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Internet Engineering Task Force (IETF)                      L. JohanssonRequest for Comments: 6880                                         SUNETCategory: Standards Track                                     March 2013ISSN: 2070-1721An Information Model for Kerberos Version 5Abstract   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 Key Distribution Center   (KDC).  This document describes the services exposed by an   administrative interface to a KDC.Status of This Memo   This is an Internet Standards Track document.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Further information on   Internet Standards is available inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6880.Johansson                    Standards Track                    [Page 1]

RFC 6880                  KDC Information Model               March 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Johansson                    Standards Track                    [Page 2]

RFC 6880                  KDC Information Model               March 2013Table of Contents1. Introduction ....................................................32. Requirements Notation ...........................................43. Information Model Demarcation ...................................54. Information Model Specification .................................64.1. Principal ..................................................64.1.1. Principal: Attributes ...............................64.1.2. Principal: Associations .............................74.2. KeySet .....................................................84.2.1. KeySet: Attributes ..................................84.2.2. KeySet: Associations ................................84.3. Key ........................................................94.3.1. Key: Attributes .....................................94.3.2. Key: Associations ..................................104.3.3. Key: Remarks .......................................104.4. Policy ....................................................104.4.1. Policy: Attributes .................................104.4.2. Mandatory-to-Implement Policy ......................115. Implementation Scenarios .......................................115.1. LDAP Backend to KDC .......................................125.2. LDAP Frontend to KDC ......................................125.3. SOAP ......................................................125.4. NETCONF ...................................................126. Security Considerations ........................................127. Acknowledgments ................................................138. References .....................................................138.1. Normative References ......................................138.2. Informative References ....................................141.  Introduction   The Kerberos version 5 authentication service described in [RFC4120]   describes how a Key Distribution Center (KDC) provides authentication   to clients.RFC 4120 does not stipulate how a KDC is managed, and   several "kadmin" servers have evolved sinceRFC 4120 was written.   This document describes the services required to administer a KDC and   the underlying information model assumed by a kadmin-type service.   The information model is written in terms of "attributes" and either   "services" or "interfaces", but the use of these particular words   must not be taken to imply any particular modeling paradigm.  Neither   an object-oriented model nor a Lightweight Directory Access Protocol   (LDAP) [RFC4510] schema is intended.  The author has attempted to   describe, in prose, the intended semantics and syntax of the   components of the model.  For instance, an LDAP schema based on this   model will be more precise in the expression of the syntax while   preserving the semantics of this model.Johansson                    Standards Track                    [Page 3]

RFC 6880                  KDC Information Model               March 2013   Implementations of this document MAY decide to change the names used   (e.g., principalName).  If so, an implementation MUST provide a name-   to-name mapping to this document.  In particular, schema languages   may have different typographical conventions, e.g., the use of an   uppercase letter (as seen in camelCase) versus the use of '_' and '-'   to separate 'words' in a name.  Implementations MUST call out such   conventions explicitly.   Implementations of this document MUST be able to support default   values for attributes and have the ability to specify syntax for   attribute values.2.  Requirements Notation   This document uses the standard key words ("MUST", "MUST NOT",   "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",   "RECOMMENDED", "MAY", and "OPTIONAL") that are defined in [RFC2119],   but with modifications to those definitions as described below.  The   reason for this (which was discussed extensively in the Kerberos   working group) is as follows:   This document describes an information model for Kerberos 5, but it   does not directly describe any mapping onto a particular schema or   modeling language.  Hence, an implementation of this model consists   of a mapping to such a language, e.g., an LDAP or SQL schema.   Therefore, the standard normative key words require precise   definition.   The terms "MUST" and "REQUIRED" mean that a schema implementing this   model must have a way to represent a feature (i.e., that it is   mandatory to implement it in the schema), but that, unless otherwise   specified, the feature may represent an optional element in the   chosen schema definition language.   However, "MUST" also means that a KDC or administrative interface   implementing this information model has to provide the feature and   associated behavior consistent with the schema.   For instance, principalName (seeSection 4.1.1.1) represents the name   of a principal.  In an LDAP schema (for instance), this may be   represented as an optional attribute even though all KDCs   implementing this specification must support this attribute.   The terms "MAY" and "OPTIONAL" mean that it is optional for a KDC or   administrative interface implementing this information model to   implement this feature.  These terms also mean that implementing the   feature in the schema is optional.Johansson                    Standards Track                    [Page 4]

RFC 6880                  KDC Information Model               March 2013   Implementers of the schema should be aware that, unless the schema   definition can represent critical but optional elements, language   confusion may arise when optional elements are used but not   understood by all implementations in a particular deployment.   The expression "MUST NOT be OPTIONAL" means that it is mandatory that   a feature be implemented ("MUST" per the definition in [RFC2119]) and   additionally that it must not be marked as optional in the schema   language.  In particular, this expression means that the feature is   both mandatory to implement and must be present in all   representations of the object to which it applies.   The terms "SHOULD" and "RECOMMENDED" mean that the consequences of   not implementing the feature as if it were described with the "MUST"   keyword must be carefully weighed before choosing a different course.   In particular, these terms imply that interoperability concerns may   arise from not following the recommended practice in schema that   implement this model.   Context will determine whether the "SHOULD" key word applies to the   schema, to the underlying behavior of the KDC, or both.  For   instance, when this document states that principalIsDisabled (seeSection 4.1.1.4) SHOULD default to FALSE, this statement implies both   a recommendation for the behavior of KDCs as well as a recommendation   for the representation of that behavior in schema.3.  Information Model Demarcation   The information model specified inSection 4 describes objects, their   properties, and the relationships between the 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 the ordering specified between the   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 should always use   a normative reference to the relevant RFCs in detailing the change in   KDC function.   This model describes a number of elements related to password policy   management.  Not all of the elements in this model are unique to   Kerberos.  For example, an LDAP implementation of this model should   incorporate existing LDAP schema where functional overlap exists,   rather than defining additional Kerberos-specific elements.Johansson                    Standards Track                    [Page 5]

RFC 6880                  KDC Information Model               March 20134.  Information Model Specification4.1.  Principal   The fundamental entity stored in a KDC is the principal.  The   principal is associated with keys and generalizes the "user" concept.   The principal MUST be implemented in full and MUST NOT be OPTIONAL in   an implementation4.1.1.  Principal: Attributes4.1.1.1.  principalName   The principalName MUST uniquely identify the principal within the   administrative context of the KDC.  The principalName MUST be   equivalent to the string representation of the principal name (seeSection 2.1.1 of [RFC1964]), including, if applicable for the name   type, the realm.   The attribute MAY be multivalued if the implementation supports   aliases, enterprise names, or both.  In this case, exactly one of the   principalName values MAY be designated as the canonical   principalName.  If the implementation supports encryption types   (enctypes) that require salt, exactly one of the values of   principalName MAY be designated as the canonical salting   principalName.   Implementations (i.e., schema) that support enterprise names,   aliases, or both, SHOULD provide for efficient lookup of principal   objects based on the alias or enterprise name.4.1.1.2.  principalNotUsedBefore   The principal MUST NOT be used before this date.  The syntax of the   attribute MUST be Internet date/time format from [RFC3339].  The   attribute MUST be single-valued.4.1.1.3.  principalNotUsedAfter   The principal MUST NOT be used after this date.  The syntax of the   attribute MUST be Internet date/time format from [RFC3339].  The   attribute MUST be single-valued.4.1.1.4.  principalIsDisabled   A boolean attribute used to disable a principal.  The attribute   SHOULD default to boolean FALSE.Johansson                    Standards Track                    [Page 6]

RFC 6880                  KDC Information Model               March 20134.1.1.5.  principalLastCredentialChangeTime   This single-valued attribute contains the time of the last successful   change of credentials (e.g., password or private key) associated with   this principal.  The syntax of the attribute MUST be Internet   date/time format from [RFC3339].4.1.1.6.  principalCreateTime   This single-valued attribute contains the time and date when this   principal was created.  The syntax of the attribute MUST be Internet   date/time format from [RFC3339].4.1.1.7.  principalModifyTime   This single-valued attribute contains the time and date when this   principal was last modified, excluding changes to credentials.  The   syntax of the attribute MUST be Internet date/time format from   [RFC3339].4.1.1.8.  principalMaximumTicketLifetime   This single-valued attribute contains the time, in seconds,   representing the maximum lifetime of a ticket issued for this   principal.4.1.1.9.  principalMaximumRenewableTicketLifetime   This single-valued attribute contains the delta time, in seconds,   representing the maximum amount of time a ticket may be renewed for   this principal.4.1.1.10.  principalAllowedEnctype   This OPTIONAL multivalued attribute lists the enctypes allowed for   this principal.  If empty or absent, any enctype supported by the   implementation is allowed for this principal.   This attribute is intended as a policy attribute and restricts all   uses of enctypes, including server, client, and session keys.  Data   models MAY choose to use policy objects in order to represent more   complex decision mechanisms.4.1.2.  Principal: Associations   Each principal MAY be associated with 0 or more KeySets and MAY be   associated with 0 or more Policies.  The KeySet is represented as an   object in this model, because it has attributes associated with itJohansson                    Standards Track                    [Page 7]

RFC 6880                  KDC Information Model               March 2013   (the key version number).  In typical situations, the principal is   associated with exactly one KeySet, but implementations MUST NOT   assume this case.  That is, an implementation of this standard MUST   be able to handle the general case of multiple KeySets associated   with each principal.  Multiple KeySets may, for instance, be useful   when performing a key rollover for a principal.4.2.  KeySet   In Kerberos, principals are associated with zero or more symmetric   secret keys, and each key has a key version number (kvno) and an   enctype.  In this model, we group keys by kvno into KeySet objects.   A principal can have zero or more KeySet objects associated with it,   each of which MUST have one or more keys.  Each KeySet is associated   with exactly one principal.  A schema derived from this model MAY   lack a direct analogue of KeySet, as described in this document.   It is expected that most Kerberos implementations will use a special-   purpose interface for setting and changing principal passwords and   keys.   If a server supports an enctype for a principal, that enctype must be   present in at least one key for the principal in question.  For any   given enctype, a KeySet MUST NOT contain more than one key with that   enctype.   The security of Kerberos 5 depends absolutely on the confidentiality   and integrity of the key objects stored in the KDC.  Implementations   of this standard MUST facilitate, to the extent possible, an   administrator's ability to place more restrictive access controls on   KeySets than on other principal data, and to arrange for more secure   backup for KeySets.4.2.1.  KeySet: Attributes4.2.1.1.  kvno   The key version number.  This is a single-valued attribute containing   a non-negative integer.  This number is incremented by one each time   a key in the KeySet is changed.4.2.2.  KeySet: Associations   Each KeySet MUST be associated with a set of one or more Keys.Johansson                    Standards Track                    [Page 8]

RFC 6880                  KDC Information Model               March 20134.3.  Key   Implementations of this model MUST NOT force keys to be represented.   That is, a schema that REQUIRED a key to be present would not meet   this constraint.4.3.1.  Key: Attributes4.3.1.1.  keyEncryptionType   The enctype SHOULD be represented as an enumeration of the enctypes   supported by the KDC using the string name ("encryption type") of the   enctype from the IANA registry of Kerberos Encryption Type Numbers.   One example is "aes128-cts-hmac-sha1-96".4.3.1.2.  keyValue   The binary representation of the key data.  This MUST be a single-   valued octet string.4.3.1.3.  keySaltValue   The binary representation of the key salt.  This MUST be a single-   valued octet string.4.3.1.4.  keyStringToKeyParameter   This MUST be a single-valued octet string representing an opaque   parameter associated with the enctype.  This parameter is specified   using the string-to-key method defined inSection 3 of [RFC3961].4.3.1.5.  keyNotUsedBefore   The key MUST NOT be used before this date.  The syntax of the   attribute MUST be semantically equivalent to the standard ISO date   format ([RFC3339]).  This attribute MUST be single-valued.4.3.1.6.  keyNotUsedAfter   The key MUST NOT be used after this date.  The syntax of the   attribute MUST be semantically equivalent to the standard ISO date   format ([RFC3339]).  This attribute MUST be single-valued.4.3.1.7.  keyIsDisabled   This is a boolean attribute that SHOULD be set to FALSE by default.   If this attribute is TRUE, the key MUST NOT be used.  The   keyIsDisabled attributed is used to temporarily disable a key.Johansson                    Standards Track                    [Page 9]

RFC 6880                  KDC Information Model               March 20134.3.2.  Key: Associations   None4.3.3.  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 is   REQUIRED of the implementation.4.4.  Policy   Implementations SHOULD implement policies, but MAY allow them to be   OPTIONAL.  The policy should be thought of as a "typed hole", i.e.,   as an opaque binary value paired with an identifier of the type of   data contained in the binary value.  Both attributes (type and value)   must be present.4.4.1.  Policy: Attributes4.4.1.1.  policyIdentifier   The policyIdentifier MUST be globally unique.  Possible types of   identifiers include:   o  An Object Identifier (OID) [RFC4517]   o  A URI [RFC3986]   o  A UUID [RFC4122]   Implementations of this specification are expected to assign globally   unique identifiers to the list of the standard policy below in   accordance with best practices for identifier management for the   schema language used.4.4.1.2.  policyIsCritical   This boolean attribute indicates that the KDC MUST be able to   correctly interpret and apply the policy for the principal to be   used.4.4.1.3.  policyContent   This optional single opaque binary value is used to store a   representation of the policy.  In general, a policy cannot be fully   expressed using attribute-value pairs.  The policyContent is OPTIONALJohansson                    Standards Track                   [Page 10]

RFC 6880                  KDC Information Model               March 2013   in the sense that an implementation MAY use it to store an opaque   value for the policy types that are not directly representable in   that implementation.4.4.1.4.  policyUse   This optional single enumerated string value is used to describe the   use of the policy.  Implementations SHOULD provide this attribute and   MUST (if the attribute is implemented) describe the enumerated set of   possible values.  The intent is that this attribute provide an   initial context-based filtering.4.4.2.  Mandatory-to-Implement Policy   All implementations that represent policy objects MUST be able to   represent the policies listed in this section.  Implementations are   not required to use the same underlying data representation for the   policyContent binary value, but SHOULD use the same OIDs as the   policyIdentifier.  In general, the expression of policy may require a   Turing-complete language.  This specification does not attempt to   model policy-expression language.4.4.2.1.  Password Quality Policy   Password quality policy controls the requirements placed by the KDC   on new passwords.4.4.2.2.  Password Management Policy   Password management policy controls how passwords are changed.4.4.2.3.  Keying Policy   A keying policy specifies the association of enctypes with new   principals.  For example, when a principal is created, one of the   applicable keying policies is used to determine the set of keys to   associate with the principal.4.4.2.4.  Ticket Flag Policy   A ticket flag policy specifies the ticket flags allowed for tickets   issued for a principal.5.  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.Johansson                    Standards Track                   [Page 11]

RFC 6880                  KDC Information Model               March 20135.1.  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 [PASSWORD].5.2.  LDAP Frontend to KDC   An alternative way to provide a directory interface to the KDC is to   implement an LDAP frontend to the KDC that exposes all non-key   objects as entries and attributes.  As in the example above, all keys   are modified using the set/change password protocol [PASSWORD].  In   this scenario, the implementation would typically not use a   traditional LDAP implementation, but would treat LDAP as a protocol   to access data in the native KDC database.5.3.  SOAP   Given an XML schema implementation of this information model, it   would be possible to build a SOAP interface to the KDC.  This   situation demonstrates the value of creating an abstract information   model that is mappable to multiple schema representations.5.4.  NETCONF   Given a YAML (YAML Ain't Markup Language) implementation of this   information model, it would be possible to create a NETCONF-based   interface to the KDC, enabling management of the KDC from standard   network management applications.6.  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   be treated as write-only attributes that SHALL be modified (using the   administrative interface) only by the change-password protocol   [PASSWORD].   Exposing the object model of a KDC typically implies that objects can   be modified, deleted, or both.  In a KDC, not all principals are   created equal.  For instance, deleting krbtgt/EXAMPLE.COM@EXAMPLE.COMJohansson                    Standards Track                   [Page 12]

RFC 6880                  KDC Information Model               March 2013   effectively disables the EXAMPLE.COM realm.  Hence, access control is   paramount to the security of any implementation.  This document does   not mandate access control.  This situation implies only 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 are 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 that   have elevated rights.7.  Acknowledgments   The author wishes to extend his thanks to Love Hoernquist-Aestrand   and Sam Hartman for their important contributions to this document.8.  References8.1.  Normative References   [RFC1964]  Linn, J., "The Kerberos Version 5 GSS-API Mechanism",RFC1964, June 1996.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3339]  Klyne, G., Ed. and C. Newman, "Date and Time on the              Internet: Timestamps",RFC 3339, July 2002.   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for              Kerberos 5",RFC 3961, February 2005.   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform              Resource Identifier (URI): Generic Syntax", STD 66,RFC3986, January 2005.   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The              Kerberos Network Authentication Service (V5)",RFC 4120,              July 2005.   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally              Unique IDentifier (UUID) URN Namespace",RFC 4122, July              2005.   [RFC4517]  Legg, S., "Lightweight Directory Access Protocol (LDAP):              Syntaxes and Matching Rules",RFC 4517, June 2006.Johansson                    Standards Track                   [Page 13]

RFC 6880                  KDC Information Model               March 20138.2.  Informative References   [PASSWORD] Williams, N., "Kerberos Set/Change Key/Password Protocol              Version 2", Work in Progress, November 2008.   [RFC4510]  Zeilenga, K., "Lightweight Directory Access Protocol              (LDAP): Technical Specification Road Map",RFC 4510, June              2006.Author's Address   Leif Johansson   Swedish University Network   Thulegatan 11   Stockholm   Sweden   EMail: leifj@sunet.se   URI:http://www.sunet.seJohansson                    Standards Track                   [Page 14]