KERBEROS WORKING GROUP Johansson Internet-Draft Stockholm university Intended status: Standards Track November 3, 2008 Expires: May 7, 2009 An information model for Kerberos version 5 draft-ietf-krb-wg-kdc-model-03 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on May 7, 2009. Johansson Expires May 7, 2009 [Page 1] Internet-Draft KDC Information Model November 2008 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 . . . . . . . . . . . . . . . 9 6.1.3. Principal: Remarks . . . . . . . . . . . . . . . . . . 10 6.2. KeySet . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6.2.1. KeySet: Attributes . . . . . . . . . . . . . . . . . . 10 6.2.2. KeySet: Associations . . . . . . . . . . . . . . . . . 10 6.2.3. KeySet: Remarks . . . . . . . . . . . . . . . . . . . 10 6.3. Key . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 6.3.1. Key: Attributes . . . . . . . . . . . . . . . . . . . 11 6.3.2. Key: Associations . . . . . . . . . . . . . . . . . . 12 6.3.3. Key: Remarks . . . . . . . . . . . . . . . . . . . . . 12 6.4. Policy . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.4.1. Policy: Attributes . . . . . . . . . . . . . . . . . . 12 6.4.2. Mandatory-to-implement Policy . . . . . . . . . . . . 13 7. Implementation Scenarios . . . . . . . . . . . . . . . . . . . 14 7.1. LDAP backend to KDC . . . . . . . . . . . . . . . . . . . 14 7.2. LDAP frontend to KDC . . . . . . . . . . . . . . . . . . . 14 7.3. SOAP . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 8. Security Considerations . . . . . . . . . . . . . . . . . . . 15 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 10.1. Normative References . . . . . . . . . . . . . . . . . . . 17 10.2. Informative References . . . . . . . . . . . . . . . . . . 17 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18 Intellectual Property and Copyright Statements . . . . . . . . . . 19 Johansson Expires May 7, 2009 [Page 2] Internet-Draft KDC Information Model November 2008 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]. Johansson Expires May 7, 2009 [Page 3] Internet-Draft KDC Information Model November 2008 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. Johansson Expires May 7, 2009 [Page 4] Internet-Draft KDC Information Model November 2008 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. Johansson Expires May 7, 2009 [Page 5] Internet-Draft KDC Information Model November 2008 4. Acknowledgments Love Hoernquist-Aestrand for important contributions. Johansson Expires May 7, 2009 [Page 6] Internet-Draft KDC Information Model November 2008 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. Johansson Expires May 7, 2009 [Page 7] Internet-Draft KDC Information Model November 2008 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 byte string which can be compared for equality. The attribute SHOULD be single valued. If an implementation supports multiple values it MUST treat one of the values as special and allow it to be fetched as if it was a single value. 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. The attribute MUST be single valued. 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. The attribute MUST be single valued. 6.1.1.4. principalIsDisabled A boolean attribute used to (temporarily) disable a principal. The attribute MUST default to false. 6.1.1.5. principalAliases This multivalued attribute contains an unordered set of aliases for the principal. Each alias SHOULD be unique within the administrative domain represented by the KDC. The syntax of an alias is an opaque identifier which can be compared for equality. Johansson Expires May 7, 2009 [Page 8] Internet-Draft KDC Information Model November 2008 6.1.1.6. principalNumberOfFailedAuthenticationAttempts This single valued integer attribute contains a count of the number of times an authentication attempt was unsuccessful for this principal. Implementations SHOULD NOT allow this counter to be reset. 6.1.1.7. principalLastFailedAuthentication This single valued attribute contains the time and date for the last failed authentication attempt for this principal. 6.1.1.8. principalLastSuccessfulAuthentication This single valued attribute contains the time and date for the last successful authentication attempt for this principal. 6.1.1.9. principalLastCredentialChange This single valued attribute contains the time and date for the last successful change of credential (eg password) this principal. 6.1.1.10. principalCreateTime This single valued attribute contains the time and date when this principal was created 6.1.1.11. principalModdifyTime This single valued attribute contains the time and date when this principal was modified excluding credentials change. 6.1.1.12. principalMaximumTicketLifetime This single valued attribute contains the delta time in seconds representing the maximum ticket lifetime for tickets issued for this principal. 6.1.1.13. principalMaximumRenewableTicketLifetime This single valued attribute contains the delta time in seconds representing the maximum amount of time a ticket may be renewed for. 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 Johansson Expires May 7, 2009 [Page 9] Internet-Draft KDC Information Model November 2008 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 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). This is a single valued attribute containing a positive integer. 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. Johansson Expires May 7, 2009 [Page 10] Internet-Draft KDC Information Model November 2008 Implementations of this standard (eg an LDAP schema) MUST make a clear separation between the representation of KeySet from other information objects. 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 SHOULD be represented as an enumeration of the enctypes supported by the KDC. 6.3.1.2. keyValue The binary representation of the key data. This MUST be a single valued octet string. 6.3.1.3. keySaltValue The binary representation of the key salt. This MUST be a single valued octet string. 6.3.1.4. keyStringToKeyParameter This MUST be a single valued octet string representing an opaque parameter associated with the enctype. 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. This MUST be a single-valued attribute. 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. This MUST be a single-valued attribute. 6.3.1.7. keyIsDisabled This is a boolean attribute which must be set to false by default. If this attribute is true the key MUST NOT be used. This is used to temporarily disable a key. Johansson Expires May 7, 2009 [Page 11] Internet-Draft KDC Information Model November 2008 6.3.2. Key: Associations None 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. The Policy should be thought of as a 'typed hole'. i.e an opaque binary value paired with an identifier of type of data contained in the binary value. Both attributes (type and value) must be present. 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 The use of OIDs is recommended for this purpose. 6.4.1.2. policyIsCritical This boolean 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 single 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. Johansson Expires May 7, 2009 [Page 12] Internet-Draft KDC Information Model November 2008 6.4.1.4. policyUse This is an optional single enumerated string value used to describe the applicability of the policy. Implementations SHOULD provide this attribute and MUST (if the attribute is implemented) describe the enumerated set of possible values. 6.4.2. Mandatory-to-implement Policy All implementations 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. 6.4.2.1. Password Quality Policy Password quality policy controls the requirements placed by the KDC on new passwords. This policy SHOULD be identified by the OID . 6.4.2.2. Password Management Policy Password management policy controls how passwords are changed. This policy SHOULD be identified by the OID . 6.4.2.3. Keying Policy 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. This policy SHOULD be identified by the OID . Johansson Expires May 7, 2009 [Page 13] Internet-Draft KDC Information Model November 2008 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. Johansson Expires May 7, 2009 [Page 14] Internet-Draft KDC Information Model November 2008 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. Johansson Expires May 7, 2009 [Page 15] Internet-Draft KDC Information Model November 2008 9. IANA Considerations None Johansson Expires May 7, 2009 [Page 16] Internet-Draft KDC Information Model November 2008 10. References 10.1. 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. 10.2. 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-07 (work in progress), September 2007. Johansson Expires May 7, 2009 [Page 17] Internet-Draft KDC Information Model November 2008 Author's Address Leif Johansson Stockholm university Avdelningen foer IT och Media Stockholm SE-106 91 Email: leifj@it.su.se URI: http://people.su.se/~leifj/ Johansson Expires May 7, 2009 [Page 18] Internet-Draft KDC Information Model November 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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