolev91]. Use of
a random confounder prepended to the plaintext is recommended. It should not
be possible to determine if two ciphertexts correspond to the same
plaintext, without knowledge of the key.

A checksum mechanism[6.1] must provide proof of the integrity of the
associated message, and must preserve the confidentiality of the message in
case it is not sent in the clear. It should be infeasible to find two
plaintexts which have the same checksum. It is NOT required that an
eavesdropper be unable to determine if two checksums are for the same
message; it is assumed that the messages themselves will be visible to any
such eavesdropper.

Due to advances in cryptography, it is considered unwise by some
cryptographers to use the same key for multiple purposes [@@reference??].
Since keys are used in performing a number of different functions in
Kerberos, it is desirable to use different keys for each of these purposes,
even though we start with a single long-term or session key.

We do this by enumerating the different uses of keys within Kerberos, and
making the "usage number" an input to the encryption or checksum mechanisms.
Later sections define simplified profile templates for encryption and
checksum mechanisms that use a key derivation function applied to a CBC mode
(or similar) cipher and a checksum or hash algorithm.

We distinguish the "base key" used in the EncryptedKey object from the
"specific key" to be used for a particular instance of encryption or
checksum operations. It is expected but not required that the specific key
will be one or more separate keys derived from the original protocol key and
the key usage number. The specific key is not explicitly referenced outside
of section 6 in this document; when other sections mention encrypting or
decrypting data with a given key, that key is the "base key", and the
"specific key" generation and use is implicit, as is the use of a key usage
number.


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Key Usage Values

This is a list of key usage number definitions and reserved ranges,
including values for all places keys are used in the Kerberos protocol and
associated section numbers.

    1.     AS-REQ PA-ENC-TIMESTAMP padata timestamp, encrypted with the
           client key (section 5.4.1)
    2.     AS-REP Ticket and TGS-REP Ticket (includes TGS session key or
           application session key), encrypted with the service key
           (section 5.4.2)
    3.     AS-REP encrypted part (includes TGS session key or application
           session key), encrypted with the client key (section 5.4.2)
    4.     TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS
           session key (section 5.4.1)
    5.     TGS-REQ KDC-REQ-BODY AuthorizationData, encrypted with the TGS
           authenticator subkey (section 5.4.1)
    6.     TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator cksum, keyed with
           the TGS session key (sections 5.3.2, 5.4.1)
    7.     TGS-REQ PA-TGS-REQ padata AP-REQ Authenticator (includes TGS
           authenticator subkey), encrypted with the TGS session key
           (section 5.3.2)
    8.     TGS-REP encrypted part (includes application session key),
           encrypted with the TGS session key (section 5.4.2)
    9.     TGS-REP encrypted part (includes application session key),
           encrypted with the TGS authenticator subkey (section 5.4.2)
    10.    AP-REQ Authenticator cksum, keyed with the application session
           key (section 5.3.2)
    11.    AP-REQ Authenticator (includes application authenticator
           subkey), encrypted with the application session key (section
           5.3.2)
    12.    AP-REP encrypted part (includes application session subkey),
           encrypted with the application session key (section 5.5.2)
    13.    KRB-PRIV encrypted part, encrypted with a key chosen by the
           application (section 5.7.1)
    14.    KRB-CRED encrypted part, encrypted with a key chosen by the
           application (section 5.6.1)
    15.    KRB-SAFE cksum, keyed with a key chosen by the application
           (section 5.8.1)
    18.    KRB-ERROR checksum (e-cksum in section 5.9.1)
    19.    AD-KDCIssued checksum (ad-checksum in appendix B.4)
    20.    Checksum for Mandatory Ticket Extensions (appendix B.6)
    21.    Checksum in Authorization Data in Ticket Extensions (appendix
           B.7)
  22-24.   Reserved for use in GSSAPI mechanisms derived from RFC 1964.
           (raeburn/MIT)
  25-511.  Reserved for future use in Kerberos and related protocols.
 512-1023. Reserved for uses internal to a Kerberos implementation. [6.2]

A few of these key usages need a little clarification. A service which
receives an AP-REQ has no way to know if the enclosed Ticket was part of an
AS-REP or TGS-REP. Therefore, key usage 2 must always be used for generating
a Ticket, whether it is in response to an AS-REQ or TGS-REQ.

Key usage values between 1024 and 2047 (inclusive) are reserved for
application use. Applications should use even values for encryption and odd
values for checksums within this range.


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There might exist other documents which define protocols in terms of the
RFC1510 encryption types or checksum types. Such documents would not know
about key usages. In order that these documents continue to be meaningful
until they are updated, key usages 1024 and 1025 must be used to derive keys
for encryption and checksums, respectively.[6.3] New protocols defined in
terms of the Kerberos encryption and checksum types should use their own key
usage values. Key usages are unsigned 32 bit integers; zero is not
permitted. Usage numbers may be registered with IANA to avoid conflicts.

6.2. Encryption mechanism attributes

An encryption mechanism profile must define the following attributes and
operations:

protocol key format
     This describes what OCTET STRING values represent valid keys. For
     encryption mechanisms that don't have perfectly dense key spaces, this
     will describe the representation used for encoding keys. It need not
     describe specific values that are not valid or desirable for use; such
     values should be avoid by all key generation routines.
specific key structure
     This is not a protocol format at all, but a description of the keying
     material derived from the chosen key and used to encrypt or decrypt
     data or compute or verify a checksum. It may be a single key, a set of
     keys, or a combination of the original key with additional data. The
     authors recommend using one or more keys derived from the original key
     via one-way functions.
required checksum mechanism
     This indicates a checksum mechanism that must be available when this
     encryption mechanism is used. Since Kerberos has no built in mechanism
     for negotiating checksum mechanisms, once an encryption mechanism has
     been decided upon, the corresponding checksum mechanism can simply be
     used.
key-generation seed length, K
     This is the length of the random bitstring needed to generate a key
     with the encryption scheme's random-to-key function (described below).
     This must be a fixed value so that various techniques for producing a
     random bitstring of a given length may be used with key generation
     functions.
key generation functions
     Keys must be generated in a number of cases, from different types of
     inputs. All function specifications must indicate how to generate keys
     in the proper wire format, and must avoid generation of "weak" keys if
     the cryptosystem has such. Entropy from each source should be preserved
     as much as possible. Many of the inputs, while unknown, may be at least
     partly predictable (e.g., a password string is likely to be entirely in
     the ASCII subset and of fairly short length in many environments; a
     semi-random string may include timestamps); the benefit of such
     predictability to an attacker must be minimized.

     string-to-key (UTF8String, UTF8String, integer)->(protocol-key)
          This function generates a key from two UTF-8 strings and an
          integer. One of the strings is normally the principal's password,
          but is in general merely a secret string. The other string is a
          "salt" intended to produce different keys from the same password
          for different users or realms. The integer is known for historical
          reasons as the "salt type", but may be used for selection between
          multiple string-to-key algorithms. The salt string and type may be
          altered by preauth data from the KDC; in the default case, the
          salt string is simply a concatenation of the principal's realm and
          name components, with no separators, and the salt type is zero.

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          This should be a one-way function, so that compromising a user's
          key in one realm does not compromise the user's key in another
          realm, even if the same password (but a different salt string) is
          used.

     random-to-key (bitstring[K])->(protocol-key)
          This function generates a key from a random bit string of a
          specific size. It may be assumed that all the bits of the input
          string are equally random, even though the entropy present in the
          random source may be limited.
     combine-keys (protocol-key, protocol-key)->(protocol-key)
          This function takes two input keys and produces a new key. This
          function is not used in this RFC, but may be used by
          preauthentication methods or other applications to be defined
          later. This operation must be commutative; this requirement lets
          us specify "combine keys A and B" in other documents without
          worrying about ordering.
     key-derivation (protocol-key, integer)->(specific-key)
          In this function, the byte string input is based on the key usage
          values specified above; the usage values must be assumed to be
          known to an attacker. For cryptosystems with dense key spaces,
          this function should be the key derivation function outlined in
          section 6.1.
cipher state
initial cipher state (specific-key)->(state)
     This describes any initial state setup needed before encrypting
     arbitrary amounts of data with a given specific key; the specific key
     must be the only input needed for this initialization. For example, a
     block cipher used in CBC mode must specify an initial vector. (For
     security reasons, the key itself should not be used as the IVEC.) This
     data may be carried over from one encryption or decryption operation to
     another. Unless otherwise specified, however, each encryption or
     decryption operation in this RFC uses a freshly initialized state and
     is thus independent of all other encryptions and decryptions.

     This state should be treated as opaque in any uses outside of an
     encryption algorithm definition.
encrypt (specific-key, state, bytestring)->(state, bytestring)
     This function takes the specific key, cipher state, and plaintext as
     input, and generates ciphertext and a new cipher state as outputs. If
     the basic encryption algorithm itself does not provide for integrity
     protection (as DES in CBC mode does not do), then some form of MAC or
     checksum must be included that can be verified by the receiver. Some
     random factor such as a confounder should be included so that an
     observer cannot know if two messages contain the same plaintext, even
     if the cipher state and specific keys are the same. The exact length of
     the plaintext need not be encoded, but if it is not and if padding is
     required, the padding must be added at the end of the string so that
     the decrypted version may be parsed from the beginning.

     The output will be used as the OCTET STRING in the EncryptedData type,
     defined in section 5.1.


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decrypt (specific-key, state, bytestring)->(state, bytestring)
     This function takes the specific key, cipher state, and ciphertext as
     inputs, and verifies the integrity of the supplied ciphertext. If the
     ciphertext's integrity is intact, this function produces the plaintext
     and a new cipher state as outputs; otherwise, an error indication must
     be returned, and the data discarded.

     This function's byte string input is the OCTET STRING from the
     EncryptedData type, defined in section 5.1.

     The result of the decryption may be longer than the original plaintext,
     if the encryption mode requires padding to a multiple of a block size.
     If this is the case, any extra padding will be after the decoded
     plaintext. An application protocol which needs to know the exact length
     of the message must encode a length or recognizable "end of message"
     marker within the plaintext.

These operations and attributes are all that should be required to support
Kerberos and various proposed preauthentication schemes.

The reader is reminded that cryptography is a complex and growing field, and
proof of the security of an algorithm is often difficult, even for those
with extensive training in the field. Merely making an algorithm complicated
is more likely to make it hard to analyze than it is to make it secure. This
should be kept in mind when defining functions for new cryptosystems; simple
applications of existing, trusted algorithms are more likely to be secure
than a complicated home-grown scheme.

6.3. Checksum mechanism attributes

A checksum mechanism profile must define the following attributes and
operations:

associated encryption algorithm(s)
     This essentially indicates the type of encryption key this checksum
     mechanism can be used with. A single checksum mechanism may have more
     than one associated encryption algorithm if they share the same wire
     key format, string-to-key function, and key derivation function. (This
     combination means that, for example, a checksum type and password are
     adequate to get the specific key used to compute a checksum.)
get_mic function
     This function generates a MIC token for a given specific key (see
     section 6.2), and message (represented as an octet string), that may be
     used to verify the integrity of the associated message. This function
     is not required to return the same deterministic result on every use;
     it need only generate a token that the verify_mic routine can check.

     The output of this function will also dictate the size of the checksum.
     It must be a fixed size.

verify_mic function
     Given a specific key, message, and MIC, this function ascertains
     whether the message integrity has been compromised. For a deterministic
     get_mic routine, the corresponding verify_mic may simply generate
     another checksum and compare them.

The get_mic and verify_mic operations must be able to handle inputs of
arbitrary length; if any padding is needed, the padding scheme must be
specified as part of these functions.

These operations and attributes are all that should be required to support
Kerberos and various proposed preauthentication schemes.


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6.4. A simplified profile for CBC-mode ciphers using key derivation

The profile outlines in sections 6.2 and 6.3 describes a large number of
operations that must be defined for encryption and checksum algorithms to be
used with Kerberos. We describe here a simpler profile from which both
encryption and checksum mechanism definitions can be generated, filling in
uses of key derivation in appropriate places, providing integrity
protection, and defining multiple operations for the cryptosystem profile
based on a smaller set of operations given in the simplified profile. Not
all of the existing cryptosystems for Kerberos fit into this simplified
profile, but we recommend that future cryptosystems use it or something
based on it.

Not all of the operations in the complete profiles are defined through this
mechanism; several must still be defined for each new algorithm pair.

A key derivation function [Horowitz]

Rather than define some scheme by which a "protocol key" is composed of a
large number of encryption keys, we use keys derived from a base key to
perform cryptographic operations. The base key must be used only for
generating the derived keys, and this derivation must be non-invertible and
entropy-preserving. Given these restrictions, compromise of one derived key
does not compromise the other subkeys. Attack of the base key is limited,
since it is only used for derivation, and is not exposed to any user data.

Since the derived key has as much entropy as the base keys (if the
cryptosystem is good), password-derived keys have the full benefit of all
the entropy in the password.

To generate a derived key from a base key, we generate a pseudorandom byte
string, using an algorithm DR described below, and generate a key from that
byte string using a function dependent on the encryption algorithm; the
input length needed for that function, which is also dependent on the
encryption algorithm, dictates the length of the string to be generated by
the DR algorithm (the value "k" below).

   Derived Key = DK(Base Key, Well-Known Constant)

   DK(Key, Constant) = random-to-key(DR(Key, Constant))

   DR(Key, Constant) = k-truncate(E(Key, Constant))

Here DR is the random-byte generation function described below, and DK is
the key-derivation function produced from it. In this construction, E(Key,
Plaintext) is a block cipher, Constant is a well-known constant determined
by the specific usage of this function, and k-truncate truncates its
argument by taking the first k bits. Here, k is the key generation seed
length needed for the encryption system.

The output of the DR function is a string of bits; the actual key is
produced by applying the cryptosystem's random-to-key operation on this
bitstring.

If the output of E is shorter than k bits, then some entropy in the key will
be lost. If the Constant is smaller than the block size of E, then it must
be padded so it may be encrypted.

In either of these situations, a variation of the above construction is
used, where the folded Constant is encrypted, and the resulting output is
fed back into the encryption as necessary (the | indicates concatentation):


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   K1 = E(Key, n-fold(Constant))
   K2 = E(Key, K1)
   K3 = E(Key, K2)
   K4 = ...

   DR(Key, Constant) = k-truncate(K1 | K2 | K3 | K4 ...)

n-fold is an algorithm which takes m input bits and ``stretches'' them to
form n output bits with equal contribution from each input bit to the
output, as described in [Blumenthal96]:

     We first define a primitive called n-folding, which takes a
     variable-length input block and produces a fixed-length output
     sequence. The intent is to give each input bit approximately equal
     weight in determining the value of each output bit. Note that
     whenever we need to treat a string of bytes as a number, the
     assumed representation is Big-Endian -- Most Significant Byte
     first.

     To n-fold a number X, replicate the input value to a length that
     is the least common multiple of n and the length of X. Before each
     repetition, the input is rotated to the right by 13 bit positions.
     The successive n-bit chunks are added together using
     1's-complement addition (that is, with end-around carry) to yield
     a n-bit result....

Test vectors for n-fold are supplied in Appendix Z. [6.4]

In this document, n-fold is always used to produce n bits of output, where n
is the block size of E.

The size of the Constant must not be larger than the block size of E,
because reducing the length of the Constant by n-folding can cause
collisions.

If the size of the Constant is smaller than the block size of E, then the
Constant must be n-folded to the block size of E. This string is used as
input to E. If the block size of E is less than the key size, then the
output from E is taken as input to a second invocation of E. This process is
repeated until the number of bits accumulated is greater than or equal to
the key size of E. When enough bits have been computed, the first k are
taken as the derived key.

Since the derived key is the result of one or more encryptions in the base
key, deriving the base key from the derived key is equivalent to determining
the key from a very small number of plaintext/ciphertext pairs. Thus, this
construction is as strong as the cryptosystem itself.

6.4.1. Simplified profile parameters

These are the operations and attributes that must be defined:

protocol key format
string-to-key function
key-generation seed length, k
random-to-key function
     As above for the normal encryption mechanism profile.
unkeyed hash algorithm, H
     This should be a collision-resistant hash algorithm such as SHA-1,
     suitable for use in an HMAC. It must support inputs of arbitrary
     length.

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encryption block size, n
encryption/decryption functions, E and D
     These are basic encryption and decryption functions for messages of
     sizes that are multiples of the block size. No integrity checking or
     confounder should be included here. They take as input the IV or
     similar data, a protocol-format key, and a byte string, returning a new
     IV and byte string.

     The encryption function is not required to use CBC mode, but is assumed
     to be using something with similar properties. In particular,
     prepending a one-block confounder to the plaintext should alter the
     entire ciphertext (comparable to choosing and including a random
     initial vector for CBC mode).

While there are still a number of properties to specify, they are fewer and
simpler than in the full profile.

6.4.2. Cryptosystem profile based on simplified profile

 protocol key   As given.
 format
 specific key   Three protocol-format keys: { Kc, Ke, Ki }.
 structure
 key-generation As given.
 seed length
 required       The checksum mechanism defined by the simplified checksum
 checksum       profile given later.
 mechanism
 cipher state   Initial vector, initialized to all zero.
 encryption     The ciphertext output is the concatenation of the output of
 function       the basic encryption function E and an HMAC using the
                specified hash function H, both applied to the padded
                plaintext with a confounder:

                  ciphertext =    E(Ke, confounder | plaintext | padding) |
                               HMAC(Ki, confounder | plaintext | padding)
                  HMAC(K,M) = H(K | H(K | M))

                One block of random confounder data is prepended to the
                plaintext, and padding added to the end to bring the length
                to a multiple of the encryption algorithm's block size. The
                initial vector for encryption is supplied by the cipher
                state, and the last block of the output of E is the new
                IVEC for the new cipher state.
 decryption     Decryption is performed by extracting the encrypted portion
 function       of the ciphertext, decrypting using key Ke from the
                specific key, and verifying the HMAC. If the HMAC is
                incorrect, an error must be reported. Otherwise, the
                confounder and padding are discarded and the remaining
                plaintext returned. As with encryption, the cipher state
                input indicates the IVEC to use, and the last block of the
                encrypted portion of the ciphertext is put into the new
                cipher state to be used as the next IVEC.
 key generation
 functions:
 string-to-key  As given.
 function
 random-to-key  As given.
 function

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 combine-keys   @@ Needs to be specified. How about:
 function
                combine-keys(K1,K2)
                    /* First, protect original keys against exposure
                       with DR.  */
                    R1 = DR(K1, n-fold(K2))    /* length k */
                    R2 = DR(K2, n-fold(K1))    /* length k */
                    /* Using k-fold on length 2k means just add with
                       wrap-around carry.  */
                    rnd = k-fold(R1 | R2)
                    tkey = random-to-key(rnd)
                    key = DK(tkey, CombineConstant)

                @@ This should be commutative, which keeps the
                specifications simpler, and I think should protect each
                key's contribution from exposure of the other key. Or
                should we just go with an XOR combination of the random
                bytes? (But that works poorly should the two keys happen to
                be the same due to sloppy or weird protocol specs, or
                parallel construction of contributed keys.) Do we need the
                two DK invocations at the start? Would one at the end be
                better? Do we need either?

                Here CombineConstant is the byte string {0x63 0x6f 0x6d
                0x62 0x69 0x6e 0x65} corresponding to the ASCII encoding of
                the string "combine".

                @@ Need a cryptographer to review this. Asked Uri
                Blumenthal, he said he'd look it over when he has time.
 key-derivation Three keys are generated, using the DK function described
 function       above, and the key usage number, represented as a 32-bit
                integer in big-endian byte order. One is used for
                generating checksums only; the other two are used for
                encrypting and integrity protection for ciphertext. These
                keys are generated as follows (with "|" indicating
                concatenation):

                        Kc = DK(base-key, usage|0x99));
                        Ke = DK(base-key, usage|0xAA);
                        Ki = DK(base-key, usage|0x55);

6.4.3. Checksum profiles based on simplified profile

When an encryption system is defined using the simplified profile given in
section 6.4.1, a checksum algorithm may be defined for it as follows:

                associated cryptosystem as defined above
                get_mic                 HMAC(Kc, message)
                verify_mic              get_mic and compare

The HMAC function and key Kc are as described in section 6.4.2.

6.5. Profiles for Kerberos encryption systems

These are the currently defined encryption systems for Kerberos. The astute
reader will notice that some of them do not fulfill all of the requirements
outlined above. These weaker encryption systems are defined for backwards
compatibility; newer implementations should attempt to make use of the
stronger encryption systems when possible.

The full list of encryption type number assignments is given in section 8.3.


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6.5.1. null

If no encryption is in use, the encryption system is said to be the NULL
encryption system. In the NULL encryption system there is no checksum,
confounder or padding. The ciphertext is simply the plaintext. The NULL Key
is used by the null encryption system and is zero octets in length.

This encryption system should not be used for protection of data. It exists
primarily to associate with the rsa-md5 checksum type, but may also be
useful for testing protocol implementations.

         protocol key format          zero-length bit string
         specific key structure       empty
         required checksum mechanism  rsa-md5
         key-generation seed length   0
         cipher state                 none
         initial cipher state         none
         encryption function          identity
         decryption function          identity, no integrity check
         key generation functions:
         string-to-key                empty string
         random-to-key                empty string
         combine-keys                 empty string
         key-derivation               empty string

The null encryption algorithm is assigned the etype value zero (0).

6.5.2. des-cbc-md5

The des-cbc-md5 encryption mode encrypts information under the Data
Encryption Standard [DES77] using the cipher block chaining mode [DESM80].
An MD5 checksum (described in [MD5-92]) is applied to the confounder and
message sequence (msg-seq) and placed in the cksum field. DES blocks are 8
bytes. As a result, the data to be encrypted (the concatenation of
confounder, checksum, and message) must be padded to an 8 byte boundary
before encryption.

Plaintext and DES ciphtertext are encoded as blocks of 8 octets which are
concatenated to make the 64-bit inputs for the DES algorithms. The first
octet supplies the 8 most significant bits (with the octet's MSbit used as
the DES input block's MSbit, etc.), the second octet the next 8 bits, ...,
and the eighth octet supplies the 8 least significant bits.

Encryption under DES using cipher block chaining requires an additional
input in the form of an initialization vector. Unless otherwise specified,
zero should be used as the initialization vector. Kerberos' use of DES
requires an 8 octet confounder.

The DES specifications identify some 'weak' and 'semi-weak' keys; those keys
shall not be used for encrypting messages for use in Kerberos. Additionally,
because of the way that keys are derived for the encryption of checksums,
keys shall not be used that yield 'weak' or 'semi-weak' keys when
eXclusive-ORed with the hexadecimal constant F0F0F0F0F0F0F0F0.

A DES key is 8 octets of data, with keytype one (1). This consists of 56
bits of key, and 8 parity bits (one per octet). The key is encoded as a
series of 8 octets written in MSB-first order. The bits within the key are
also encoded in MSB order. For example, if the encryption key is
(B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8) where
B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8 are the parity
bits, the first octet of the key would be B1,B2,...,B7,P1 (with B1 as the
MSbit). [See the FIPS 81 introduction for reference.]


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Encryption data format

The format for the data to be encrypted includes a one-block confounder, a
checksum, the encoded plaintext, and any necessary padding, as described in
the following diagram. The msg-seq field contains the part of the protocol
message described in section 5 which is to be encrypted. The confounder,
checksum, and padding are all untagged and untyped.

      +-----------+----------+-------------+-----+
      |confounder |   check  |   msg-seq   | pad |
      +-----------+----------+-------------+-----+

One generates a random confounder of one block, placing it in confounder;
zeroes out check; calculates the appropriate checksum over confounder,
check, and msg-seq, placing the result in check; adds the necessary padding;
then encrypts using the specified encryption type and the appropriate key.

String to key transformation

To generate a DES key from a UTF-8 text string (password), a "salt" is
concatenated to the text string, and then padded with ASCII nulls to an 8
byte boundary.

This string is then fan-folded and eXclusive-ORed with itself to form an 8
byte DES key. Before eXclusive-ORing a block, every byte is shifted one bit
to the left to leave the lowest bit zero. The key is the "corrected" by
correcting the parity on the key, and if the key matches a 'weak' or
'semi-weak' key as described in the DES specification, it is eXclusive-ORed
with the constant 00000000000000F0. This key is then used to generate a DES
CBC checksum on the initial string (with the salt appended). The result of
the CBC checksum is the "corrected" as described above to form the result
which is return as the key.

Pseudocode follows:

     key_correction(key) {
          fixparity(key);
          if (is_weak_key_key(key))
               key = key XOR 0xF0;
          return(key);
     }

     mit_des_string_to_key(string,salt) {
          odd = 1;
          s = string + salt;
          tempkey = NULL;
          pad(s); /* with nulls to 8 byte boundary */
          for (8byteblock in s) {
               if (odd == 0)  {
                   odd = 1;
                   reverse(8byteblock)
               }
               else odd = 0;
               left shift every byte in 8byteblock one bit;
               tempkey = tempkey XOR 8byteblock;
          }
          tempkey = key_correction(tempkey);
          key = key_correction(DES-CBC-check(s,tempkey));
          return(key);
     }


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     des_string_to_key(type,string,salt) {
          if (type == 0)
               mit_des_string_to_key(string,salt);
          else if (type == 1)
               afs_des_string_to_key(string,salt);
     }

The AFS string-to-key algorithm is not defined here, but salt type value one
(1) is reserved for that use.

The encryption system parameters for des-cbc-md5 are:

  protocol key format          8 bytes, parity in low bit of each
  specific key structure       copy of original key
  required checksum mechanism  rsa-md5-des
  key-generation seed length   8 bytes
  cipher state                 8 bytes (CBC initial vector)
  initial cipher state         all-zero
  encryption function          des-cbc(confounder | checksum | msg | pad)

                               with checksum computed as described above
  decryption function          decrypt encrypted text and verify checksum
  key generation functions:
  string-to-key                des_string_to_key
  random-to-key                copy input, then fix parity bits
                               (discards low bit of each input byte)
  combine-keys                 bitwise XOR, then fix parity bits
  key-derivation               identity

The des-cbc-md5 encryption type is assigned the etype value three (3).

6.5.3. des-cbc-md4

The des-cbc-md4 encryption mode encrypts information under the Data
Encryption Standard [DES77] using the cipher block chaining mode [DESM80].
An MD4 checksum (described in [MD492]) is applied to the confounder and
message sequence (msg-seq) and placed in the cksum field. DES blocks are 8
bytes. As a result, the data to be encrypted (the concatenation of
confounder, checksum, and message) must be padded to an 8 byte boundary
before encryption. The details of the encryption of this data are identical
to those for the des-cbc-md5 encryption mode.

  protocol key format          8 bytes, parity in low bit of each
  specific key structure       copy of original key
  required checksum mechanism  rsa-md4-des
  key-generation seed length   8 bytes
  cipher state                 8 bytes (CBC initial vector)
  initial cipher state         all-zero
  encryption function          des-cbc(confounder | checksum | msg | pad)

                               with checksum computed as described above
  decryption function          decrypt encrypted text and verify checksum
  key generation functions:
  string-to-key                des_string_to_key
  random-to-key                copy input, then fix parity bits
  combine-keys                 bitwise XOR, then fix parity bits
  key-derivation               identity

The des-cbc-md4 encryption algorithm is assigned the etype value two (2).


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6.5.4. des-cbc-crc

The des-cbc-crc encryption mode encrypts information under the Data
Encryption Standard [DES77] using the cipher block chaining mode [DESM80]. A
4-octet CRC-32 checksum (described in ISO 3309 [ISO3309]) is computed over
the confounder and message sequence (msg-seq) and placed in the cksum field.
DES blocks are 8 bytes. As a result, the data to be encrypted (the
concatenation of confounder, checksum, and message) must be padded to an 8
byte boundary before encryption. Unless otherwise specified, the key should
be used as the initialization vector, unlike for the other Kerberos DES
encryption schemes. The other details of the encryption of this data are
identical to those for the des-cbc-md5 encryption mode.

Note that, since the CRC-32 checksum is not collision-proof, an attacker
could use a probabilistic chosen-plaintext attack to generate a valid
message even if a confounder is used [SG92]. The use of collision-proof
checksums is recommended for environments where such attacks represent a
significant threat. The use of the CRC-32 as the checksum for ticket or
authenticator is no longer mandated as an interoperability requirement for
Kerberos Version 5 Specification 1 (See section 9.1 for specific details).

  protocol key format          8 bytes, parity in low bit of each
  specific key structure       copy of original key
  required checksum mechanism  rsa-md5-des
  key-generation seed length   8 bytes
  cipher state                 8 bytes (CBC initial vector)
  initial cipher state         copy of original key
  encryption function          des-cbc(confounder | checksum | msg | pad)

                               with checksum computed as described above
  decryption function          decrypt encrypted text and verify checksum
  key generation functions:
  string-to-key                des_string_to_key
  random-to-key                copy input, then fix parity bits
  combine-keys                 bitwise XOR, then fix parity bits
  key-derivation               identity

The des-cbc-crc encryption algorithm is assigned the etype value one (1).

6.5.5. des3-cbc-hmac-sha1-kd

This encryption type is based on the Triple DES cryptosystem in Outer-CBC
mode, and the HMAC-SHA1 [Krawczyk96] message authentication algorithm.

A Triple DES key is the concatenation of three DES keys as described above
for des-cbc-md5. A Triple DES key is generated from random data by creating
three DES keys from separate sequences of random data.

EncryptedData using this type must be generated as described in section
6.4.2. If the length of the input data is not a multiple of the block size,
zero octets must be used to pad the plaintext to the next eight-octet
boundary. The counfounder must be eight random octets (one block).


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The simplified profile for Triple DES, with key derivation as defined in
section 6.4, is as follows:

   protocol key format               24 bytes, parity in low bit of each
   key-generation seed length        21 bytes
   hash function                     SHA-1
   block size                        8 bytes
   encryption, decryption functions  triple-DES EDE in outer CBC mode
   key generation functions:
   random-to-key                     see below
   string-to-key                     DES3string-to-key (see below)

The des3-cbc-hmac-sha1-kd encryption type is assigned the value sixteen
(16).

Triple DES Key Production (random-to-key, string-to-key)

The 168 bits of random key data are converted to a protocol key value as
follows. First, the 168 bits are divided into three groups of 56 bits, which
are expanded individually into 64 bits as follows:

 1  2  3  4  5  6  7  p
 9 10 11 12 13 14 15  p
17 18 19 20 21 22 23  p
25 26 27 28 29 30 31  p
33 34 35 36 37 38 39  p
41 42 43 44 45 46 47  p
49 50 51 52 53 54 55  p
56 48 40 32 24 16  8  p

The "p" bits are parity bits computed over the data bits. The output of the
three expansions are concatenated to form the protocol key value.

When the HMAC-SHA1 of a string is computed, the key is used in the protocol
key form.

The string-to-key function is used to tranform UTF-8 passwords into DES3
keys. The DES3 string-to-key function relies on the "N-fold" algorithm and
DK function, described in section 6.4.

The n-fold algorithm is applied to the password string concatenated with a
salt value. For 3-key triple DES, the operation will involve a 168-fold of
the input password string, to generate an intermediate key, from which the
user's long-term key will be derived with the DK function. The DES3
string-to-key function is shown here in pseudocode:

DES3string-to-key(passwordString, salt, key)

    s = passwordString + salt
    tmpKey = random-to-key(168-fold(s))
    key = DK (tmpKey, KerberosConstant)

No weak-key checking is performed. The KerberosConstant value is the byte
string {0x6b 0x65 0x72 0x62 0x65 0x72 0x6f 0x73}. These values correspond to
the ASCII encoding for the string "kerberos".

6.6. Profiles for Kerberos checksums

These are the checksum types currently defined for Kerberos. The full list
of checksum type number assignments is given in section 8.3.


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6.6.1. RSA MD4 Cryptographic Checksum Using DES (rsa-md4-des)

The RSA-MD4-DES checksum calculates a keyed collision-proof checksum by
prepending an 8 octet confounder before the text, applying the RSA MD4
checksum algorithm [MD4-92], and encrypting the confounder and the checksum
using DES in cipher-block-chaining (CBC) mode using a variant of the key,
where the variant is computed by eXclusive-ORing the key with the constant
F0F0F0F0F0F0F0F0[39]. The initialization vector should be zero. The
resulting checksum is 24 octets long. This checksum is tamper-proof and
believed to be collision-proof.

The DES specifications identify some weak keys' and 'semi-weak keys'; those
keys shall not be used for generating RSA-MD4 checksums for use in Kerberos.

The format for the checksum is described in the following diagram:

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|  des-cbc(confounder   +   rsa-md4(confounder+msg),key=var(key),iv=0)  |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

 associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
 get_mic                 des-cbc(key XOR F0F0F0F0F0F0F0F0,
                                 confounder | rsa-md4(confounder | msg))

 verify_mic              decrypt and verify rsa-md4 checksum

The rsa-md4-des checksum algorithm is assigned a checksum type number of
three (3).

6.6.2. The RSA MD5 Checksum (rsa-md5)

The RSA-MD5 checksum calculates a checksum using the RSA MD5 algorithm
[MD5-92]. The algorithm takes as input an input message of arbitrary length
and produces as output a 128-bit (16 octet) checksum. RSA-MD5 is believed to
be collision-proof. However, since it is unkeyed, it must be used with
caution. Currently it is used by some implementations in places where the
checksum itself is part of a larger message that will be encrypted. Its use
is not recommended.

 associated cryptosystem null
 get_mic                 rsa-md5(msg)
 verify_mic              get_mic and compare

The rsa-md5 checksum algorithm is assigned a checksum type number of seven
(7).

6.6.3. RSA MD5 Cryptographic Checksum Using DES (rsa-md5-des)

The RSA-MD5-DES checksum calculates a keyed collision-proof checksum by
prepending an 8 octet confounder before the text, applying the RSA MD5
checksum algorithm, and encrypting the confounder and the checksum using DES
in cipher-block-chaining (CBC) mode using a variant of the key, where the
variant is computed by eXclusive-ORing the key with the hexadecimal constant
F0F0F0F0F0F0F0F0. The initialization vector should be zero. The resulting
checksum is 24 octets long. This checksum is tamper-proof and believed to be
collision-proof.


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The DES specifications identify some 'weak keys' and 'semi-weak keys'; those
keys shall not be used for encrypting RSA-MD5 checksums for use in Kerberos.

The format for the checksum is described in the following diagram:

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|  des-cbc(confounder   +   rsa-md5(confounder+msg),key=var(key),iv=0)  |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

 associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
 get_mic                 des-cbc(key XOR F0F0F0F0F0F0F0F0,
                                 confounder | rsa-md5(confounder | msg))

 verify_mic              decrypt and verify rsa-md5 checksum

The rsa-md5-des checksum algorithm is assigned a checksum type number of
eight (8).

6.6.4. The HMAC-SHA1-DES3-KD Checksum (hmac-sha1-des3-kd)

This checksum type is defined as outlined in section 6.3 above, using the
des3-hmac-sha1-kd encryption algorithm parameters. The checksum is thus a
SHA-1 HMAC using the computed key Kc over the message to be protected.

The hmac-sha1-des3-kd checksum algorithm is assigned a checksum type number
of twelve (12).

6.7. Use of Kerberos encryption outside this specification

Several Kerberos-based application protocols and preauthentication systems
have been designed and deployed that perform encryption and message
integrity checks in various ways. While in some cases there may be good
reason for specifying these protocols in terms of specific encryption or
checksum algorithms, we anticipate that in many cases this will not be true,
and more generic approaches independent of particular algorithms will be
desirable. Rather than having each protocol designer reinvent schemes for
protecting data, using multiple keys, etc, we have attempted to present in
this section a general framework that should be sufficient not only for the
Kerberos protocol itself but also for many preauthentication systems and
application protocols, while trying to avoid some of the assumptions that
can work their way into such protocol designs.[6.5] Such assumptions, while
they may hold for any given set of encryption and checksum algorithms, may
not be true of the next algorithms to be defined, leaving the application
protocol unable to make use of those algorithms without updates to its
specification.

The Kerberos protocol uses only the attributes and operations described in
sections 6.2 and 6.3. Preauthentication systems and application protocols
making use of Kerberos are encouraged to use them as well.

While we don't recommend it, undoubtedly some application protocols will
continue to use the key data directly, even if only in some of the currently
existing protocol specifications. An implementation intended to support
general Kerberos applications may therefore need to make the key data
available, as well as the attributes and operations described in sections
6.2 and 6.3. [6.6]


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7. Naming Constraints

7.1. Realm Names

Although realm names are encoded as GeneralStrings and although a realm can
technically select any name it chooses, interoperability across realm
boundaries requires agreement on how realm names are to be assigned, and
what information they imply.

To enforce these conventions, each realm must conform to the conventions
itself, and it must require that any realms with which inter-realm keys are
shared also conform to the conventions and require the same from its
neighbors.

Kerberos realm names are case sensitive. Realm names that differ only in the
case of the characters are not equivalent. There are presently four styles
of realm names: domain, X500, other, and reserved. Examples of each style
follow:

     domain:   ATHENA.MIT.EDU (example)
       X500:   C=US/O=OSF (example)
      other:   NAMETYPE:rest/of.name=without-restrictions (example)
   reserved:   reserved, but will not conflict with above

Domain names must look like domain names: they consist of components
separated by periods (.) and they contain neither colons (:) nor slashes
(/). Though domain names themselves are case insensitive, in order for
realms to match, the case must match as well. When establishing a new realm
name based on an internet domain name it is recommended by convention that
the characters be converted to upper case.

X.500 names contain an equal (=) and cannot contain a colon (:) before the
equal. The realm names for X.500 names will be string representations of the
names with components separated by slashes. Leading and trailing slashes
will not be included. Note that the slash separator is consistent with
Kerberos implementations based on RFC1510, but it is different from the
separator recommended in RFC2253.

Names that fall into the other category must begin with a prefix that
contains no equal (=) or period (.) and the prefix must be followed by a
colon (:) and the rest of the name. All prefixes must be assigned before
they may be used. Presently none are assigned.

The reserved category includes strings which do not fall into the first
three categories. All names in this category are reserved. It is unlikely
that names will be assigned to this category unless there is a very strong
argument for not using the 'other' category.

These rules guarantee that there will be no conflicts between the various
name styles. The following additional constraints apply to the assignment of
realm names in the domain and X.500 categories: the name of a realm for the
domain or X.500 formats must either be used by the organization owning (to
whom it was assigned) an Internet domain name or X.500 name, or in the case
that no such names are registered, authority to use a realm name may be
derived from the authority of the parent realm. For example, if there is no
domain name for E40.MIT.EDU, then the administrator of the MIT.EDU realm can
authorize the creation of a realm with that name.


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This is acceptable because the organization to which the parent is assigned
is presumably the organization authorized to assign names to its children in
the X.500 and domain name systems as well. If the parent assigns a realm
name without also registering it in the domain name or X.500 hierarchy, it
is the parent's responsibility to make sure that there will not in the
future exist a name identical to the realm name of the child unless it is
assigned to the same entity as the realm name.

7.2. Principal Names

As was the case for realm names, conventions are needed to ensure that all
agree on what information is implied by a principal name. The name-type
field that is part of the principal name indicates the kind of information
implied by the name. The name-type should be treated as a hint. Ignoring the
name type, no two names can be the same (i.e. at least one of the
components, or the realm, must be different). The following name types are
defined:

  name-type      value   meaning

   NT-UNKNOWN        0   Name type not known
   NT-PRINCIPAL      1   General principal name (e.g. username, or 
                                                  DCE principal)
   NT-SRV-INST       2   Service and other unique instance (krbtgt)
   NT-SRV-HST        3   Service with host name as instance (telnet, rcommands)
   NT-SRV-XHST       4   Service with slash-separated host name components
   NT-UID            5   Unique ID
   NT-X500-PRINCIPAL 6   Encoded X.509 Distingished name [RFC 1779]
   NT-SMTP-NAME      7   Name in form of SMTP email name (e.g. user@foo.com)
   NT-ENTERPRISE    10   Enterprise name - may be mapped to principal name

When a name implies no information other than its uniqueness at a particular
time the name type PRINCIPAL should be used. The principal name type should
be used for users, and it might also be used for a unique server. If the
name is a unique machine generated ID that is guaranteed never to be
reassigned then the name type of UID should be used (note that it is
generally a bad idea to reassign names of any type since stale entries might
remain in access control lists).

If the first component of a name identifies a service and the remaining
components identify an instance of the service in a server specified manner,
then the name type of SRV-INST should be used. An example of this name type
is the Kerberos ticket-granting service whose name has a first component of
krbtgt and a second component identifying the realm for which the ticket is
valid.

If instance is a single component following the service name and the
instance identifies the host on which the server is running, then the name
type SRV-HST should be used. This type is typically used for Internet
services such as telnet and the Berkeley R commands. If the separate
components of the host name appear as successive components following the
name of the service, then the name type SRV-XHST should be used. This type
might be used to identify servers on hosts with X.500 names where the slash
(/) might otherwise be ambiguous.

A name type of NT-X500-PRINCIPAL should be used when a name from an X.509
certificate is translated into a Kerberos name. The encoding of the X.509
name as a Kerberos principal shall conform to the encoding rules specified
in RFC 2253.


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A name type of SMTP allows a name to be of a form that resembles a SMTP
email name. This name, including an "@" and a domain name, is used as the
one component of the principal name.

A name type of UNKNOWN should be used when the form of the name is not
known. When comparing names, a name of type UNKNOWN will match principals
authenticated with names of any type. A principal authenticated with a name
of type UNKNOWN, however, will only match other names of type UNKNOWN.

Names of any type with an initial component of 'krbtgt' are reserved for the
Kerberos ticket granting service. See section 8.2.3 for the form of such
names.

7.2.1. Name of server principals

The principal identifier for a server on a host will generally be composed
of two parts: (1) the realm of the KDC with which the server is registered,
and (2) a two-component name of type NT-SRV-HST if the host name is an
Internet domain name or a multi-component name of type NT-SRV-XHST if the
name of the host is of a form such as X.500 that allows slash (/)
separators. The first component of the two- or multi-component name will
identify the service and the latter components will identify the host. Where
the name of the host is not case sensitive (for example, with Internet
domain names) the name of the host must be lower case. If specified by the
application protocol for services such as telnet and the Berkeley R commands
which run with system privileges, the first component may be the string
'host' instead of a service specific identifier. When a host has an official
name and one or more aliases and the official name can be reliably
determined, the official name of the host should be used when constructing
the name of the server principal.

8. Constants and other defined values

8.1. Host address types

All negative values for the host address type are reserved for local use.
All non-negative values are reserved for officially assigned type fields and
interpretations.

The values of the types for the following addresses are chosen to match the
defined address family constants in the Berkeley Standard Distributions of
Unix. They can be found in with symbolic names AF_xxx (where xxx is an
abbreviation of the address family name).

Internet (IPv4) Addresses

Internet (IPv4) addresses are 32-bit (4-octet) quantities, encoded in MSB
order. The IPv4 loopback address should not appear in a Kerberos packet. The
type of IPv4 addresses is two (2).

Internet (IPv6) Addresses [Westerlund]

IPv6 addresses are 128-bit (16-octet) quantities, encoded in MSB order. The
type of IPv6 addresses is twenty-four (24). [RFC2373]. The following
addresses (see [RFC1884]) MUST not appear in any Kerberos packet:

   * the Unspecified Address
   * the Loopback Address
   * Link-Local addresses


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IPv4-mapped IPv6 addresses MUST be represented as addresses of type 2.

CHAOSnet addresses

CHAOSnet addresses are 16-bit (2-octet) quantities, encoded in MSB order.
The type of CHAOSnet addresses is five (5).

ISO addresses

ISO addresses are variable-length. The type of ISO addresses is seven (7).

Xerox Network Services (XNS) addresses

XNS addresses are 48-bit (6-octet) quantities, encoded in MSB order. The
type of XNS addresses is six (6).

AppleTalk Datagram Delivery Protocol (DDP) addresses

AppleTalk DDP addresses consist of an 8-bit node number and a 16-bit network
number. The first octet of the address is the node number; the remaining two
octets encode the network number in MSB order. The type of AppleTalk DDP
addresses is sixteen (16).

DECnet Phase IV addresses

DECnet Phase IV addresses are 16-bit addresses, encoded in LSB order. The
type of DECnet Phase IV addresses is twelve (12).

Netbios addresses

Netbios addresses are 16-octet addresses typically composed of 1 to 15
characters, trailing blank (ascii char 20) filled, with a 16th octet of 0x0.
The type of Netbios addresses is 20 (0x14).

8.2. KDC messages

8.2.1. UDP/IP transport

When contacting a Kerberos server (KDC) for a KRB_KDC_REQ request using UDP
IP transport, the client shall send a UDP datagram containing only an
encoding of the request to port 88 (decimal) at the KDC's IP address; the
KDC will respond with a reply datagram containing only an encoding of the
reply message (either a KRB_ERROR or a KRB_KDC_REP) to the sending port at
the sender's IP address. Kerberos servers supporting IP transport must
accept UDP requests on port 88 (decimal). The response to a request made
through UDP/IP transport must also use UDP/IP transport.

8.2.2. TCP/IP transport [Westerlund,Danielsson]

Kerberos servers (KDC's) should accept TCP requests on port 88 (decimal) and
clients should support the sending of TCP requests on port 88 (decimal).
When the KRB_KDC_REQ message is sent to the KDC over a TCP stream, a new
connection will be established for each authentication exchange (request and
response). The KRB_KDC_REP or KRB_ERROR message will be returned to the
client on the same TCP stream that was established for the request. The
response to a request made through TCP/IP transport must also use TCP/IP
transport. Implementors should note that some extensions to the Kerberos
protocol will not work if any implementation not supporting the TCP
transport is involved (client or KDC). Implementors are strongly urged to
support the TCP transport on both the client and server and are advised that

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

the current notation of "should" support will likely change in the future to
must support. The KDC may close the TCP stream after sending a response, but
may leave the stream open if it expects a followup - in which case it may
close the stream at any time if resource constraints or other factors make
it desirable to do so. Care must be taken in managing TCP/IP connections
with the KDC to prevent denial of service attacks based on the number of
TCP/IP connections with the KDC that remain open. If multiple exchanges with
the KDC are needed for certain forms of preauthentication, multiple TCP
connections may be required. A client may close the stream after receiving
response, and should close the stream if it does not expect to send followup
messages. The client must be prepared to have the stream closed by the KDC
at anytime, in which case it must simply connect again when it is ready to
send subsequent messages.

The first four octets of the TCP stream used to transmit the request request
will encode in network byte order the length of the request (KRB_KDC_REQ),
and the length will be followed by the request itself. The response will
similarly be preceded by a 4 octet encoding in network byte order of the
length of the KRB_KDC_REP or the KRB_ERROR message and will be followed by
the KRB_KDC_REP or the KRB_ERROR response. If the sign bit is set on the
integer represented by the first 4 octets, then the next 4 octets will be
read, extending the length of the field by another 4 octets (less the sign
bit of the additional four octets which is reserved for future expansion and
which at present must be zero).

8.2.3. OSI transport

During authentication of an OSI client to an OSI server, the mutual
authentication of an OSI server to an OSI client, the transfer of
credentials from an OSI client to an OSI server, or during exchange of
private or integrity checked messages, Kerberos protocol messages may be
treated as opaque objects and the type of the authentication mechanism will
be:

OBJECT IDENTIFIER ::= {iso (1), org(3), dod(6),internet(1), security(5),                kerberosv5(2)}

Depending on the situation, the opaque object will be an authentication
header (KRB_AP_REQ), an authentication reply (KRB_AP_REP), a safe message
(KRB_SAFE), a private message (KRB_PRIV), or a credentials message
(KRB_CRED). The opaque data contains an application code as specified in the
ASN.1 description for each message. The application code may be used by
Kerberos to determine the message type.

8.2.4. Name of the TGS

The principal identifier of the ticket-granting service shall be composed of
three parts: (1) the realm of the KDC issuing the TGS ticket (2) a two-part
name of type NT-SRV-INST, with the first part "krbtgt" and the second part
the name of the realm which will accept the ticket-granting ticket. For
example, a ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be
used to get tickets from the ATHENA.MIT.EDU KDC has a principal identifier
of "ATHENA.MIT.EDU" (realm), ("krbtgt", "ATHENA.MIT.EDU") (name). A
ticket-granting ticket issued by the ATHENA.MIT.EDU realm to be used to get
tickets from the MIT.EDU realm has a principal identifier of
"ATHENA.MIT.EDU" (realm), ("krbtgt", "MIT.EDU") (name).


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

8.3. Protocol constants and associated values

The following tables list constants used in the protocol and define their
meanings. Ranges are specified in the "specification" section that limit the
values of constants for which values are defined here. This allows
implementations to make assumptions about the maximum values that will be
received for these constants. Implementation receiving values outside the
range specified in the "specification" section may reject the request, but
they must recover cleanly.

Encryption type       etype value block size  minimum pad size  confounder size
NULL                           0     1           0                 0
des-cbc-crc                    1     8           4                 8
des-cbc-md4                    2     8           0                 8
des-cbc-md5                    3     8           0                 8
[reserved]                     4
des3-cbc-md5                   5     8           0                 8
[reserved]                     6
des3-cbc-sha1                  7     8           0                 8
dsaWithSHA1-CmsOID             9                                 (pkinit)
md5WithRSAEncryption-CmsOID   10                                 (pkinit)
sha1WithRSAEncryption-CmsOID  11                                 (pkinit)
rc2CBC-EnvOID                 12                                 (pkinit)
rsaEncryption-EnvOID          13                      (pkinit from PKCS#1 v1.5)
rsaES-OAEP-ENV-OID            14                      (pkinit from PKCS#1 v2.0)
des-ede3-cbc-Env-OID          15                                 (pkinit)
des3-cbc-sha1-kd              16                                 (Tom Yu)
rc4-hmac                      23                                 (swift)
rc4-hmac-exp                  24                                 (swift)
subkey-keynaterial            65                                (opaque mhur)

[reserved]       0x8003

Checksum type              sumtype value       checksum size
CRC32                      1                   4
rsa-md4                    2                   16
rsa-md4-des                3                   24
des-mac                    4                   16
des-mac-k                  5                   8
rsa-md4-des-k              6                   16 (drop rsa ?)
rsa-md5                    7                   16 (drop rsa ?)
rsa-md5-des                8                   24 (drop rsa ?)
rsa-md5-des3               9                   24 (drop rsa ?)
hmac-sha1-des3-kd          12                  20
hmac-sha1-des3             13                  20
sha1 (unkeyed)             14                  20

padata and data types           padata-type value  comment

PA-TGS-REQ                      1
PA-ENC-TIMESTAMP                2
PA-PW-SALT                      3
[reserved]                      4
PA-ENC-UNIX-TIME                5                  (depricated)
PA-SANDIA-SECUREID              6
PA-SESAME                       7
PA-OSF-DCE                      8
PA-CYBERSAFE-SECUREID           9
PA-AFS3-SALT                    10
PA-ETYPE-INFO                   11
PA-SAM-CHALLENGE                12                  (sam/otp)
PA-SAM-RESPONSE                 13                  (sam/otp)
PA-PK-AS-REQ                    14                  (pkinit)

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

PA-PK-AS-REP                    15                  (pkinit)
PA-USE-SPECIFIED-KVNO           20
PA-SAM-REDIRECT                 21                  (sam/otp)
PA-GET-FROM-TYPED-DATA          22                  (embedded in typed data)
TD-PADATA                       22                  (embeds padata)
PA-SAM-ETYPE-INFO               23                  (sam/otp)
PA-ALT-PRINC                    24       (crawdad@fnal.gov)
TD-PKINIT-CMS-CERTIFICATES      101      CertificateSet from CMS
TD-KRB-PRINCIPAL                102      PrincipalName (see Sec.5.9.1)
TD-KRB-REALM                    103      Realm (see Sec.5.9.1)
TD-TRUSTED-CERTIFIERS           104      from PKINIT
TD-CERTIFICATE-INDEX            105      from PKINIT
TD-APP-DEFINED-ERROR            106      application specific (see Sec.5.9.1)
TD-REQ-NONCE                    107      INTEGER (see Sec.5.9.1)
TD-REQ-SEQ                      108      INTEGER (see Sec.5.9.1)
PA-PAC-REQUEST                  128      (jbrezak@exchange.microsoft.com)

authorization data type         ad-type value
AD-IF-RELEVANT                     1
AD-INTENDED-FOR-SERVER             2
AD-INTENDED-FOR-APPLICATION-CLASS  3
AD-KDC-ISSUED                      4
AD-OR                              5
AD-MANDATORY-TICKET-EXTENSIONS     6
AD-IN-TICKET-EXTENSIONS            7
reserved values                    8-63
OSF-DCE                            64
SESAME                             65
AD-OSF-DCE-PKI-CERTID              66         (hemsath@us.ibm.com)
AD-WIN2K-PAC                      128         (jbrezak@exchange.microsoft.com)

Ticket Extension Types

TE-TYPE-NULL                  0      Null ticket extension
TE-TYPE-EXTERNAL-ADATA        1      Integrity protected authorization data
[reserved]                    2      TE-TYPE-PKCROSS-KDC  (I have reservations)
TE-TYPE-PKCROSS-CLIENT        3      PKCROSS cross realm key ticket
TE-TYPE-CYBERSAFE-EXT         4      Assigned to CyberSafe Corp
[reserved]                    5      TE-TYPE-DEST-HOST (I have reservations)

alternate authentication type   method-type value
reserved values                 0-63
ATT-CHALLENGE-RESPONSE          64

transited encoding type         tr-type value
DOMAIN-X500-COMPRESS            1
reserved values                 all others

Label               Value   Meaning or MIT code

pvno                    5   current Kerberos protocol version number


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

message types (Will be updated to match section 5)

KRB_AS_REQ             10   Request for initial authentication
KRB_AS_REP             11   Response to KRB_AS_REQ request
KRB_TGS_REQ            12   Request for authentication based on TGT
KRB_TGS_REP            13   Response to KRB_TGS_REQ request
KRB_AP_REQ             14   application request to server
KRB_AP_REP             15   Response to KRB_AP_REQ_MUTUAL
KRB_SAFE               20   Safe (checksummed) application message
KRB_PRIV               21   Private (encrypted) application message
KRB_CRED               22   Private (encrypted) message to forward credentials
KRB_ERROR              30   Error response

name types

KRB_NT_UNKNOWN        0  Name type not known
KRB_NT_PRINCIPAL      1  Just the name of the principal as in DCE, or for users
KRB_NT_SRV_INST       2  Service and other unique instance (krbtgt)
KRB_NT_SRV_HST        3  Service with host name as instance (telnet, rcommands)
KRB_NT_SRV_XHST       4  Service with host as remaining components
KRB_NT_UID            5  Unique ID
KRB_NT_X500_PRINCIPAL 6  Encoded X.509 Distingished name [RFC 2253]

error codes

KDC_ERR_NONE                    0   No error
KDC_ERR_NAME_EXP                1   Client's entry in database has expired
KDC_ERR_SERVICE_EXP             2   Server's entry in database has expired
KDC_ERR_BAD_PVNO                3   Requested protocol version number
                                    not supported
KDC_ERR_C_OLD_MAST_KVNO         4   Client's key encrypted in old master key
KDC_ERR_S_OLD_MAST_KVNO         5   Server's key encrypted in old master key
KDC_ERR_C_PRINCIPAL_UNKNOWN     6   Client not found in Kerberos database
KDC_ERR_S_PRINCIPAL_UNKNOWN     7   Server not found in Kerberos database
KDC_ERR_PRINCIPAL_NOT_UNIQUE    8   Multiple principal entries in database
KDC_ERR_NULL_KEY                9   The client or server has a null key
KDC_ERR_CANNOT_POSTDATE        10   Ticket not eligible for postdating
KDC_ERR_NEVER_VALID            11   Requested start time is later than end time
KDC_ERR_POLICY                 12   KDC policy rejects request
KDC_ERR_BADOPTION              13   KDC cannot accommodate requested option
KDC_ERR_ETYPE_NOSUPP           14   KDC has no support for encryption type
KDC_ERR_SUMTYPE_NOSUPP         15   KDC has no support for checksum type
KDC_ERR_PADATA_TYPE_NOSUPP     16   KDC has no support for padata type
KDC_ERR_TRTYPE_NOSUPP          17   KDC has no support for transited type
KDC_ERR_CLIENT_REVOKED         18   Clients credentials have been revoked
KDC_ERR_SERVICE_REVOKED        19   Credentials for server have been revoked
KDC_ERR_TGT_REVOKED            20   TGT has been revoked
KDC_ERR_CLIENT_NOTYET          21   Client not yet valid - try again later
KDC_ERR_SERVICE_NOTYET         22   Server not yet valid - try again later
KDC_ERR_KEY_EXPIRED            23   Password has expired - change password
                                    to reset
KDC_ERR_PREAUTH_FAILED         24   Pre-authentication information was invalid
KDC_ERR_PREAUTH_REQUIRED       25   Additional pre-authenticationrequired [40]
KDC_ERR_SERVER_NOMATCH         26   Requested server and ticket don't match
KDC_ERR_MUST_USE_USER2USER     27   Server principal valid for user2user only
KDC_ERR_PATH_NOT_ACCPETED      28   KDC Policy rejects transited path
KDC_ERR_SVC_UNAVAILABLE        29   A service is not available
KRB_AP_ERR_BAD_INTEGRITY       31   Integrity check on decrypted field failed
KRB_AP_ERR_TKT_EXPIRED         32   Ticket expired
KRB_AP_ERR_TKT_NYV             33   Ticket not yet valid

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

KRB_AP_ERR_REPEAT              34   Request is a replay
KRB_AP_ERR_NOT_US              35   The ticket isn't for us
KRB_AP_ERR_BADMATCH            36   Ticket and authenticator don't match
KRB_AP_ERR_SKEW                37   Clock skew too great
KRB_AP_ERR_BADADDR             38   Incorrect net address
KRB_AP_ERR_BADVERSION          39   Protocol version mismatch
KRB_AP_ERR_MSG_TYPE            40   Invalid msg type
KRB_AP_ERR_MODIFIED            41   Message stream modified
KRB_AP_ERR_BADORDER            42   Message out of order
KRB_AP_ERR_BADKEYVER           44   Specified version of key is not available
KRB_AP_ERR_NOKEY               45   Service key not available
KRB_AP_ERR_MUT_FAIL            46   Mutual authentication failed
KRB_AP_ERR_BADDIRECTION        47   Incorrect message direction
KRB_AP_ERR_METHOD              48   Alternative authentication method required
KRB_AP_ERR_BADSEQ              49   Incorrect sequence number in message
KRB_AP_ERR_INAPP_CKSUM         50   Inappropriate type of checksum in message
KRB_AP_PATH_NOT_ACCEPTED       51   Policy rejects transited path
KRB_ERR_RESPONSE_TOO_BIG       52   Response too big for UDP, retry with TCP
KRB_ERR_GENERIC                60   Generic error (description in e-text)
KRB_ERR_FIELD_TOOLONG          61   Field is too long for this implementation
KDC_ERROR_CLIENT_NOT_TRUSTED            62 (pkinit)
KDC_ERROR_KDC_NOT_TRUSTED               63 (pkinit)
KDC_ERROR_INVALID_SIG                   64 (pkinit)
KDC_ERR_KEY_TOO_WEAK                    65 (pkinit)
KDC_ERR_CERTIFICATE_MISMATCH            66 (pkinit)
KRB_AP_ERR_NO_TGT                       67 (user-to-user)
KDC_ERR_WRONG_REALM                     68 (user-to-user)
KRB_AP_ERR_USER_TO_USER_REQUIRED        69 (user-to-user)
KDC_ERR_CANT_VERIFY_CERTIFICATE         70 (pkinit)
KDC_ERR_INVALID_CERTIFICATE             71 (pkinit)
KDC_ERR_REVOKED_CERTIFICATE             72 (pkinit)
KDC_ERR_REVOCATION_STATUS_UNKNOWN       73 (pkinit)
KDC_ERR_REVOCATION_STATUS_UNAVAILABLE   74 (pkinit)
KDC_ERR_CLIENT_NAME_MISMATCH            75 (pkinit)
KDC_ERR_KDC_NAME_MISMATCH               76 (pkinit)


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

9. Interoperability requirements

Version 5 of the Kerberos protocol supports a myriad of options. Among these
are multiple encryption and checksum types, alternative encoding schemes for
the transited field, optional mechanisms for pre-authentication, the
handling of tickets with no addresses, options for mutual authentication,
user to user authentication, support for proxies, forwarding, postdating,
and renewing tickets, the format of realm names, and the handling of
authorization data.

In order to ensure the interoperability of realms, it is necessary to define
a minimal configuration which must be supported by all implementations. This
minimal configuration is subject to change as technology does. For example,
if at some later date it is discovered that one of the required encryption
or checksum algorithms is not secure, it will be replaced.

9.1. Specification 2

This section defines the second specification of these options.
Implementations which are configured in this way can be said to support
Kerberos Version 5 Specification 2 (5.1). Specification 1 (deprecated) may
be found in RFC1510.

Transport

TCP/IP and UDP/IP transport must be supported by KDCs claiming conformance
to specification 2. Kerberos clients claiming conformance to specification 2
must support UDP/IP transport for messages with the KDC and should support
TCP/IP transport.

Encryption and checksum methods

The following encryption and checksum mechanisms must be supported.
Implementations may support other mechanisms as well, but the additional
mechanisms may only be used when communicating with principals known to also
support them: This list is to be determined and should correspond to section
6.

Encryption: DES-CBC-MD5, DES3-CBC-SHA1-KD, RIJNDAEL(decide identifier)
Checksums: CRC-32, DES-MAC, DES-MAC-K, DES-MD5, HMAC-SHA1-DES3-KD

Realm Names

All implementations must understand hierarchical realms in both the Internet
Domain and the X.500 style. When a ticket granting ticket for an unknown
realm is requested, the KDC must be able to determine the names of the
intermediate realms between the KDCs realm and the requested realm.

Transited field encoding

DOMAIN-X500-COMPRESS (described in section 3.3.3.2) must be supported.
Alternative encodings may be supported, but they may be used only when that
encoding is supported by ALL intermediate realms.

Pre-authentication methods

The TGS-REQ method must be supported. The TGS-REQ method is not used on the
initial request. The PA-ENC-TIMESTAMP method must be supported by clients
but whether it is enabled by default may be determined on a realm by realm
basis. If not used in the initial request and the error

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

KDC_ERR_PREAUTH_REQUIRED is returned specifying PA-ENC-TIMESTAMP as an
acceptable method, the client should retry the initial request using the
PA-ENC-TIMESTAMP preauthentication method. Servers need not support the
PA-ENC-TIMESTAMP method, but if not supported the server should ignore the
presence of PA-ENC-TIMESTAMP pre-authentication in a request.

Mutual authentication

Mutual authentication (via the KRB_AP_REP message) must be supported.

Ticket addresses and flags

All KDC's must pass through tickets that carry no addresses (i.e. if a TGT
contains no addresses, the KDC will return derivative tickets), but each
realm may set its own policy for issuing such tickets, and each application
server will set its own policy with respect to accepting them.

Proxies and forwarded tickets must be supported. Individual realms and
application servers can set their own policy on when such tickets will be
accepted.

All implementations must recognize renewable and postdated tickets, but need
not actually implement them. If these options are not supported, the
starttime and endtime in the ticket shall specify a ticket's entire useful
life. When a postdated ticket is decoded by a server, all implementations
shall make the presence of the postdated flag visible to the calling server.

User-to-user authentication

Support for user to user authentication (via the ENC-TKT-IN-SKEY KDC option)
must be provided by implementations, but individual realms may decide as a
matter of policy to reject such requests on a per-principal or realm-wide
basis.

Authorization data

Implementations must pass all authorization data subfields from
ticket-granting tickets to any derivative tickets unless directed to
suppress a subfield as part of the definition of that registered subfield
type (it is never incorrect to pass on a subfield, and no registered
subfield types presently specify suppression at the KDC).

Implementations must make the contents of any authorization data subfields
available to the server when a ticket is used. Implementations are not
required to allow clients to specify the contents of the authorization data
fields.

Constant ranges

All protocol constants are constrained to 32 bit (signed) values unless
further constrained by the protocol definition. This limit is provided to
allow implementations to make assumptions about the maximum values that will
be received for these constants. Implementation receiving values outside
this range may reject the request, but they must recover cleanly.


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

9.2. Recommended KDC values

Following is a list of recommended values for a KDC implementation, based on
the list of suggested configuration constants (see section 4.4).

minimum lifetime              5 minutes
maximum renewable lifetime    1 week
maximum ticket lifetime       1 day
empty addresses               only when suitable  restrictions  appear
                              in authorization data
proxiable, etc.               Allowed.

10. IANA considerations

An appendix will be created with the tables that IANA will need to start
maintaining.

   * cryptosystem registration
   * usage number registration

11. ACKNOWLEDGEMENTS

T.B.S.

12. REFERENCES

[Blumenthal96]
     Blumenthal, U., "A Better Key Schedule for DES-Like Ciphers",
     Proceedings of PRAGOCRYPT '96, 1996.
[Bellare98]
     Bellare, M., Desai, A., Pointcheval, D., Rogaway, P., "Relations Among
     Notions of Security for Public-Key Encryption Schemes". Extended
     abstract published in Advances in Cryptology- Crypto 98 Proceedings,
     Lecture Notes in Computer Science Vol. 1462, H. Krawcyzk ed.,
     Springer-Verlag, 1998.
[DES77]
     National Bureau of Standards, U.S. Department of Commerce, "Data
     Encryption Standard," Federal Information Processing Standards
     Publication 46, Washington, DC (1977).
[DESM80]
     National Bureau of Standards, U.S. Department of Com- merce, "DES Modes
     of Operation," Federal Information Processing Standards Publication 81,
     Springfield, VA (December 1980).
[Dolev91]
     Dolev, D., Dwork, C., Naor, M., "Non-malleable cryptography",
     Proceedings of the 23rd Annual Symposium on Theory of Computing, ACM,
     1991.
[DS81]
     Dorothy E. Denning and Giovanni Maria Sacco, "Time- stamps in Key
     Distribution Protocols," Communications of the ACM, Vol. 24(8), pp.
     533-536 (August 1981).
[DS90]
     Don Davis and Ralph Swick, "Workstation Services and Kerberos
     Authentication at Project Athena," Technical Memorandum TM-424, MIT
     Laboratory for Computer Science (February 1990).
[Horowitz96]
     Horowitz, M., "Key Derivation for Authentication, Integrity, and
     Privacy", draft-horowitz-key-derivation-02.txt, August 1998.
[HorowitzB96]
     Horowitz, M., "Key Derivation for Kerberos V5", draft-

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

     horowitz-kerb-key-derivation-01.txt, September 1998.
[IS3309]
     International Organization for Standardization, "ISO Information
     Processing Systems - Data Communication - High-Level Data Link Control
     Procedure - Frame Struc- ture," IS 3309 (October 1984). 3rd Edition.
[KBC96]
     H. Krawczyk, M. Bellare, and R. Canetti, "HMAC: Keyed- Hashing for
     Message Authentication," Working Draft
     draft-ietf-ipsec-hmac-md5-01.txt, (August 1996).
[KNT92]
     John T. Kohl, B. Clifford Neuman, and Theodore Y. Ts'o, "The Evolution
     of the Kerberos Authentication Service," in an IEEE Computer Society
     Text soon to be published (June 1992).
[Krawczyk96]
     Krawczyk, H., Bellare, and M., Canetti, R., "HMAC: Keyed-Hashing for
     Message Authentication", draft-ietf-ipsec-hmac- md5-01.txt, August,
     1996.
[LGDSR87]
     P. J. Levine, M. R. Gretzinger, J. M. Diaz, W. E. Som- merfeld, and K.
     Raeburn, Section E.1: Service Manage- ment System, M.I.T. Project
     Athena, Cambridge, Mas- sachusetts (1987).
[MD4-92]
     R. Rivest, "The MD4 Message Digest Algorithm," RFC 1320, MIT Laboratory
     for Computer Science (April 1992).
[MD5-92]
     R. Rivest, "The MD5 Message Digest Algorithm," RFC 1321, MIT Laboratory
     for Computer Science (April 1992).
[MNSS87]
     S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. Saltzer, Section
     E.2.1: Kerberos Authentication and Authorization System, M.I.T. Project
     Athena, Cambridge, Massachusetts (December 21, 1987).
[Neu93]
     B. Clifford Neuman, "Proxy-Based Authorization and Accounting for
     Distributed Systems," in Proceedings of the 13th International
     Conference on Distributed Com- puting Systems, Pittsburgh, PA (May,
     1993).
[NS78]
     Roger M. Needham and Michael D. Schroeder, "Using Encryption for
     Authentication in Large Networks of Com- puters," Communications of the
     ACM, Vol. 21(12), pp. 993-999 (December, 1978).
[NT94]
     B. Clifford Neuman and Theodore Y. Ts'o, "An Authenti- cation Service
     for Computer Networks," IEEE Communica- tions Magazine, Vol. 32(9), pp.
     33-38 (September 1994).
[Pat92].
     J. Pato, Using Pre-Authentication to Avoid Password Guessing Attacks,
     Open Software Foundation DCE Request for Comments 26 (December 1992).
[SG92]
     Stuart G. Stubblebine and Virgil D. Gligor, "On Message Integrity in
     Cryptographic Protocols," in Proceedings of the IEEE Symposium on
     Research in Security and Privacy, Oakland, California (May 1992).
[SNS88]
     J. G. Steiner, B. C. Neuman, and J. I. Schiller, "Ker- beros: An
     Authentication Service for Open Network Sys- tems," pp. 191-202 in
     Usenix Conference Proceedings, Dallas, Texas (February, 1988).
[X509-88]
     CCITT, Recommendation X.509: The Directory Authentica- tion Framework,
     December 1988.


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

A. Pseudo-code for protocol processing

This appendix provides pseudo-code describing how the messages are to be
constructed and interpreted by clients and servers. [we are waiting on
verification code from Microsoft for possible replacement]

A.1. KRB_AS_REQ generation

        request.pvno := protocol version; /* pvno = 5 */
        request.msg-type := message type; /* type = KRB_AS_REQ */

        if(pa_enc_timestamp_required) then
                request.padata.padata-type = PA-ENC-TIMESTAMP;
                get system_time;
                padata-body.patimestamp,pausec = system_time;
                encrypt padata-body into request.padata.padata-value
                        using client.key; /* derived from password */
        endif

        body.kdc-options := users's preferences;
        body.cname := user's name;
        body.realm := user's realm;
        body.sname := service's name; /* usually "krbtgt",  =
"localrealm" */
        if (body.kdc-options.POSTDATED is set) then
                body.from := requested starting time;
        else
                omit body.from;
        endif
        body.till := requested end time;
        if (body.kdc-options.RENEWABLE is set) then
                body.rtime := requested final renewal time;
        endif
        body.nonce := random_nonce();
        body.etype := requested etypes;
        if (user supplied addresses) then
                body.addresses := user's addresses;
        else
                omit body.addresses;
        endif
        omit body.enc-authorization-data;
        request.req-body := body;

        kerberos := lookup(name of local kerberos server (or =
servers));
        send(packet,kerberos);

        wait(for response);
        if (timed_out) then
                retry or use alternate server;
        endif

A.2. KRB_AS_REQ verification and KRB_AS_REP generation

        decode message into req;

        client := lookup(req.cname,req.realm);
        server := lookup(req.sname,req.realm);

        get system_time;

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        kdc_time := system_time.seconds;

        if (!client) then
                /* no client in Database */
                error_out(KDC_ERR_C_PRINCIPAL_UNKNOWN);
        endif
        if (!server) then
                /* no server in Database */
                error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
        endif

        if(client.pa_enc_timestamp_required and
           pa_enc_timestamp not present) then
                error_out(KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP));
        endif

        if(pa_enc_timestamp present) then
                decrypt req.padata-value into decrypted_enc_timestamp
                        using client.key;
                        using auth_hdr.authenticator.subkey;
                if (decrypt_error()) then
                        error_out(KRB_AP_ERR_BAD_INTEGRITY);
                if(decrypted_enc_timestamp is not within allowable skew) =
then
                        error_out(KDC_ERR_PREAUTH_FAILED);
                endif
                if(decrypted_enc_timestamp and usec is replay)
                        error_out(KDC_ERR_PREAUTH_FAILED);
                endif
                add decrypted_enc_timestamp and usec to replay cache;
        endif

        use_etype := first supported etype in req.etypes;

        if (no support for req.etypes) then
                error_out(KDC_ERR_ETYPE_NOSUPP);
        endif

        new_tkt.vno := ticket version; /* = 5 */
        new_tkt.sname := req.sname;
        new_tkt.srealm := req.srealm;
        reset all flags in new_tkt.flags;

        /* It should be noted that local policy may affect the  */
        /* processing of any of these flags.  For example, some */
        /* realms may refuse to issue renewable tickets         */

        if (req.kdc-options.FORWARDABLE is set) then
                set new_tkt.flags.FORWARDABLE;
        endif
        if (req.kdc-options.PROXIABLE is set) then
                set new_tkt.flags.PROXIABLE;
        endif

        if (req.kdc-options.ALLOW-POSTDATE is set) then
                set new_tkt.flags.MAY-POSTDATE;
        endif
        if ((req.kdc-options.RENEW is set) or
            (req.kdc-options.VALIDATE is set) or
            (req.kdc-options.PROXY is set) or

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

            (req.kdc-options.FORWARDED is set) or
            (req.kdc-options.ENC-TKT-IN-SKEY is set)) then
                error_out(KDC_ERR_BADOPTION);
        endif

        new_tkt.session := random_session_key();
        new_tkt.cname := req.cname;
        new_tkt.crealm := req.crealm;
        new_tkt.transited := empty_transited_field();

        new_tkt.authtime := kdc_time;

        if (req.kdc-options.POSTDATED is set) then
           if (against_postdate_policy(req.from)) then
                error_out(KDC_ERR_POLICY);
           endif
           set new_tkt.flags.POSTDATED;
           set new_tkt.flags.INVALID;
           new_tkt.starttime := req.from;
        else
           omit new_tkt.starttime; /* treated as authtime when omitted =
*/
        endif
        if (req.till = 0) then
                till := infinity;
        else
                till := req.till;
        endif

        new_tkt.endtime := min(till,
                              new_tkt.starttime+client.max_life,
                              new_tkt.starttime+server.max_life,
                              new_tkt.starttime+max_life_for_realm);

        if ((req.kdc-options.RENEWABLE-OK is set) and
            (new_tkt.endtime < req.till)) then
                /* we set the RENEWABLE option for later processing */
                set req.kdc-options.RENEWABLE;
                req.rtime := req.till;
        endif

        if (req.rtime = 0) then
                rtime := infinity;
        else
                rtime := req.rtime;
        endif

        if (req.kdc-options.RENEWABLE is set) then
                set new_tkt.flags.RENEWABLE;
                new_tkt.renew-till := min(rtime,
                                       new_tkt.starttime+client.max_rlife,
                                       new_tkt.starttime+server.max_rlife,
                                       new_tkt.starttime+max_rlife_for_realm);
        else
                omit new_tkt.renew-till; /* only present if RENEWABLE */
        endif

        if (req.addresses) then
                new_tkt.caddr := req.addresses;
        else

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

                omit new_tkt.caddr;
        endif

        new_tkt.authorization_data := empty_authorization_data();

        encode to-be-encrypted part of ticket into OCTET STRING;
        new_tkt.enc-part := encrypt OCTET STRING
                using etype_for_key(server.key), server.key, server.p_kvno;

        /* Start processing the response */

        resp.pvno := 5;
        resp.msg-type := KRB_AS_REP;
        resp.cname := req.cname;
        resp.crealm := req.realm;
        resp.ticket := new_tkt;

        resp.key := new_tkt.session;
        resp.last-req := fetch_last_request_info(client);
        resp.nonce := req.nonce;
        resp.key-expiration := client.expiration;
        resp.flags := new_tkt.flags;

        resp.authtime := new_tkt.authtime;
        resp.starttime := new_tkt.starttime;
        resp.endtime := new_tkt.endtime;

        if (new_tkt.flags.RENEWABLE) then
                resp.renew-till := new_tkt.renew-till;
        endif

        resp.realm := new_tkt.realm;
        resp.sname := new_tkt.sname;

        resp.caddr := new_tkt.caddr;

        encode body of reply into OCTET STRING;

        resp.enc-part := encrypt OCTET STRING
                         using use_etype, client.key, client.p_kvno;
        send(resp);

A.3. KRB_AS_REP verification

        decode response into resp;

        if (resp.msg-type = KRB_ERROR) then
                if(error = KDC_ERR_PREAUTH_REQUIRED(PA_ENC_TIMESTAMP)) then
                        set pa_enc_timestamp_required;
                        goto KRB_AS_REQ;
                endif
                process_error(resp);
                return;
        endif

        /* On error, discard the response, and zero the session key */
        /* from the response immediately */

        key = get_decryption_key(resp.enc-part.kvno, resp.enc-part.etype,
                                 resp.padata);

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        unencrypted part of resp := decode of decrypt of resp.enc-part
                                using resp.enc-part.etype and key;
        zero(key);

        if (common_as_rep_tgs_rep_checks fail) then
                destroy resp.key;
                return error;
        endif

        if near(resp.princ_exp) then
                print(warning message);
        endif
        save_for_later(ticket,session,client,server,times,flags);

A.4. KRB_AS_REP and KRB_TGS_REP common checks

        if (decryption_error() or
            (req.cname != resp.cname) or
            (req.realm != resp.crealm) or
            (req.sname != resp.sname) or
            (req.realm != resp.realm) or
            (req.nonce != resp.nonce) or
            (req.addresses != resp.caddr)) then
                destroy resp.key;
                return KRB_AP_ERR_MODIFIED;
        endif

        /* make sure no flags are set that shouldn't be, and that all that */
        /* should be are set */
        if (!check_flags_for_compatability(req.kdc-options,resp.flags)) then
                destroy resp.key;
                return KRB_AP_ERR_MODIFIED;
        endif

        if ((req.from = 0) and
            (resp.starttime is not within allowable skew)) then
                destroy resp.key;
                return KRB_AP_ERR_SKEW;
        endif
        if ((req.from != 0) and (req.from != resp.starttime)) then
                destroy resp.key;
                return KRB_AP_ERR_MODIFIED;
        endif
        if ((req.till != 0) and (resp.endtime > req.till)) then
                destroy resp.key;
                return KRB_AP_ERR_MODIFIED;
        endif

        if ((req.kdc-options.RENEWABLE is set) and
            (req.rtime != 0) and (resp.renew-till > req.rtime)) then
                destroy resp.key;
                return KRB_AP_ERR_MODIFIED;
        endif
        if ((req.kdc-options.RENEWABLE-OK is set) and
            (resp.flags.RENEWABLE) and
            (req.till != 0) and
            (resp.renew-till > req.till)) then
                destroy resp.key;
                return KRB_AP_ERR_MODIFIED;
        endif

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f


A.5. KRB_TGS_REQ generation

        /* Note that make_application_request might have to recursivly  */
        /* call this routine to get the appropriate ticket-granting ticket */

        request.pvno := protocol version; /* pvno = 5 */
        request.msg-type := message type; /* type = KRB_TGS_REQ */

        body.kdc-options := users's preferences;
        /* If the TGT is not for the realm of the end-server  */
        /* then the sname will be for a TGT for the end-realm */
        /* and the realm of the requested ticket (body.realm) */
        /* will be that of the TGS to which the TGT we are    */
        /* sending applies                                    */
        body.sname := service's name;
        body.realm := service's realm;

        if (body.kdc-options.POSTDATED is set) then
                body.from := requested starting time;
        else
                omit body.from;
        endif
        body.till := requested end time;
        if (body.kdc-options.RENEWABLE is set) then
                body.rtime := requested final renewal time;
        endif
        body.nonce := random_nonce();
        body.etype := requested etypes;
        if (user supplied addresses) then
                body.addresses := user's addresses;
        else
                omit body.addresses;
        endif

        body.enc-authorization-data := user-supplied data;
        if (body.kdc-options.ENC-TKT-IN-SKEY) then
                body.additional-tickets_ticket := second TGT;
        endif

        request.req-body := body;
        check := generate_checksum (req.body,checksumtype);

        request.padata[0].padata-type := PA-TGS-REQ;
        request.padata[0].padata-value := create a KRB_AP_REQ using
                                      the TGT and checksum

        /* add in any other padata as required/supplied */

        kerberos := lookup(name of local kerberose server (or servers));
        send(packet,kerberos);

        wait(for response);
        if (timed_out) then
                retry or use alternate server;
        endif

A.6. KRB_TGS_REQ verification and KRB_TGS_REP generation

        /* note that reading the application request requires first

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        determining the server for which a ticket was issued, and choosing the
        correct key for decryption.  The name of the server appears in the
        plaintext part of the ticket. */

        if (no KRB_AP_REQ in req.padata) then
                error_out(KDC_ERR_PADATA_TYPE_NOSUPP);
        endif
        verify KRB_AP_REQ in req.padata;

        /* Note that the realm in which the Kerberos server is operating is
        determined by the instance from the ticket-granting ticket.  The realm
        in the ticket-granting ticket is the realm under which the ticket
        granting ticket was issued.  It is possible for a single Kerberos
        server to support more than one realm. */

        auth_hdr := KRB_AP_REQ;
        tgt := auth_hdr.ticket;

        if (tgt.sname is not a TGT for local realm and is not req.sname) then
                error_out(KRB_AP_ERR_NOT_US);

        realm := realm_tgt_is_for(tgt);

        decode remainder of request;

        if (auth_hdr.authenticator.cksum is missing) then
                error_out(KRB_AP_ERR_INAPP_CKSUM);
        endif

        if (auth_hdr.authenticator.cksum type is not supported) then
                error_out(KDC_ERR_SUMTYPE_NOSUPP);
        endif
        if (auth_hdr.authenticator.cksum is not both collision-proof and keyed) then
                error_out(KRB_AP_ERR_INAPP_CKSUM);
        endif

        set computed_checksum := checksum(req);
        if (computed_checksum != auth_hdr.authenticatory.cksum) then
                error_out(KRB_AP_ERR_MODIFIED);
        endif

        server := lookup(req.sname,realm);

        if (!server) then
                if (is_foreign_tgt_name(req.sname)) then
                        server := best_intermediate_tgs(req.sname);
                else
                        /* no server in Database */
                        error_out(KDC_ERR_S_PRINCIPAL_UNKNOWN);
                endif
        endif

        session := generate_random_session_key();

        use_etype := first supported etype in req.etypes;

        if (no support for req.etypes) then
                error_out(KDC_ERR_ETYPE_NOSUPP);
        endif


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        new_tkt.vno := ticket version; /* = 5 */
        new_tkt.sname := req.sname;
        new_tkt.srealm := realm;
        reset all flags in new_tkt.flags;

        /* It should be noted that local policy may affect the  */
        /* processing of any of these flags.  For example, some */
        /* realms may refuse to issue renewable tickets         */

        new_tkt.caddr := tgt.caddr;
        resp.caddr := NULL; /* We only include this if they change */
        if (req.kdc-options.FORWARDABLE is set) then
                if (tgt.flags.FORWARDABLE is reset) then
                        error_out(KDC_ERR_BADOPTION);
                endif
                set new_tkt.flags.FORWARDABLE;
        endif
        if (req.kdc-options.FORWARDED is set) then
                if (tgt.flags.FORWARDABLE is reset) then
                        error_out(KDC_ERR_BADOPTION);
                endif
                set new_tkt.flags.FORWARDED;
                new_tkt.caddr := req.addresses;
                resp.caddr := req.addresses;
        endif
        if (tgt.flags.FORWARDED is set) then
                set new_tkt.flags.FORWARDED;
        endif

        if (req.kdc-options.PROXIABLE is set) then
                if (tgt.flags.PROXIABLE is reset)
                        error_out(KDC_ERR_BADOPTION);
                endif
                set new_tkt.flags.PROXIABLE;
        endif
        if (req.kdc-options.PROXY is set) then
                if (tgt.flags.PROXIABLE is reset) then
                        error_out(KDC_ERR_BADOPTION);
                endif
                set new_tkt.flags.PROXY;
                new_tkt.caddr := req.addresses;
                resp.caddr := req.addresses;
        endif

        if (req.kdc-options.ALLOW-POSTDATE is set) then
                if (tgt.flags.MAY-POSTDATE is reset)
                        error_out(KDC_ERR_BADOPTION);
                endif
                set new_tkt.flags.MAY-POSTDATE;
        endif
        if (req.kdc-options.POSTDATED is set) then
                if (tgt.flags.MAY-POSTDATE is reset) then
                        error_out(KDC_ERR_BADOPTION);
                endif
                set new_tkt.flags.POSTDATED;
                set new_tkt.flags.INVALID;
                if (against_postdate_policy(req.from)) then
                        error_out(KDC_ERR_POLICY);
                endif
                new_tkt.starttime := req.from;

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        endif

        if (req.kdc-options.VALIDATE is set) then
                if (tgt.flags.INVALID is reset) then
                        error_out(KDC_ERR_POLICY);
                endif
                if (tgt.starttime > kdc_time) then
                        error_out(KRB_AP_ERR_NYV);
                endif
                if (check_hot_list(tgt)) then
                        error_out(KRB_AP_ERR_REPEAT);
                endif
                tkt := tgt;
                reset new_tkt.flags.INVALID;
        endif

        if (req.kdc-options.(any flag except ENC-TKT-IN-SKEY, RENEW,
                             and those already processed) is set) then
                error_out(KDC_ERR_BADOPTION);
        endif

        new_tkt.authtime := tgt.authtime;

        if (req.kdc-options.RENEW is set) then
          /* Note that if the endtime has already passed, the ticket would  */
          /* have been rejected in the initial authentication stage, so     */
          /* there is no need to check again here                           */
                if (tgt.flags.RENEWABLE is reset) then
                        error_out(KDC_ERR_BADOPTION);
                endif
                if (tgt.renew-till < kdc_time) then
                        error_out(KRB_AP_ERR_TKT_EXPIRED);
                endif
                tkt := tgt;
                new_tkt.starttime := kdc_time;
                old_life := tgt.endttime - tgt.starttime;
                new_tkt.endtime := min(tgt.renew-till,
                                       new_tkt.starttime + old_life);
        else
                new_tkt.starttime := kdc_time;
                if (req.till = 0) then
                        till := infinity;
                else
                        till := req.till;
                endif
                new_tkt.endtime := min(till,
                                       new_tkt.starttime+client.max_life,
                                       new_tkt.starttime+server.max_life,
                                       new_tkt.starttime+max_life_for_realm,
                                       tgt.endtime);

                if ((req.kdc-options.RENEWABLE-OK is set) and
                    (new_tkt.endtime < req.till) and
                    (tgt.flags.RENEWABLE is set) then
                        /* we set the RENEWABLE option for later processing */
                        set req.kdc-options.RENEWABLE;
                        req.rtime := min(req.till, tgt.renew-till);
                endif
        endif


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        if (req.rtime = 0) then
                rtime := infinity;
        else
                rtime := req.rtime;
        endif

        if ((req.kdc-options.RENEWABLE is set) and
            (tgt.flags.RENEWABLE is set)) then
                set new_tkt.flags.RENEWABLE;
                new_tkt.renew-till := min(rtime,
                                         new_tkt.starttime+client.max_rlife,
                                         new_tkt.starttime+server.max_rlife,
                                         new_tkt.starttime+max_rlife_for_realm,
                                         tgt.renew-till);
        else
                new_tkt.renew-till := OMIT; /* leave the renew-till field out */
        endif
        if (req.enc-authorization-data is present) then
                decrypt req.enc-authorization-data into decrypted_authorization_data
                        using auth_hdr.authenticator.subkey;
                if (decrypt_error()) then
                        error_out(KRB_AP_ERR_BAD_INTEGRITY);
                endif
        endif
        new_tkt.authorization_data := req.auth_hdr.ticket.authorization_data +
                                 decrypted_authorization_data;

        new_tkt.key := session;
        new_tkt.crealm := tgt.crealm;
        new_tkt.cname := req.auth_hdr.ticket.cname;

        if (realm_tgt_is_for(tgt) := tgt.realm) then
                /* tgt issued by local realm */
                new_tkt.transited := tgt.transited;
        else
                /* was issued for this realm by some other realm */
                if (tgt.transited.tr-type not supported) then
                        error_out(KDC_ERR_TRTYPE_NOSUPP);
                endif
                new_tkt.transited := compress_transited(tgt.transited + tgt.realm)
                /* Don't check tranited field if TGT for foreign realm,=20
                 * or requested not to check */
                if (is_not_foreign_tgt_name(new_tkt.server)=20
                   && req.kdc-options.DISABLE-TRANSITED-CHECK not set) then
                        /* Check it, so end-server does not have to=20
                         * but don't fail, end-server may still accept it */
                        if (check_transited_field(new_tkt.transited) == OK)
                              set =
new_tkt.flags.TRANSITED-POLICY-CHECKED;
                        endif
                endif
        endif

        encode encrypted part of new_tkt into OCTET STRING;
        if (req.kdc-options.ENC-TKT-IN-SKEY is set) then
                if (server not specified) then
                        server = req.second_ticket.client;
                endif
                if ((req.second_ticket is not a TGT) or
                    (req.second_ticket.client != server)) then

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

                        error_out(KDC_ERR_POLICY);
                endif

                new_tkt.enc-part := encrypt OCTET STRING using
                        using etype_for_key(second-ticket.key), second-ticket.key;
        else
                new_tkt.enc-part := encrypt OCTET STRING
                        using etype_for_key(server.key), server.key, server.p_kvno;
        endif

        resp.pvno := 5;
        resp.msg-type := KRB_TGS_REP;
        resp.crealm := tgt.crealm;
        resp.cname := tgt.cname;
        resp.ticket := new_tkt;

        resp.key := session;
        resp.nonce := req.nonce;
        resp.last-req := fetch_last_request_info(client);
        resp.flags := new_tkt.flags;

        resp.authtime := new_tkt.authtime;
        resp.starttime := new_tkt.starttime;
        resp.endtime := new_tkt.endtime;

        omit resp.key-expiration;

        resp.sname := new_tkt.sname;
        resp.realm := new_tkt.realm;

        if (new_tkt.flags.RENEWABLE) then
                resp.renew-till := new_tkt.renew-till;
        endif

        encode body of reply into OCTET STRING;

        if (req.padata.authenticator.subkey)
                resp.enc-part := encrypt OCTET STRING using use_etype,
                        req.padata.authenticator.subkey;
        else resp.enc-part := encrypt OCTET STRING using use_etype, tgt.key;

        send(resp);

=09

A.7. KRB_TGS_REP verification

        decode response into resp;

        if (resp.msg-type = KRB_ERROR) then
                process_error(resp);
                return;
        endif

        /* On error, discard the response, and zero the session key from
        the response immediately */

        if (req.padata.authenticator.subkey)
                unencrypted part of resp := decode of decrypt of resp.enc-part
                        using resp.enc-part.etype and subkey;

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        else unencrypted part of resp := decode of decrypt of resp.enc-part
                                using resp.enc-part.etype and tgt's session key;
        if (common_as_rep_tgs_rep_checks fail) then
                destroy resp.key;
                return error;
        endif

        check authorization_data as necessary;
        save_for_later(ticket,session,client,server,times,flags);

A.8. Authenticator generation

        body.authenticator-vno := authenticator vno; /* = 5 */
        body.cname, body.crealm := client name;
        if (supplying checksum) then
                body.cksum := checksum;
        endif
        get system_time;
        body.ctime, body.cusec := system_time;
        if (selecting sub-session key) then
                select sub-session key;
                body.subkey := sub-session key;
        endif
        if (using sequence numbers) then
                select initial sequence number;
                body.seq-number := initial sequence;
        endif

A.9. KRB_AP_REQ generation

        obtain ticket and session_key from cache;

        packet.pvno := protocol version; /* 5 */
        packet.msg-type := message type; /* KRB_AP_REQ */

        if (desired(MUTUAL_AUTHENTICATION)) then
                set packet.ap-options.MUTUAL-REQUIRED;
        else
                reset packet.ap-options.MUTUAL-REQUIRED;
        endif
        if (using session key for ticket) then
                set packet.ap-options.USE-SESSION-KEY;
        else
                reset packet.ap-options.USE-SESSION-KEY;
        endif
        packet.ticket := ticket; /* ticket */
        generate authenticator;
        encode authenticator into OCTET STRING;
        encrypt OCTET STRING into packet.authenticator using session_key;

A.10. KRB_AP_REQ verification

        receive packet;
        if (packet.pvno != 5) then
                either process using other protocol spec
                or error_out(KRB_AP_ERR_BADVERSION);
        endif
        if (packet.msg-type != KRB_AP_REQ) then
                error_out(KRB_AP_ERR_MSG_TYPE);
        endif

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        if (packet.ticket.tkt_vno != 5) then
                either process using other protocol spec
                or error_out(KRB_AP_ERR_BADVERSION);
        endif
        if (packet.ap_options.USE-SESSION-KEY is set) then
                retrieve session key from ticket-granting ticket for
                 packet.ticket.{sname,srealm,enc-part.etype};
        else
                retrieve service key for
                 packet.ticket.{sname,srealm,enc-part.etype,enc-part.skvno};
        endif
        if (no_key_available) then
                if (cannot_find_specified_skvno) then
                        error_out(KRB_AP_ERR_BADKEYVER);
                else
                        error_out(KRB_AP_ERR_NOKEY);
                endif
        endif
        decrypt packet.ticket.enc-part into decr_ticket using retrieved key;
        if (decryption_error()) then
                error_out(KRB_AP_ERR_BAD_INTEGRITY);
        endif
        decrypt packet.authenticator into decr_authenticator
                using decr_ticket.key;
        if (decryption_error()) then
                error_out(KRB_AP_ERR_BAD_INTEGRITY);
        endif
        if (decr_authenticator.{cname,crealm} !=
            decr_ticket.{cname,crealm}) then
                error_out(KRB_AP_ERR_BADMATCH);
        endif
        if (decr_ticket.caddr is present) then
                if (sender_address(packet) is not in decr_ticket.caddr) then
                        error_out(KRB_AP_ERR_BADADDR);
                endif
        elseif (application requires addresses) then
                error_out(KRB_AP_ERR_BADADDR);
        endif
        if (not in_clock_skew(decr_authenticator.ctime,
                              decr_authenticator.cusec)) then
                error_out(KRB_AP_ERR_SKEW);
        endif
        if (repeated(decr_authenticator.{ctime,cusec,cname,crealm})) then
                error_out(KRB_AP_ERR_REPEAT);
        endif
        save_identifier(decr_authenticator.{ctime,cusec,cname,crealm});
        get system_time;
        if ((decr_ticket.starttime-system_time > CLOCK_SKEW) or
            (decr_ticket.flags.INVALID is set)) then
                /* it hasn't yet become valid */
                error_out(KRB_AP_ERR_TKT_NYV);
        endif
        if (system_time-decr_ticket.endtime > CLOCK_SKEW) then
                error_out(KRB_AP_ERR_TKT_EXPIRED);
        endif
        if (decr_ticket.transited) then
            /* caller may ignore the TRANSITED-POLICY-CHECKED and do
             * check anyway */
            if (decr_ticket.flags.TRANSITED-POLICY-CHECKED not set) then
                 if (check_transited_field(decr_ticket.transited) then

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

                      error_out(KDC_AP_PATH_NOT_ACCPETED);
                 endif
            endif
        endif
        /* caller must check decr_ticket.flags for any pertinent details */
        return(OK, decr_ticket, packet.ap_options.MUTUAL-REQUIRED);

A.11. KRB_AP_REP generation

        packet.pvno := protocol version; /* 5 */
        packet.msg-type := message type; /* KRB_AP_REP */

        body.ctime := packet.ctime;
        body.cusec := packet.cusec;
        if (selecting sub-session key) then
                select sub-session key;
                body.subkey := sub-session key;
        endif
        if (using sequence numbers) then
                select initial sequence number;
                body.seq-number := initial sequence;
        endif

        encode body into OCTET STRING;

        select encryption type;
        encrypt OCTET STRING into packet.enc-part;

A.12. KRB_AP_REP verification

        receive packet;
        if (packet.pvno != 5) then
                either process using other protocol spec
                or error_out(KRB_AP_ERR_BADVERSION);
        endif
        if (packet.msg-type != KRB_AP_REP) then
                error_out(KRB_AP_ERR_MSG_TYPE);
        endif
        cleartext := decrypt(packet.enc-part) using ticket's session key;
        if (decryption_error()) then
                error_out(KRB_AP_ERR_BAD_INTEGRITY);
        endif
        if (cleartext.ctime != authenticator.ctime) then
                error_out(KRB_AP_ERR_MUT_FAIL);
        endif
        if (cleartext.cusec != authenticator.cusec) then
                error_out(KRB_AP_ERR_MUT_FAIL);
        endif
        if (cleartext.subkey is present) then
                save cleartext.subkey for future use;
        endif
        if (cleartext.seq-number is present) then
                save cleartext.seq-number for future verifications;
        endif
        return(AUTHENTICATION_SUCCEEDED);

A.13. KRB_SAFE generation

        collect user data in buffer;


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        /* assemble packet: */
        packet.pvno := protocol version; /* 5 */
        packet.msg-type := message type; /* KRB_SAFE */

        body.user-data := buffer; /* DATA */
        if (using timestamp) then
                get system_time;
                body.timestamp, body.usec := system_time;
        endif
        if (using sequence numbers) then
                body.seq-number := sequence number;
        endif
        body.s-address := sender host addresses;
        if (only one recipient) then
                body.r-address := recipient host address;
        endif
        checksum.cksumtype := checksum type;
        compute checksum over body;
        checksum.checksum := checksum value; /* checksum.checksum */
        packet.cksum := checksum;
        packet.safe-body := body;

A.14. KRB_SAFE verification

        receive packet;
        if (packet.pvno != 5) then
                either process using other protocol spec
                or error_out(KRB_AP_ERR_BADVERSION);
        endif
        if (packet.msg-type != KRB_SAFE) then
                error_out(KRB_AP_ERR_MSG_TYPE);
        endif
        if (packet.checksum.cksumtype is not both collision-proof and keyed) then
                error_out(KRB_AP_ERR_INAPP_CKSUM);
        endif
        if (safe_priv_common_checks_ok(packet)) then
                set computed_checksum := checksum(packet.body);
                if (computed_checksum != packet.checksum) then
                        error_out(KRB_AP_ERR_MODIFIED);
                endif
                return (packet, PACKET_IS_GENUINE);
        else
                return common_checks_error;
        endif

A.15. KRB_SAFE and KRB_PRIV common checks

        if (packet.s-address != O/S_sender(packet)) then
                /* O/S report of sender not who claims to have sent it */
                error_out(KRB_AP_ERR_BADADDR);
        endif
        if ((packet.r-address is present) and
            (packet.r-address != local_host_address)) then
                /* was not sent to proper place */
                error_out(KRB_AP_ERR_BADADDR);
        endif
        if (((packet.timestamp is present) and
             (not in_clock_skew(packet.timestamp,packet.usec))) or
            (packet.timestamp is not present and timestamp expected)) then
                error_out(KRB_AP_ERR_SKEW);

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        endif
        if (repeated(packet.timestamp,packet.usec,packet.s-address)) then
                error_out(KRB_AP_ERR_REPEAT);
        endif

        if (((packet.seq-number is present) and
             ((not in_sequence(packet.seq-number)))) or
            (packet.seq-number is not present and sequence expected)) then
                error_out(KRB_AP_ERR_BADORDER);
        endif
        if (packet.timestamp not present and packet.seq-number not present) then
                error_out(KRB_AP_ERR_MODIFIED);
        endif

        save_identifier(packet.{timestamp,usec,s-address},
                        sender_principal(packet));

        return PACKET_IS_OK;

A.16. KRB_PRIV generation

        collect user data in buffer;

        /* assemble packet: */
        packet.pvno := protocol version; /* 5 */
        packet.msg-type := message type; /* KRB_PRIV */

        packet.enc-part.etype := encryption type;

        body.user-data := buffer;
        if (using timestamp) then
                get system_time;
                body.timestamp, body.usec := system_time;
        endif
        if (using sequence numbers) then
                body.seq-number := sequence number;
        endif
        body.s-address := sender host addresses;
        if (only one recipient) then
                body.r-address := recipient host address;
        endif

        encode body into OCTET STRING;

        select encryption type;
        encrypt OCTET STRING into packet.enc-part.cipher;

A.17. KRB_PRIV verification

        receive packet;
        if (packet.pvno != 5) then
                either process using other protocol spec
                or error_out(KRB_AP_ERR_BADVERSION);
        endif
        if (packet.msg-type != KRB_PRIV) then
                error_out(KRB_AP_ERR_MSG_TYPE);
        endif

        cleartext := decrypt(packet.enc-part) using negotiated key;
        if (decryption_error()) then

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

                error_out(KRB_AP_ERR_BAD_INTEGRITY);
        endif

        if (safe_priv_common_checks_ok(cleartext)) then
                return(cleartext.DATA, PACKET_IS_GENUINE_AND_UNMODIFIED);
        else
                return common_checks_error;
        endif

A.18. KRB_CRED generation

        invoke KRB_TGS; /* obtain tickets to be provided to peer */

        /* assemble packet: */
        packet.pvno := protocol version; /* 5 */
        packet.msg-type := message type; /* KRB_CRED */

        for (tickets[n] in tickets to be forwarded) do
                packet.tickets[n] = tickets[n].ticket;
        done

        packet.enc-part.etype := encryption type;

        for (ticket[n] in tickets to be forwarded) do
                body.ticket-info[n].key = tickets[n].session;
                body.ticket-info[n].prealm = tickets[n].crealm;
                body.ticket-info[n].pname = tickets[n].cname;
                body.ticket-info[n].flags = tickets[n].flags;
                body.ticket-info[n].authtime = tickets[n].authtime;
                body.ticket-info[n].starttime = tickets[n].starttime;
                body.ticket-info[n].endtime = tickets[n].endtime;
                body.ticket-info[n].renew-till = tickets[n].renew-till;
                body.ticket-info[n].srealm = tickets[n].srealm;
                body.ticket-info[n].sname = tickets[n].sname;
                body.ticket-info[n].caddr = tickets[n].caddr;
        done

        get system_time;
        body.timestamp, body.usec := system_time;

        if (using nonce) then
                body.nonce := nonce;
        endif

        if (using s-address) then
                body.s-address := sender host addresses;
        endif
        if (limited recipients) then
                body.r-address := recipient host address;
        endif

        encode body into OCTET STRING;

        select encryption type;
        encrypt OCTET STRING into packet.enc-part.cipher
               using negotiated encryption key;

A.19. KRB_CRED verification

        receive packet;

draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

        if (packet.pvno != 5) then
                either process using other protocol spec
                or error_out(KRB_AP_ERR_BADVERSION);
        endif
        if (packet.msg-type != KRB_CRED) then
                error_out(KRB_AP_ERR_MSG_TYPE);
        endif

        cleartext := decrypt(packet.enc-part) using negotiated key;
        if (decryption_error()) then
                error_out(KRB_AP_ERR_BAD_INTEGRITY);
        endif
        if ((packet.r-address is present or required) and
           (packet.s-address != O/S_sender(packet)) then
                /* O/S report of sender not who claims to have sent it */
                error_out(KRB_AP_ERR_BADADDR);
        endif
        if ((packet.r-address is present) and
            (packet.r-address != local_host_address)) then
                /* was not sent to proper place */
                error_out(KRB_AP_ERR_BADADDR);
        endif
        if (not in_clock_skew(packet.timestamp,packet.usec)) then
                error_out(KRB_AP_ERR_SKEW);
        endif
        if (repeated(packet.timestamp,packet.usec,packet.s-address)) then
                error_out(KRB_AP_ERR_REPEAT);
        endif
        if (packet.nonce is required or present) and
           (packet.nonce != expected-nonce) then
                error_out(KRB_AP_ERR_MODIFIED);
        endif

        for (ticket[n] in tickets that were forwarded) do
                save_for_later(ticket[n],key[n],principal[n],
                               server[n],times[n],flags[n]);
        return

A.20. KRB_ERROR generation

        /* assemble packet: */
        packet.pvno := protocol version; /* 5 */
        packet.msg-type := message type; /* KRB_ERROR */

        get system_time;
        packet.stime, packet.susec := system_time;
        packet.realm, packet.sname := server name;

        if (client time available) then
                packet.ctime, packet.cusec := client_time;
        endif
        packet.error-code := error code;
        if (client name available) then
                packet.cname, packet.crealm := client name;
        endif
        if (error text available) then
                packet.e-text := error text;
        endif
        if (error data available) then
                packet.e-data := error data;
        endif


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

B. Definition of common authorization data elements

This appendix contains the definitions of common authorization data
elements. These common authorization data elements are recursivly defined,
meaning the ad-data for these types will itself contain a sequence of
authorization data whose interpretation is affected by the encapsulating
element. Depending on the meaning of the encapsulating element, the
encapsulated elements may be ignored, might be interpreted as issued
directly by the KDC, or they might be stored in a separate plaintext part of
the ticket. The types of the encapsulating elements are specified as part of
the Kerberos specification because the behavior based on these values should
be understood across implementations whereas other elements need only be
understood by the applications which they affect.

In the definitions that follow, the value of the ad-type for the element
will be specified in the subsection number, and the value of the ad-data
will be as shown in the ASN.1 structure that follows the subsection heading.

B.1. If relevant

AD-IF-RELEVANT   AuthorizationData

AD elements encapsulated within the if-relevant element are intended for
interpretation only by application servers that understand the particular
ad-type of the embedded element. Application servers that do not understand
the type of an element embedded within the if-relevant element may ignore
the uninterpretable element. This element promotes interoperability across
implementations which may have local extensions for authorization.

B.2. Intended for server

AD-INTENDED-FOR-SERVER   SEQUENCE {
         intended-server[0]     SEQUENCE OF PrincipalName
         elements[1]            AuthorizationData
}

AD elements encapsulated within the intended-for-server element may be
ignored if the application server is not in the list of principal names of
intended servers. Further, a KDC issuing a ticket for an application server
can remove this element if the application server is not in the list of
intended servers.

Application servers should check for their principal name in the
intended-server field of this element. If their principal name is not found,
this element should be ignored. If found, then the encapsulated elements
should be evaluated in the same manner as if they were present in the top
level authorization data field. Applications and application servers that do
not implement this element should reject tickets that contain authorization
data elements of this type.

B.3. Intended for application class

AD-INTENDED-FOR-APPLICATION-CLASS SEQUENCE { intended-application-class[0]
SEQUENCE OF GeneralString elements[1] AuthorizationData } AD elements
encapsulated within the intended-for-application-class element may be
ignored if the application server is not in one of the named classes of
application servers. Examples of application server classes include
"FILESYSTEM", and other kinds of servers.

This element and the elements it encapulates may be safely ignored by
applications, application servers, and KDCs that do not implement this
element.


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

B.4. KDC Issued

AD-KDCIssued   SEQUENCE {
               ad-checksum[0]    Checksum,
                i-realm[1]       Realm OPTIONAL,
                i-sname[2]       PrincipalName OPTIONAL,
               elements[3]       AuthorizationData.
}

ad-checksum
     A checksum over the elements field using a cryptographic checksum
     method that is identical to the checksum used to protect the ticket
     itself (i.e. using the same hash function and the same encryption
     algorithm used to encrypt the ticket) and using a key derived from the
     same key used to protect the ticket.
i-realm, i-sname
     The name of the issuing principal if different from the KDC itself.
     This field would be used when the KDC can verify the authenticity of
     elements signed by the issuing principal and it allows this KDC to
     notify the application server of the validity of those elements.
elements
     A sequence of authorization data elements issued by the KDC.

The KDC-issued ad-data field is intended to provide a means for Kerberos
principal credentials to embed within themselves privilege attributes and
other mechanisms for positive authorization, amplifying the priveleges of
the principal beyond what can be done using a credentials without such an
a-data element.

This can not be provided without this element because the definition of the
authorization-data field allows elements to be added at will by the bearer
of a TGT at the time that they request service tickets and elements may also
be added to a delegated ticket by inclusion in the authenticator.

For KDC-issued elements this is prevented because the elements are signed by
the KDC by including a checksum encrypted using the server's key (the same
key used to encrypt the ticket - or a key derived from that key). Elements
encapsulated with in the KDC-issued element will be ignored by the
application server if this "signature" is not present. Further, elements
encapsulated within this element from a ticket granting ticket may be
interpreted by the KDC, and used as a basis according to policy for
including new signed elements within derivative tickets, but they will not
be copied to a derivative ticket directly. If they are copied directly to a
derivative ticket by a KDC that is not aware of this element, the signature
will not be correct for the application ticket elements, and the field will
be ignored by the application server.

This element and the elements it encapulates may be safely ignored by
applications, application servers, and KDCs that do not implement this
element.


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

B.5. And-Or

AD-AND-OR           SEQUENCE {
                        condition-count[0]    INTEGER,
                        elements[1]           AuthorizationData
}

When restrictive AD elements encapsulated within the and-or element are
encountered, only the number specified in condition-count of the
encapsulated conditions must be met in order to satisfy this element. This
element may be used to implement an "or" operation by setting the
condition-count field to 1, and it may specify an "and" operation by setting
the condition count to the number of embedded elements. Application servers
that do not implement this element must reject tickets that contain
authorization data elements of this type.

B.6. Mandatory ticket extensions

AD-Mandatory-Ticket-Extensions           SEQUENCE {
                        te-type[0]       INTEGER,
                        te-checksum[0]    Checksum
}

An authorization data element of type mandatory-ticket-extensions specifies
the type and a collision-proof checksum using the same hash algorithm used
to protect the integrity of the ticket itself. This checksum will be
calculated over an individual extension field of the type indicated. If
there are more than one extension, multiple Mandatory-Ticket-Extensions
authorization data elements may be present, each with a checksum for a
different extension field. This restriction indicates that the ticket should
not be accepted if a ticket extension is not present in the ticket for which
the type and checksum do not match that checksum specified in the
authorization data element. Note that although the type is redundant for the
purposes of the comparison, it makes the comparison easier when multiple
extensions are present. Application servers that do not implement this
element must reject tickets that contain authorization data elements of this
type.

B.7. Authorization Data in ticket extensions

AD-IN-Ticket-Extensions   Checksum

An authorization data element of type in-ticket-extensions specifies a
collision-proof checksum using the same hash algorithm used to protect the
integrity of the ticket itself. This checksum is calculated over a separate
external AuthorizationData field carried in the ticket extensions.
Application servers that do not implement this element must reject tickets
that contain authorization data elements of this type. Application servers
that do implement this element will search the ticket extensions for
authorization data fields, calculate the specified checksum over each
authorization data field and look for one matching the checksum in this
in-ticket-extensions element. If not found, then the ticket must be
rejected. If found, the corresponding authorization data elements will be
interpreted in the same manner as if they were contained in the top level
authorization data field.

Note that if multiple external authorization data fields are present in a
ticket, each will have a corresponding element of type in-ticket-extensions
in the top level authorization data field, and the external entries will be
linked to the corresponding element by their checksums.


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

C. Definition of common ticket extensions

This appendix contains the definitions of common ticket extensions. Support
for these extensions is optional. However, certain extensions have
associated authorization data elements that may require rejection of a
ticket containing an extension by application servers that do not implement
the particular extension. Other extensions have been defined beyond those
described in this specification. Such extensions are described elswhere and
for some of those extensions the reserved number may be found in the list of
constants.

It is known that older versions of Kerberos did not support this field, and
that some clients will strip this field from a ticket when they parse and
then reassemble a ticket as it is passed to the application servers. The
presence of the extension will not break such clients, but any functionaly
dependent on the extensions will not work when such tickets are handled by
old clients. In such situations, some implementation may use alternate
methods to transmit the information in the extensions field.

C.1. Null ticket extension

TE-NullExtension   OctetString -- The empty Octet String

The te-data field in the null ticket extension is an octet string of lenght
zero. This extension may be included in a ticket granting ticket so that the
KDC can determine on presentation of the ticket granting ticket whether the
client software will strip the extensions field. =20

C.2. External Authorization Data

TE-ExternalAuthorizationData   AuthorizationData

The te-data field in the external authorization data ticket extension is
field of type AuthorizationData containing one or more authorization data
elements. If present, a corresponding authorization data element will be
present in the primary authorization data for the ticket and that element
will contain a checksum of the external authorization data ticket extension.

D. Significant changes since RFC 1510

Section 1: The preamble and introduction does not define the protocol,
mention is made in the introduction regarding the ability to rely on the KDC
to check the transited field, and on the inclusion of a flag in a ticket
indicating that this check has occurred. This is a new capability not
present in RFC1510. Pre-existing implementation may ignore or not set this
flag without negative security implications.

The definition of the secret key says that in the case of a user the key may
be derived from a password. In 1510, it said that the key was derived from
the password. This change was made to accommodate situations where the user
key might be stored on a smart-card, or otherwise obtained independent of a
password.

The introduction also mentions the use of public key for initial
authentication in Kerberos by reference. RFC1510 did not include such a
reference.

Section 1.2 was added to explain that while Kerberos provides authentication
of a named principal, it is still the responsibility of the application to
ensure that the authenticated name is the entity with which the application
wishes to communicate.


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

Section 2: No changes were made to existing options and flags specified in
RFC1510, though text was revised to make the description and intent of
existing options clearer, especially with respect to the ENC-TKT-IN-SKEY
option (now section 2.9.3) which is used for user-to-user authentication.
New options and ticket flags added since RFC1510 include transited policy
checking (section 2.7), anonymous tickets (section 2.8) and name
canonicalization (section since dropped, but flags remain).

Section 3: Added mention of the optional checksum field in the KRB-ERROR
message. Added mention of name canonicalization (since deleted) and
anonymous tickets in exposition on KDC options. Mention of the name
canonicalization case was included in the description of the KDC reply
(3.1.3). A warning regarding generation of session keys for application use
was added, urging the inclusion of key entropy from the KDC generated
session key in the ticket. An example regarding use of the subsession key
was added to section 3.2.6. Descriptions of the pa-etype-info, and
pa-pw-salt preauthentication data items were added.

Changes to section 4: Added language about who has access to the keys in the
Kerberos database. Also made it clear that KDC's may obtain the information
from some database field through other means - for example, one form of
pkinit may extract some of these fields from a certificate.

Section 5: The message specification section has undergone a major rewrite
to eliminate confusion regarding different versions of ASN.1, and to
highlight those areas where the Kerberos protocol does not strictly follow
ASN.1. These are changes to make the description more clear, rather than
changes to the protocol.

Major changes were made to message numbers, providing a clearer
interoperability path for messages that have new optional fields. The
specific additions to these messages are listed specifically elswhere in
this document.

A statement regarding the carrying of unrecognized additional fields in
ASN.1 encoding through in tickets was added (still waiting on some better
text regarding this).

Ticket flags and KDC options were added to support the new functions
described elsewhere in this document. The encoding of the options flags are
now described to be no less than 32 bits, and the smallest number of bits
beyond 32 needed to encode any set bits. It also describes the encoding of
the bitstring as using "unnamed" bits.

An optional ticket extensions field was added to support the carrying of
auxiliary data that allows the passing of auxiliary that is to accompany a
ticket to the verifier.

(Still pending, Tom Yu's request to change the application codes on KDC
message to indicate which minor rev of the protocol - I think this might
break things, but am not sure).

Definition of the PA-USE-SPECIFIED-KVNO preauthentication data field was
added.

The optional e-cksum field was added to the KRB-ERROR message and the e-data
filed was generalized for use in other than the KDC_ERR_PREAUTH_REQUIRED
error. The TypedData structure was defined. Type tags for TypedData are
defined in the same sequence as the PA-DATA type space to avoid confusion
with the use of the PA-DATA namespace previously used for the e-data field
for the KDC_ERR_PREAUTH_REQUIRED error.


draft-ietf-cat-kerberos-revisions-10                     Expires 20 May 2002\f

Section 6: Section 6 has undergone a major rewrite to more redily convey how
to add new encryption and checksum methods to Kerberos. New encryption
methods were added. Existing methods that are in use have not changed, but
their descriptions have to yield bettwer symmetry with the discussions of
the new methods.

Section 7: Words were added describing the convention that domain based
realm names for newly created realms should be specified as upper case. This
recommendation does not make lower case realm names illegal. Words were
added highlighting that the slash separated components in the X500 style of
realm names is consistent with existing RFC1510 based implementations, but
that it conflicts with the general recommendation of X.500 name
representation specified in RFC2253.

There were suggestions on the list regarding extensions to or new name
types. These require discussion at the IETF meeting. My own feeling at this
point is that in the absence of a strong consensus for adding new types at
this time, I would rather not add new name types in the current draft, but
leave things open for additions later.

Section 8: Since RFC1510, the definition of the TCP transport for Kerberos
messages was added.

Section 9: Requirements for supporting DES3-CBC-SHA1-KD encryption and
HMAC-SHA1-DES3-KD checksums were added.

I would like to make support for Rijndael mandatory and for us to have a
SINGLE standard for use of Rijndale in these revisions.

Y. Deprecated checksum types

     Work is still needed on this section.

        * More detailed list of differences from RFC 1510, to update
          the "Significant changes" appendix.
        * Compare specs against actual implementations (e.g., choice of
          ivec) and make sure they match.

     See end notes for other issues.

     -- Ken 2001-11-20

Give me an appendix letter?

This section describes some checksum mechanisms defined in RFC 1510 but
considered deprecated in this specification. While the authors believe the
mechanisms currently in common use have all been included in section 6,
these deprecated mechanisms may still be available in older implementations,
so we include them here, modified to fit the framework outlined in 6. They
are not required for an implementation to be conformant to this
specification.


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Y.1. The CRC-32 Checksum (crc32)

The CRC-32 checksum calculates a checksum based on a cyclic redundancy check
as described in ISO 3309 [14]. The resulting checksum is four (4) octets in
length. The CRC-32 is neither keyed nor collision-proof. The use of this
checksum is not recommended. An attacker using a probabilistic
chosen-plaintext attack as described in [13] might be able to generate an
alternative message that satisfies the checksum. The use of collision-proof
checksums is recommended for environments where such attacks represent a
significant threat.
 associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
 get_mic                 crc32(msg)

 verify_mic              compute checksum and compare
The crc32 checksum algorithm is assigned a checksum type number of one (1).

Y.2. The RSA MD4 Checksum (rsa-md4)

The RSA-MD4 checksum calculates a checksum using the RSA MD4 algorithm [15].
The algorithm takes as input an input message of arbitrary length and
produces as output a 128-bit (16 octet) checksum. RSA-MD4 is believed to be
collision-proof.
 associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
 get_mic                 md4(msg)

 verify_mic              compute checksum and compare
The rsa-md4 checksum algorithm is assigned a checksum type number of two
(2).

Y.3. DES cipher-block chained checksum (des-mac)

The DES-MAC checksum is computed by prepending an 8 octet confounder to the
plaintext, performing a DES CBC-mode encryption on the result using the key
and an initialization vector of zero, taking the last block of the
ciphertext, prepending the same confounder and encrypting the pair using DES
in cipher-block-chaining (CBC) mode using a a variant of the key, where the
variant is computed by eXclusive-ORing the key with the constant
F0F0F0F0F0F0F0F0. The initialization vector should be zero. The resulting
checksum is 128 bits (16 octets) long, 64 bits of which are redundant. This
checksum is tamper-proof and collision-proof. The DES specifications
identify some "weak" and "semiweak" keys; those keys shall not be used for
generating DES-MAC checksums for use in Kerberos, nor shall a key be used
whose veriant is "weak" or "semi-weak".
 associated    des-cbc-md5, des-cbc-md4, des-cbc-crc
 cryptosystem
 get_mic       des-cbc(key XOR F0F0F0F0F0F0F0F0,
                       confounder | des-mac(key, confounder | msg, ivec=0),
                       ivec=0)

 verify_mic    decrypt, compute DES MAC using confounder, compare
The des-mac checksum algorithm is assigned a checksum type number of four
(4).


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Y.4. RSA MD4 Cryptographic Checksum Using DES alternative (rsa-md4-des-k)

The RSA-MD4-DES-K checksum calculates a keyed collision-proof checksum by
applying the RSA MD4 checksum algorithm and encrypting the results using DES
in cipherblock-chaining (CBC) mode using a DES key as both key and
initialization vector. The resulting checksum is 16 octets long. This
checksum is tamper-proof and believed to be collision-proof. Note that this
checksum type is the old method for encoding the RSA-MD4-DES checksum and it
is no longer recommended.
 associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
 get_mic                 des-cbc(key, md4(msg), ivec=key)

 verify_mic              compute CRC-32 and compare
The rsa-md4-des-k checksum algorithm is assigned a checksum type number of
six (6).

Y.5. DES cipher-block chained checksum alternative (des-mac-k)

The DES-MAC-K checksum is computed by performing a DES CBC-mode encryption
of the plaintext, and using the last block of the ciphertext as the checksum
value. It is keyed with an encryption key and an initialization vector; any
uses which do not specify an additional initialization vector will use the
key as both key and initialization vector. The resulting checksum is 64 bits
(8 octets) long. This checksum is tamper-proof and collision-proof. Note
that this checksum type is the old method for encoding the DESMAC checksum
and it is no longer recommended. The DES specifications identify some "weak
keys"; those keys shall not be used for generating DES-MAC checksums for use
in Kerberos.
 associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
 get_mic                 des-mac(key, msg, ivec=key or given)

 verify_mic              compute MAC and compare
The des-mac-k checksum algorithm is assigned a checksum type number of five
(5).


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Appendix Z. Test Vectors

This section provides test vectors for various functions defined or
described in section 6. For convenience, most inputs are ASCII strings,
though some UTF-8 samples should be provided for string-to-key functions.
Keys and other binary data are specified as hexadecimal strings.

Z.1. n-fold

The n-fold function is defined in section 6.4. As noted there, the sample
vector in the original paper defining the algorithm appears to be incorrect.
Here are values provided by Marc Horowitz:

   64-fold("012345") =
   64-fold(303132333435) = be072631276b1955

   56-fold("password") =
   56-fold(70617373776f7264) = 78a07b6caf85fa

   64-fold("Rough Consensus, and Running Code") =
   64-fold(526f75676820436f6e73656e7375732c20616e642052756e
           6e696e6720436f6465) = bb6ed30870b7f0e0

   168-fold("password") =
   168-fold(70617373776f7264) =
            59e4a8ca7c0385c3c37b3f6d2000247cb6e6bd5b3e

   192-fold("MASSACHVSETTS INSTITVTE OF TECHNOLOGY"
   192-fold(4d41535341434856534554545320494e5354495456544520
            4f4620544543484e4f4c4f4759) =
            db3b0d8f0b061e603282b308a50841229ad798fab9540c1b

Z.2. mit_des_string_to_key

The function mit_des_string_to_key is defined in section 6.5.2. We present
here several test values, with some of the intermediate results. The fourth
test demonstrates the use of UTF-8 with three characters. The last two tests
are specifically constructed so as to trigger the weak-key fixups for the
intermediate key produced by fan-folding; we have no test cases that cause
such fixups for the final key.

UTF-8 encodings:
 eszett  C3 9F
 s-caron C5 A1
 c-acute C4 87

Test vector:

salt:     "ATHENA.MIT.EDUraeburn"  415448454e412e4d49542e4544557261656275726e
password: "password"               70617373776f7264
fan-fold result:                   c01e38688ac86c2e
intermediate key:                  c11f38688ac86d2f
DES key:                           cbc22fae235298e3

salt:     "WHITEHOUSE.GOVdanny"    5748495445484f5553452e474f5664616e6e79
password: "potatoe"                706f7461746f65
fan-fold result:                   a028944ee63c0416
intermediate key:                  a129944fe63d0416
DES key:                           df3d32a74fd92a01


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salt:     "EXAMPLE.COMbuckaroo"    4558414d504c452e434f4d6275636b61726f6f
password: "penny"                  70656e6e79
fan-fold result:                   96d2d87e925c64ee
intermediate key:                  97d3d97f925d64ef
DES key:                           9443a2e532fdc4f1

salt:     "ATHENA.MIT.EDUJuri" + s-caron + "i" + c-acute
                           415448454e412e4d49542e4544554a757269c5a169c487
password: eszett                   c39f
fan-fold result:                   b8f6c40e305afc9e
intermediate key:                  b9f7c40e315bfd9e
DES key:                           62c81a5232b5e69d

salt:     "AAAAAAAA"               4141414141414141
password: "11119999"               3131313139393939
fan-fold result:                   e0e0e0e0f0f0f0f0
intermediate key:                  e0e0e0e0f1f1f101
DES key:                           984054d0f1a73e31

salt:     "FFFFAAAA"               4646464641414141
password: "NNNN6666"               4e4e4e4e36363636
fan-fold result:                   1e1e1e1e0e0e0e0e
intermediate key:                  1f1f1f1f0e0e0efe
DES key:                           c4bf6b25adf7a4f8

Z.3. DES3 DR and DK

These tests show the derived-random and derived-key values for the
des3-hmac-sha1-kd encryption scheme, using the DR and DK functions defined
in section 6.5.5. The input keys were randomly generated; the usage values
are ones actually used by Kerberos.

key:    dce06b1f64c857a11c3db57c51899b2cc1791008ce973b92
usage:  0000000155
DR:     935079d14490a75c3093c4a6e8c3b049c71e6ee705
DK:     925179d04591a79b5d3192c4a7e9c289b049c71f6ee604cd

key:    5e13d31c70ef765746578531cb51c15bf11ca82c97cee9f2
usage:  00000001aa
DR:     9f58e5a047d894101c469845d67ae3c5249ed812f2
DK:     9e58e5a146d9942a101c469845d67a20e3c4259ed913f207

key:    98e6fd8a04a4b6859b75a176540b9752bad3ecd610a252bc
usage:  0000000155
DR:     12fff90c773f956d13fc2ca0d0840349dbd39908eb
DK:     13fef80d763e94ec6d13fd2ca1d085070249dad39808eabf

key:    622aec25a2fe2cad7094680b7c64940280084c1a7cec92b5
usage:  00000001aa
DR:     f8debf05b097e7dc0603686aca35d91fd9a5516a70
DK:     f8dfbf04b097e6d9dc0702686bcb3489d91fd9a4516b703e

key:    d3f8298ccb166438dcb9b93ee5a7629286a491f838f802fb
usage:  6b65726265726f73
DR:     2270db565d2a3d64cfbfdc5305d4f778a6de42d9da
DK:     2370da575d2a3da864cebfdc5204d56df779a7df43d9da43

key:    b55e983467e551b3e5d0e5b6c80d45769423a873dc62b30e
usage:  636f6d62696e65
DR:     0127398bacc81a2a62bc45f8d4c151bbcdd5cb788a
DK:     0126388aadc81a1f2a62bc45f8d5c19151bacdd5cb798a3e


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key:    c1081649ada74362e6a1459d01dfd30d67c2234c940704da
usage:  0000000155
DR:     348056ec98fcc517171d2b4d7a9493af482d999175
DK:     348057ec98fdc48016161c2a4c7a943e92ae492c989175f7

key:    5d154af238f46713155719d55e2f1f790dd661f279a7917c
usage:  00000001aa
DR:     a8818bc367dadacbe9a6c84627fb60c294b01215e5
DK:     a8808ac267dada3dcbe9a7c84626fbc761c294b01315e5c1

key:    798562e049852f57dc8c343ba17f2ca1d97394efc8adc443
usage:  0000000155
DR:     c813f88b3be2b2f75424ce9175fbc8483b88c8713a
DK:     c813f88a3be3b334f75425ce9175fbe3c8493b89c8703b49

key:    26dce334b545292f2feab9a8701a89a4b99eb9942cecd016
usage:  00000001aa
DR:     f58efc6f83f93e55e695fd252cf8fe59f7d5ba37ec
DK:     f48ffd6e83f83e7354e694fd252cf83bfe58f7d5ba37ec5d

Z.4. DES3string_to_key

These are the keys generated for some of the above input strings for
triple-DES with key derivation as defined in section 6.5.5.

salt:   "ATHENA.MIT.EDUraeburn"
passwd: "password"
key:    850bb51358548cd05e86768c313e3bfef7511937dcf72c3e

salt:   "WHITEHOUSE.GOVdanny"
passwd: "potatoe"
key:    dfcd233dd0a43204ea6dc437fb15e061b02979c1f74f377a

salt:   "EXAMPLE.COMbuckaroo"
passwd: "penny"
key:    6d2fcdf2d6fbbc3ddcadb5da5710a23489b0d3b69d5d9d4a

salt:   "ATHENA.MIT.EDUJuri" + s-caron + "i" + c-acute
passwd: eszett
key:    16d5a40e1ce3bacb61b9dce00470324c831973a7b952feb0

Z.5. DES3 combine-keys

PLACEHOLDER FOR NEW DATA


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  ------------------------------------------------------------------------

Footnotes:

TM
     Project Athena, Athena, and Kerberos are trademarks of the
     Massachusetts Institute of Technology (MIT). No commercial use of these
     trademarks may be made without prior written permission of MIT.

1.1
     Note, however, that many applications use Kerberos' functions only upon
     the initiation of a stream-based network connection. Unless an
     application subsequently provides integrity protection for the data
     stream, the identity verification applies only to the initiation of the
     connection, and does not guarantee that subsequent messages on the
     connection originate from the same principal.

1.2
     Secret and private are often used interchangeably in the literature. In
     our usage, it takes two (or more) to share a secret, thus a shared DES
     key is a secret key. Something is only private when no one but its
     owner knows it. Thus, in public key cryptosystems, one has a public and
     a private key.

1.3
     Of course, with appropriate permission the client could arrange
     registration of a separately-named principal in a remote realm, and
     engage in normal exchanges with that realm's services. However, for
     even small numbers of clients this becomes cumbersome, and more
     automatic methods as described here are necessary.

2.1
     Though it is permissible to request or issue tick- ets with no network
     addresses specified.

2.2
     It is important that the KDC be sent the name as typed by the user, and
     not only the canonical form of the name. If the domain name system was
     used to find the canonical name on the client side, the mapping is
     vulnerable.
3.1
     The password-changing request must not be honored unless the requester
     can provide the old password (the user's current secret key).
     Otherwise, it would be possible for someone to walk up to an unattended
     session and change another user's password.

3.2
     To authenticate a user logging on to a local system, the credentials
     obtained in the AS exchange may first be used in a TGS exchange to
     obtain credentials for a local server. Those credentials must then be
     verified by a local server through successful completion of the
     Client/Server exchange.

3.3
     "Random" means that, among other things, it should be impossible to
     guess the next session key based on knowledge of past session keys.
     This can only be achieved in a pseudo-random number generator if it is
     based on cryptographic principles. It is more desirable to use a truly
     random number generator, such as one based on measurements of random
     physical phenomena.


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3.4
     Tickets contain both an encrypted and unencrypted portion, so cleartext
     here refers to the entire unit, which can be copied from one message
     and replayed in another without any cryptographic skill.

3.5
     Note that this can make applications based on unreliable transports
     difficult to code correctly. If the transport might deliver duplicated
     messages, either a new authenticator must be generated for each retry,
     or the application server must match requests and replies and replay
     the first reply in response to a detected duplicate.

3.6
     This allows easy implementation of user-to-user authentication [8],
     which uses ticket-granting ticket session keys in lieu of secret server
     keys in situations where such secret keys could be easily compromised.

3.7
Note also that the rejection here is restricted to authenticators from the
same principal to the same server. Other client principals communicating
with the same server principal should not be have their authenticators
rejected if the time and microsecond fields happen to match some other
client's authenticator.

3.8
     If this is not done, an attacker could subvert the authentication by
     recording the ticket and authenticator sent over the network to a
     server and replaying them following an event that caused the server to
     lose track of recently seen authenticators.

3.9
     In the Kerberos version 4 protocol, the timestamp in the reply was the
     client's timestamp plus one. This is not necessary in version 5 because
     version 5 messages are formatted in such a way that it is not possible
     to create the reply by judicious message surgery (even in encrypted
     form) without knowledge of the appropriate encryption keys.

3.10
     Note that for encrypting the KRB_AP_REP message, the sub-session key is
     not used, even if present in the Authenticator.

3.11
     Implementations of the protocol may wish to provide routines to choose
     subkeys based on session keys and random numbers and to generate a
     negotiated key to be returned in the KRB_AP_REP message.

3.12
This can be accomplished in several ways. It might be known beforehand
(since the realm is part of the principal identifier), it might be stored in
a nameserver, or it might be obtained from a configuration file. If the
realm to be used is obtained from a nameserver, there is a danger of being
spoofed if the nameservice providing the realm name is not authenticated.
This might result in the use of a realm which has been compromised, and
would result in an attacker's ability to compromise the authentication of
the application server to the client.

3.13
     If the client selects a sub-session key, care must be taken to ensure
     the randomness of the selected sub-session key. One approach would be
     to generate a random number and XOR it with the session key from the
     ticket-granting ticket.


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4.1
     The implementation of the Kerberos server need not combine the database
     and the server on the same machine; it is feasible to store the
     principal database in, say, a network name service, as long as the
     entries stored therein are protected from disclosure to and
     modification by unauthorized parties. However, we recommend against
     such strategies, as they can make system management and threat analysis
     quite complex.

4.2
     See the discussion of the padata field in section 5.4.2 for details on
     why this can be useful.

6.1
     While Message Authentication Code (MAC) or Message Integrity Check
     (MIC) would be more appropriate terms for many of the uses in this
     section, we continue to use the term "checksum" for historical reasons.
6.2
     For example, a pseudo-random number generator may be seeded with a
     session key, but to protect the original key from any accidental
     weakness in the PRNG, use possibly-known data encrypted or checksummed
     using the key rather than using the key directly. Usage numbers in this
     reserved range should help avoid accidentally seeding the PRNG with a
     value also computed and perhaps exposed to an attacker elsewhere.
6.3
     Of course, this does not apply to protocols that do their own
     encryption independent of this framework, directly using the key
     resulting from the Kerberos authentication exchange.
6.4
     It should be noted that the sample vector in Appendix B.2 of the
     original paper appears to be incorrect. Two independent implementations
     from the specification (one in C by Marc Horowitz, and another in
     Scheme by Bill Sommerfeld) agree on a value different from that in
     [Blumenthal96].
6.5
     Some problematic assumptions we've seen, and sometimes made, include:
     that a random bitstring is always valid as a key (not true for DES keys
     with parity); that the basic block encryption chaining mode provides no
     integrity checking, or can easily be separated from such checking (not
     true for many modes in development that do both simultaneously); that a
     checksum for a message always results in the same value (not true if a
     confounder is incorporated); that an initial vector is used (may not be
     true if a block cipher in CBC mode is not in use); that the key is a
     clever thing to use as the initial vector for CBC mode encryption (not
     true @@REF Bellovin paper).
6.6
     Perhaps one of the more common reasons for directly performing
     encryption is direct control over the negotiation and to select a
     "sufficiently strong" encryption algorithm (whatever that means in the
     context of a given application). While Kerberos directly provides no
     facility for negotiating encryption types between the application
     client and server, there are other means for accomplishing similar
     goals. For example, requesting only "strong" session key types from the
     KDC, and assuming that the type actually returned by the KDC will be
     understood and supported by the application server.