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[gsasl.git] / doc / specification / draft-josefsson-sasl-kerberos5-00.txt

Network Working Group                                       S. Josefsson
Internet-Draft                                             November 2002
Expires: May 2, 2003


                      A Kerberos 5 SASL Mechanism
                   draft-josefsson-sasl-kerberos5-00

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

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   This Internet-Draft will expire on May 2, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document specifies a Simple Authentication and Security Layer
   (SASL) [3] mechanism for Kerberos 5 Client/Server Authentication [1],
   with optional initial Authentication Service (AS) and/or
   Ticket-Granting-Service (TGS) exchanges.











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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Kerberos version 5 mechanism . . . . . . . . . . . . . . . . .  3
   3.  Theory of operation  . . . . . . . . . . . . . . . . . . . . .  5
   3.1 Separate SASL and Kerberos 5 server  . . . . . . . . . . . . .  5
   3.2 Combined SASL and Kerberos 5 server  . . . . . . . . . . . . .  6
   4.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
       Normative References . . . . . . . . . . . . . . . . . . . . .  9
       Informative References . . . . . . . . . . . . . . . . . . . .  9
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10






































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1. Introduction

   Kerberos 5 provides client and optional server authentication,
   usually employing symmetric encryption and a trusted (symmetric) key
   distribution center.  Specifying Kerberos 5 authentication for each
   network protocol where there is a need to use Kerberos 5 is a tedious
   task.  However, as many protocols already specify support for the
   SASL framework, by specifying a Kerberos 5 SASL mechanism, support
   for Kerberos 5 in many protocols is accomplished.  Even for protocols
   that do not support SASL, specifying SASL support (and thereby
   implicitely Kerberos 5) is often advantageous over specifying
   Kerberos 5 support directly.  The advantages include better
   flexibility if or when new Kerberos versions are released, and
   perhaps more commonly, when circumstances demand that other
   authentication methods (supported by SASL) should be used.

   It should be mentioned that Kerberos 5 authentication via SASL is
   already possible, by using the Generic Security Service Application
   Program Interface [6] framework.  However, GSSAPI adds some amount of
   overhead, both in terms of code complexity, code size and additional
   network round trips.  More importantly, GSSAPI do not support the
   authentication steps (AS and TGS).  These are some of the motivation
   behind this "slimmer" Kerberos 5 SASL mechanism.

2. Kerberos version 5 mechanism

   The mechanism name associated with Kerberos version 5 is
   "KERBEROS_V5".  The exchange consists of one initial server packet
   (containing some parameters and a challenge, described below), and
   then an unfixed number of messages containing Kerberos 5 packets,
   with the last exchange being an AP-REQ, and optional AP-REP, for the
   desired SASL service on a format described below.

   The normal packet exchange, after the required initial server packet,
   are one optional AS-REQ and AS-REP exchange, one optional TGS-REQ and
   TGS-REP exchange and then the AP-REQ packet and optional AP-REP
   reply.  The only steps that are required by this SASL mechanism is
   the initial server packet and the final AP-REQ and optional AP-REP
   exchange.  The AP-REP is sent when and only when mutual
   authentication is required.  The AP-REQ is for the SASL service that
   is requested.  The AP-REQ must contain authenticated application data
   on a format described below.  The AS and TGS exchanges is usually
   used by clients to aquire the proper tickets required for the AP
   phase.  It is not expected that any other Kerberos 5 packets will be
   exchanged, but this mechanism do not disallow such packets in order
   to make it possible to use this SASL mechanism with future Kerberos
   extensions.




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   As discussed above, the client is allowed to send any valid Kerberos
   5 message and the server should handle any message, subject to local
   policy restrictions.  If the server do not understand the meaning of
   a packet or do not wish to respond to it (e.g., it cannot proxy a
   TGS-REQ), it SHOULD respond with a empty response and await further
   packets.  Reasons for aborting the authentication phase instead of
   sending an empty response includes if the number of received packets
   exceeds a pre-defined limit.  AS-REQ and TGS-REQ packets destined for
   non-local Kerberos Key Distribution Centers (KDCs) is proxied to the
   correct server by the SASL server.  No provisions are made in the
   protocol to allow the client to specify the addresses of the KDCs,
   instead the SASL server is required to discover this information
   (usually by static configuration or by using DNS).  It is legitimate
   for the SASL server to abort the authentication phase with an error
   saying that the indicated realm was not found or is restricted by
   policy (i.e., a policy that only permits local KDC requests is
   permitted).

   Since it is expected that clients may not yet have IP addresses when
   they invoke this SASL mechanism (invoking this mechanism may be one
   step in aquiring an IP address), clients commonly leave the address
   fields in the AS-REQ empty.

   The initial server packet should contain one octet containing a bit
   mask of supported security layers, four octets indicating the maximum
   cipher-text buffer size the server is able to receive (or 0 if no
   security layers are supported) in network byte order, and then 16
   octets containing random data (see [4] on how random data might be
   generated).

   The last exchange must be an AP-REQ for the desired SASL service,
   optionaly followed by an AP-REP.  The SASL service is translated into
   a Kerberos principal and realm as follows: The first element of the
   principal is the service name specified in the protocol that uses
   SASL.  The second element is the address of the SASL server, usually
   expressed as a hostname.  The realm should be the Kerberos realm of
   the SASL server.  The checksum field in the Authenticator of the
   AP-REQ must be on a string where the first octet indicate the desired
   security layer requested by the client (a bitmask with one bit set,
   which must also be set in the server offered security layer bitmask),
   the next four octets indicate the maximum cipher-text buffer size the
   client is able to receive in network byte order (or 0 if a security
   layer is not indicated by the first octet), followed by the entire
   initial server packet, in turn followed by the desired authorization
   identity (which can be empty to indicate that the authorization
   identity should be the same as the authentication identity provided
   by the Kerberos ticket in the AP-REQ).




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   Upon decrypting and verifying the ticket and authenticator (i.e.,
   standard AP-REQ processing), the server verifies that the contained
   security layer bit mask, server maximum cipher-text buffer size, and
   the random data equals the data provided in the original server
   challenge.  The server also verify that the client selected security
   level matches one of the offered security levels.  Should the
   verification be successful, the server must verify that the principal
   identified in the Kerberos ticket is authorized to connect to the
   service as the authorization identity specified by the client (or, if
   absent, the Kerberos ticket principal).  Unless the client requested
   mutual authentication, the authentication process is complete.

   If the client requested mutual authentication, the server constructs
   an AP-REP according to Kerberos 5.  The checksum field in the
   Authenticator of the AP-REP must contain one octet with the desired
   security level (must be equal to what the client requested) followed
   by the application the client provided in the AP-REQ Authenticator
   checksum.

   The security layers and their corresponding bit-masks are as follows:

         Bit 0 No security layer
         Bit 1 Integrity (KRB-SAFE) protection
         Bit 2 Privacy (KRB-PRIV) protection
         Bit 3 Mutual authentication is required (AP option MUTUAL-
               REQUIRED must also be present).

   Other bit-masks may be defined in the future; bits which are not
   understood must be negotiated off.

3. Theory of operation

   This section describes, for illustrion only, two scenarios where this
   mechanism can be used.

3.1 Separate SASL and Kerberos 5 server

   Normally a SASL server is an application server such as a mail
   system.  The server is configured to belong to at least one Kerberos
   5 realm, and the server shares a symmetric key with the Kerberos 5
   Key Distribution Center in that realm.  The server cannot perform the
   initial Kerberos AS and TGS operation itself but rather acts as a
   proxy, and once the user acquired credentials the server it is able
   to carry out the AP-REQ/AP-REP phase using its symmetric key.  The
   steps are as follows:






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   0) Server sends initial token.

   1) Client constructs AS-REQ using username and realm supplied by user,
      in order to acquire a ticket granting ticket.

   2) Server finds address of KDC and forwards the AS-REQ to and waits
      for the AS-REP response which it forwards back to the client.

   3) Client parses AS-REP and constructs a TGS-REQ using the ticket
      granting ticket encryption key, in order to acquire a ticket for
      the server.

   4) Server finds address of KDC and forwards the TGS-REQ to and waits
      for the TGS-REP response which it forwards back to the client.

   5) Client parses TGS-REP and generates the AP-REQ using the session
      encryption key.

   4) Server parses AP-REQ and generates the AP-REP if requested.

   5) Client optionally parses AP-REP.


3.2 Combined SASL and Kerberos 5 server

   Kerberos 5 is usually a distributed security system, but we wish to
   point out that this Kerberos 5 SASL mechanism may be used in a
   standalone SASL server to provide the security advantages that the
   Kerberos 5 Authentication Service (AS) provides over other methods.

   Specifically, the SASL server may use a legacy password database such
   as a CRAM-MD5 clear text password file to generate Kerberos 5
   principals "on the fly".  Authentication may thus proceed as follows:

   0) Server sends initial token.

   1) Client constructs AS-REQ using username supplied by user, in order to
      acquire a ticket for the server directly.

   2) Server parses AS-REQ and generates AS-REP based on password in database.

   3) Client parses AS-REP and construct a ticket and the AP-REQ using
      the session encryption key.

   4) Server parses AP-REQ and generates the AP-REP if requested.

   5) Client optionally parses AP-REP.




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   This may be extended further, i.e.  by using the password and
   Kerberos 5 pre-authentication in step 1.

4. Example

   The following is one Kerberos version 5 login scenarios to the IMAP4
   protocol (note that the line breaks in the sample authenticators are
   for editorial clarity and are not in real authenticators).

        S: * OK IMAP4rev1 server
        C: . AUTHENTICATE KERBEROS_V5
        S: + AAAAAAB2ZXJ5IHJhbmRvbSBkYXRh
        C: aoGNMIGKoQMCAQWiAwIBCqMCMACkejB4oAcDBQAAAAAAoRAwDqADAgEBoQcwBRsD
           amFzog8bDUpPU0VGU1NPTi5PUkejIjAgoAMCAQGhGTAXGwZrcmJ0Z3QbDUpPU0VG
           U1NPTi5PUkelERgPMjAwMzAxMDgyMzAwMDBapwYCBEbYfGaoCzAJAgESAgEQAgED
        S: + a4ICITCCAh2gAwIBBaEDAgELow8bDUpPU0VGU1NPTi5PUkekEDAOoAMCAQGhBzAF
             GwNqYXOlggECYYH/MIH8oAMCAQWhDxsNSk9TRUZTU09OLk9SR6IiMCCgAwIBAaEZ
             MBcbBmtyYnRndBsNSk9TRUZTU09OLk9SR6OBvzCBvKADAgEQoQMCAQGiga8EgaxJ
             LzxzeR/yPCjnnac67Qk1aavvaLf3erjNFC0pDHOsWhe7aBFzsPeZsjwrpDizC/kF
             hi1Y9pQMlEDxbfs0vqSRo5LP0pnkAeX/euiqsnupwhG7eaQfcFPNqDJaAdKFa8li
             b0g4J7hf3QoTqnpIW/eYK/M+3I0E3GeOJGFrBVgumml7tDdCxjDd8OpjVh9Ejdq3
             3FaVN7c92If9izILr9Az9mADuyv6zF4QafFtpoHnMIHkoAMCARChAwIBBqKB1wSB
             1C4UxQSlQ7LlUap9QBJm/EwK8WyYXao+ScoccIXysZ2iNEVNyjE2Czv/F9rjUNNb
             60e2PMHHsZRUYIvIlgkq6dwtlWI1AW11saE5OsdGPNNl8nP5m1MlrH4iYB8f2mWt
             xmcEGSk4T9HREucyVl9DlhCyD/S+bRAjXO4u6GbMcFaVn+idonKgDUy+8TbgkICs
             e79I7e//sxdAxVduK3QFmSAXrnIts+lo8VFfnWqwZOoPdZiBxnPck35zy7jJejaU
             QysakfCgSniRfDzgW1C/UAo8QCWb
        C: boIBpTCCAaGgAwIBBaEDAgEOogcDBQAAAAAAo4IBFGGCARAwggEMoAMCAQWhDxsN
           Sk9TRUZTU09OLk9SR6IiMCCgAwIBAaEZMBcbBGltYXAbD3l4YS5leHR1bmRvLmNv
           baOBzzCBzKADAgEQoQMCAQGigb8Egbxn6oQtjVF1OlzSSm2TimVUM4xEcaHUeZ0Q
           TxqpVmmej0OhVXwtHKnrNY2v/+edOzNS8yojmmKaKN5cnUHnvLwgS/W0Xx55bhFW
           zo0I+x0kAaOef7HHf5XsfinYybp5qVaLihjJXK0IrbYkeZy/B6tpAfsIuKfIRVXL
           jI9DYt9a+sHVLpE+yHMiCxM//JWOnCJsnD2T5IeuHgjBf6D9DXVwzJTMJZRs3zVm
           Vo4NhAoqIDsFtdZ3ca+bcaBA2aR0MHKgAwIBEKEDAgEBomYEZC0aevz6IesKrIx+
           m8/qfYjYx/cho274VGzMrREKEg9lK8s1ajkeq4yrNQWxblwA3zx6Q2HzX0DHomhj
           6Lm1TnB8VKJziZmFocyG3nicOuORf1oqO+qLmr9S3yNQUseKBFOEfX8=
        S: . OK KERBEROS_V5 successful

   (XXX this is not a real example)

5. Security Considerations

   The authentication phase is belived to be no less secure than the
   Client/Server Authentication exchange described in the Kerberos 5
   protocol.

   If no security layer is negotiated, the connection is subject to
   active man-in-the-middle attackers that hijack the connection after



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   authentication has been completed.

   When security layers are used, it is believed that the communication
   channel negotiated by this specification is no less secure than the
   KRB_SAFE and KRB_PRIV primitives.  In other words, it is believed
   that if an attack that breaches integrity or privacy of this
   mechanism, the same attack also applies to the Kerberos 5
   specification, and vice versa.

   Server implementations should be aware that this proxying function
   can be abused, and MAY implement precaution against this if it is
   considered a threat.  Useful precautions include limiting the size of
   packets forwarded, and the number of packets, to abort the SASL
   exchange when the limit is reached.

   Server implementations should make sure the method to look up KDC for
   the client indicated realm does not cause security problems.  In
   particular, trusting unprotected DNS lookups to find the KDC of a
   realm may be considered as dangerous by a server.

   The forward-compatibility behaviour of returning empty responses to
   unsupported commands may be abused as a covert channel.

   The reason for the client to send, in the Authenticator checksum
   field, not only the server random number but the entire initial
   server packet with the security layer bitmask and maximum cipher-text
   buffer size accepted by server, is to prevent an attacker from
   downgrading the security layer ultimately selected.  The random
   number ties the client and server to the same network session,
   prevent man-in-the-middle attacks assuming a Kerberos 5 security
   layer is chosen and that the Kerberos 5 security layer is secure.

   The security considerations of Kerberos 5 and SASL are inherited.
   Some immediates consequences of this follows (this is an inconclusive
   summary):

   Note that some of the Kerberos 5 encryption types are considered
   weak, implementations must decide which algorithms are trusted.

   Note that Kerberos 5 do not authorize users, it only authenticate
   users.  Applications using this mechanism must thus perform checks,
   not described in detail in this document, to make sure the
   authenticated user is authorized to the service she is requesting.

   Note that the SASL framework is subject to "downgrade" attacks where
   an attacker forces a weaker SASL mechanism to be used.  The use of,
   e.g., TLS [5] can be used to mitigate this.




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Normative References

   [1]  Kohl, J. and B. Neuman, "The Kerberos Network Authentication
        Service (V5)", RFC 1510, September 1993.

   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [3]  Myers, J., "Simple Authentication and Security Layer (SASL)",
        RFC 2222, October 1997.

Informative References

   [4]  Eastlake, D., Crocker, S. and J. Schiller, "Randomness
        Recommendations for Security", RFC 1750, December 1994.

   [5]  Dierks, T., Allen, C., Treese, W., Karlton, P., Freier, A. and
        P. Kocher, "The TLS Protocol Version 1.0", RFC 2246, January
        1999.

   [6]  Linn, J., "Generic Security Service Application Program
        Interface Version 2, Update 1", RFC 2743, January 2000.


Author's Address

   Simon Josefsson
   Drottningholmsv. 70
   Stockholm  112 42
   Sweden

   EMail: simon@josefsson.org

Acknowledgements

   Text and ideas was borrowed from the Kerberos version 4 SASL
   mechanism in RFC 2222.














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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.











































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