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Network Working Group                                          Johansson
Internet-Draft                                      Stockholm university
Intended status: Standards Track                            July 8, 2007
Expires: January 9, 2008


              An information model for Kerberos version 5
                     draft-johansson-kerberos-model-03

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   Copyright (C) The IETF Trust (2007).














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Abstract

   This document describes an information model for Kerberos version 5
   from the point of view of an administrative service.  There is no
   standard for administrating a kerberos 5 KDC.  This document
   describes the services exposed by an administrative interface to a
   KDC.


Table of Contents

   1.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  How to interpret RFC2119 terms . . . . . . . . . . . . . . . .  5
   4.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Information model demarcation  . . . . . . . . . . . . . . . .  7
   6.  Information model specification  . . . . . . . . . . . . . . .  8
     6.1.  Principal  . . . . . . . . . . . . . . . . . . . . . . . .  8
       6.1.1.  Principal: Attributes  . . . . . . . . . . . . . . . .  8
       6.1.2.  Principal: Associations  . . . . . . . . . . . . . . .  8
       6.1.3.  Principal: Remarks . . . . . . . . . . . . . . . . . .  9
     6.2.  KeySet . . . . . . . . . . . . . . . . . . . . . . . . . .  9
       6.2.1.  KeySet: Attributes . . . . . . . . . . . . . . . . . .  9
       6.2.2.  KeySet: Associations . . . . . . . . . . . . . . . . .  9
       6.2.3.  KeySet: Remarks  . . . . . . . . . . . . . . . . . . .  9
     6.3.  Key  . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
       6.3.1.  Key: Attributes  . . . . . . . . . . . . . . . . . . . 10
       6.3.2.  Key: Associations  . . . . . . . . . . . . . . . . . . 10
       6.3.3.  Key: Remarks . . . . . . . . . . . . . . . . . . . . . 11
     6.4.  Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 11
       6.4.1.  Policy: Attributes . . . . . . . . . . . . . . . . . . 11
       6.4.2.  Password Quality Policy  . . . . . . . . . . . . . . . 11
       6.4.3.  Password Management Policy . . . . . . . . . . . . . . 12
       6.4.4.  Keying Policy  . . . . . . . . . . . . . . . . . . . . 12
   7.  Implementation Scenarios . . . . . . . . . . . . . . . . . . . 13
     7.1.  LDAP backend to KDC  . . . . . . . . . . . . . . . . . . . 13
     7.2.  LDAP frontend to KDC . . . . . . . . . . . . . . . . . . . 13
     7.3.  SOAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Remarks  . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 17
     11.2. Informative References . . . . . . . . . . . . . . . . . . 17
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19





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1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].














































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2.  Introduction

   The Kerberos version 5 authentication service described in [RFC4120]
   describes how a Key Distribution Service (KDC) provides
   authentication to clients.  The standard does not stipulate how a KDC
   is managed and several "kadmin" servers have evolved.  This document
   describes the services required to administrate a KDC and the
   underlying information model assumed by a kadmin-type service.

   The information model is written in terms of "attributes" and
   "services" or "interfaces" but the use of these particular words MUST
   NOT be taken to imply any particular modeling paradigm so that
   neither an object oriented model or an LDAP schema is intended.  The
   author has attempted to describe in natural language the intended
   semantics and syntax of the components of the model.  An LDAP schema
   (for instance) based on this model will be more precise in the
   expression of the syntax while preserving the semantics of this
   model.

   Implementations of this document MAY decide to change the names used
   (eg principalName).  If so an implementation MUST provide a name to
   name mapping to this document.





























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3.  How to interpret RFC2119 terms

   This document describes an information model for kerberos 5 but does
   not directly describe any mapping onto a particular schema- or
   modelling language.  Hence an implementation of this model consists
   of a mapping to such a language - eg an LDAP or SQL schema.  The
   precise interpretation of terms from [RFC2119] therefore require some
   extra explanation.  The terms MUST, MUST NOT, REQUIRED, SHALL, SHALL
   NOT mean that an implementation MUST provide a feature but does not
   mean that this feature MUST be REQUIRED by the implementation - eg an
   attribute is available in an LDAP schema but marked as OPTIONAL.  If
   a feature must be implemented and REQUIRED this is made explicit in
   this model.  The term MAY, OPTIONAL and RECOMMENDED means that an
   implementation MAY need to REQUIRE the feature due to the particular
   nature of the schema/modelling language.  In some cases this is
   expressly forbidden by this model (feature X MUST NOT be REQUIRED by
   an implementation).

   Note that any implementation of this model SHOULD be published as an
   RFC.































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4.  Acknowledgments

   Love Hoernquist-Aestrand <lha@it.su.se> for important contributions.
















































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5.  Information model demarcation

   The information model specified in the next chapter describes
   objects, properties of those objects and relations between those
   objects.  These elements comprise an abstract view of the data
   represented in a KDC.  It is important to understand that the
   information model is not a schema.  In particular the way objects are
   compared for equality beyond that which is implied by the
   specification of a syntax is not part of this specification.  Nor is
   ordering specified between elements of a particular syntax.

   Further work on Kerberos will undoubtedly prompt updates to this
   information model to reflect changes in the functions performed by
   the KDC.  Such extensions to the information model MUST always use a
   normative reference to the relevant RFCs detailing the change in KDC
   function.



































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6.  Information model specification

6.1.  Principal

   The fundamental entity stored in a KDC is the principal.  The
   principal is associated to keys and generalizes the "user" concept.
   The principal MUST be implemented in full and MUST NOT be optional in
   an implementation

6.1.1.  Principal: Attributes

6.1.1.1.  principalName

   The principalName MUST uniquely identify the principal within the
   administrative context of the KDC.  The type of the principalName is
   not described in this document.  It is a unique identifier and can be
   viewed as an opaque which can be compared for equality.

6.1.1.2.  principalNotUsedBefore

   The principal may not be used before this date.  The syntax of the
   attribute MUST be semantically equivalent with the standard ISO date
   format.

6.1.1.3.  principalNotUsedAfter

   The principal may not be used after this date.  The syntax of the
   attribute MUST be semantically equivalent with the standard ISO date
   format.

6.1.1.4.  principalIsDisabled

   A boolean attribute used to temporarily disable a principal.

6.1.1.5.  principalAliases

   This multivalued attribute contains a set of aliases for the
   principal.

6.1.2.  Principal: Associations

   Each principal MAY be associated with 1 or more KeySet and MAY be
   associated with 1 or more Policies.  The KeySet is represented as an
   object in this model since it has attributes associated with it (the
   key version number).  In typical situations the principal is
   associated with exactly 1 KeySet but implementations MUST NOT assume
   this case, i.e an implemenation of this standard (e.g an LDAP schema)
   MUST be able to handle the general case of multiple KeySet associated



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   with each principal.

6.1.3.  Principal: Remarks

   Traditionally a principal consists of a local-part and a realm
   denoted in string form by local-part@REALM.  The realm concept is
   used to provide administrative boundaries and together with cross-
   realm authentication provides scalability to Kerberos 5.  However the
   realm is not central to an administrative information model.  For
   instance the initialization or creation of a realm is equivalent to
   creating a specific set of principals (krbtgt@REALM, etc) which is
   covered by the model and services described in this document.  A
   realm is typically associated with policy covering (for instance)
   keying and password management.  The management of such policy and
   their association to realms is beyond the scope of this document.

6.2.  KeySet

   A KeySet is a set of keys associated with exactly one principal.
   This object and its associations MUST NOT be REQUIRED by an
   implementation.  It is expected that most implementations of this
   standard will use the set/change password protocol for all aspects of
   key management [I-D.ietf-krb-wg-kerberos-set-passwd].  This
   information model only includes these objects for the sake of
   completenes.

6.2.1.  KeySet: Attributes

6.2.1.1.  keySetVersionNumber

   This is traditionally called the key version number (kvno).

6.2.2.  KeySet: Associations

   To each KeySet MUST be associated a set of 1 or more Keys.

6.2.3.  KeySet: Remarks

   The reason for separating the KeySet from the Principal is security.
   The security of Kerberos 5 depends absolutely on the security of the
   keys stored in the KDC.  The KeySet type is provided to make this
   clear and to make separation of keys from other parts of the model
   clear.

   Implementations of this standard (eg an LDAP schema) MUST preserve
   this distinction.





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6.3.  Key

   Implementations of this model MUST NOT REQUIRE keys to be
   represented.

6.3.1.  Key: Attributes

6.3.1.1.  keyEncryptionType

   The enctype.  The precise representation depends on the
   implementation.

6.3.1.2.  keyValue

   The binary representation of the key data.

6.3.1.3.  keySaltValue

   The binary representation of the key salt.

6.3.1.4.  keyStringToKeyParameter

   The syntax of this opaque object is defined by the encryption type
   used.

6.3.1.5.  keyNotUsedAfter

   This key MUST NOT be used after this date.  The syntax of the
   attribute MUST be semantically equivalent with the standard ISO date
   format.

6.3.1.6.  keyNotUsedBefore

   This key MUST NOT be used before this date.  The syntax of the
   attribute MUST be semantically equivalent with the standard ISO date
   format.

6.3.1.7.  keyIsDisabled

   If this attribute is true the key MUST NOT be used.  This is used to
   temporarily disable a key.

6.3.2.  Key: Associations

   None






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6.3.3.  Key: Remarks

   The security of the keys is an absolute requirement for the operation
   of Kerberos 5.  If keys are implemented adequate protection from
   unauthorized modification and disclosure MUST be available and
   REQUIRED by the implementation.

6.4.  Policy

   Implementations SHOULD implement policy but MAY allow them to be
   OPTIONAL.

6.4.1.  Policy: Attributes

6.4.1.1.  policyIdentifier

   The policyIdentifier MUST be unique within the local administrative
   context and MUST be globally unique.  Possible types of identifiers
   include:

      An Object Identifier (OID)

      A URN

      A UUID

6.4.1.2.  policyIsMandatory

   This attribute indicates that the KDC MUST be able to correctly
   interpret and apply this policy for the key to be used.

6.4.1.3.  policyContent

   This is an optional opaque binary value used to store a
   representation of the policy.  In general a policy cannot be fully
   expressed using attribute-value pairs.  The policyContent is OPTIONAL
   in the sense that an implementation MAY use it to store an opaque
   value for those policy-types which are not directly representable in
   that implementation.

6.4.2.  Password Quality Policy

6.4.2.1.  Password Quality Policy: Attributes

   Password quality policy controls the requirements placed by the KDC
   on new passwords.  TODO: update with information from Nico





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6.4.3.  Password Management Policy

6.4.3.1.  Password Management Policy: Attributes

   Password management policy controls how passwords are changed.  TODO:
   update with information from Nico and Ludovic

6.4.4.  Keying Policy

6.4.4.1.  Keying Policy: Attributes

   A keying policy specifies the association of enctypes with new
   principals, i.e when a principal is created one of the possibly many
   applicable keying policies determine the set of keys to associate
   with the principal.  In general the expression of a keying policy may
   require a Turing-complete language.



































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7.  Implementation Scenarios

   There are several ways to implement an administrative service for
   Kerberos 5 based on this information model.  In this section we list
   a few of them.

7.1.  LDAP backend to KDC

   Given an LDAP schema implementation of this information model it
   would be possible to build an administrative service by backending
   the KDC to a directory server where principals and keys are stored.
   Using the security mechanisms available on the directory server keys
   are protected from access by anyone apart from the KDC.
   Administration of the principals, policy and other non-key data is
   done through the directory server while the keys are modified using
   the set/change password protocol
   [I-D.ietf-krb-wg-kerberos-set-passwd].

7.2.  LDAP frontend to KDC

   An alternative way to provide a directory interface to the KDC is to
   implement an LDAP-frontend to the KDC which exposes all non-key
   objects as entries and attributes.  As in the example above all keys
   are modified using the set/change password protocol
   [I-D.ietf-krb-wg-kerberos-set-passwd].  In this scenario the
   implementation would typically not use a traditional LDAP
   implementation but treat LDAP as an access-protocol to data in the
   native KDC database.

7.3.  SOAP

   Given an XML schema implementation of this information model it would
   be possible to build a SOAP-interface to the KDC.  This demonstrates
   the value of creating an abstract information model which is mappable
   to multiple schema representations.
















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8.  Security Considerations

   This document describes an abstract information model for Kerberos 5.
   The Kerberos 5 protocol depends on the security of the keys stored in
   the KDC.  The model described here assumes that keys MUST NOT be
   transported in the clear over the network and furthermore that keys
   are treated as write-only attributes that SHALL only be modified
   (using the administrative interface) by the change-password protocol
   [I-D.ietf-krb-wg-kerberos-set-passwd].

   Exposing the object model of a KDC typically implies that objects can
   be modified and/or deleted.  In a KDC not all principals are created
   equal, so that for instance deleting krbtgt/EXAMPLE.COM@EXAMPLE.COM
   effectively disables the EXAMPLE.COM realm.  Hence access control is
   paramount to the security of any implementation.  This document does
   not (at the time of writing - leifj) mandate access control.  This
   only implies that access control is beyond the scope of the standard
   information model, i.e that access control MAY NOT be accessible via
   any protocol based on this model.  If access control objects is
   exposed via an extension to this model the presence of access control
   may in itself provide points of attack by giving away information
   about principals with elevated rights etc. etc.





























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9.  IANA Considerations

   None
















































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10.  Remarks

   A few notes and TODOs:

      Do we want to model access control?  I have received a few notes
      on that from Love.  It will affect both the model and the security
      considerations but It may be relevant.  The catch is that most
      implementations (SOAP, LDAP, etc) will have acl mechanisms
      separate from the data which makes modeling acls difficult.
      Perhaps there are certain aspects of access control which can be
      modeled with relative ease - for instance the ability to make an
      object immutable.

      Explanatory text on a few of the basic attributes that doesn't
      just repeat the section title.

      Expand on the password policy types.  Is the subdivision into
      quality and management policies valid?

































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11.  References

11.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

11.2.  Informative References

   [I-D.ietf-krb-wg-kerberos-set-passwd]
              Williams, N., "Kerberos Set/Change Key/Password Protocol
              Version 2", draft-ietf-krb-wg-kerberos-set-passwd-06 (work
              in progress), March 2007.


































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Author's Address

   Leif Johansson
   Stockholm university
   Sektionen foer IT och Media
   Stockholm  SE-106 91

   Email: leifj@it.su.se
   URI:   http://people.su.se/~leifj/










































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