Network Working Group Johansson Internet-Draft Stockholm university Expires: April 8, 2004 October 9, 2003 An information model for Kerberos version 5 draft-johansson-kerberos-model-00 Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. 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 April 8, 2004. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved. 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. Johansson Expires April 8, 2004 [Page 1] Internet-Draft An information model for Kerberos version 5 October 2003 Table of Contents 1. Requirements notation . . . . . . . . . . . . . . . . . . . 3 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 5 4. Information model . . . . . . . . . . . . . . . . . . . . . 6 4.1 Principal . . . . . . . . . . . . . . . . . . . . . . . . . 6 4.1.1 Principal: Attributes . . . . . . . . . . . . . . . . . . . 6 4.1.2 Principal: Associations . . . . . . . . . . . . . . . . . . 6 4.1.3 Principal: Remarks . . . . . . . . . . . . . . . . . . . . . 6 4.2 KeySet . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.2.1 KeySet: Attributes . . . . . . . . . . . . . . . . . . . . . 7 4.2.2 KeySet: Associations . . . . . . . . . . . . . . . . . . . . 7 4.2.3 KeySet: Remarks . . . . . . . . . . . . . . . . . . . . . . 7 4.3 Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4.3.1 Key: Attributes . . . . . . . . . . . . . . . . . . . . . . 7 4.3.2 Key: Associations . . . . . . . . . . . . . . . . . . . . . 8 4.3.3 Key: Remarks . . . . . . . . . . . . . . . . . . . . . . . . 8 4.4 Policy . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.4.1 Policy: Attributes . . . . . . . . . . . . . . . . . . . . . 8 4.4.2 Password Quality Policy . . . . . . . . . . . . . . . . . . 9 4.4.3 Password Management Policy . . . . . . . . . . . . . . . . . 9 4.4.4 Keying Policy . . . . . . . . . . . . . . . . . . . . . . . 9 5. Implementation Scenarios . . . . . . . . . . . . . . . . . . 10 5.1 LDAP backend to KDC . . . . . . . . . . . . . . . . . . . . 10 5.2 LDAP frontend to KDC . . . . . . . . . . . . . . . . . . . . 10 5.3 SOAP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6. Security Considerations . . . . . . . . . . . . . . . . . . 11 7. Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . 13 Intellectual Property and Copyright Statements . . . . . . . 14 Johansson Expires April 8, 2004 [Page 2] Internet-Draft An information model for Kerberos version 5 October 2003 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 April 8, 2004 [Page 3] Internet-Draft An information model for Kerberos version 5 October 2003 2. Motivation The Kerberos version 5 authentication service described in [RFC1510] 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 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 (say) 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 April 8, 2004 [Page 4] Internet-Draft An information model for Kerberos version 5 October 2003 3. Acknowledgments Love H÷rnquist-Êstrand for important contributions. Johansson Expires April 8, 2004 [Page 5] Internet-Draft An information model for Kerberos version 5 October 2003 4. Information model 4.1 Principal The fundamental entity stored in a KDC is the principal. The principal is associated to keys and generalizes the "user" concept. 4.1.1 Principal: Attributes 4.1.1.1 principalName A principal MUST be uniquely identified withing the administrative context of the KDC by the principalName. 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 with. 4.1.1.2 principalNotUsedBefore The principal may not be used before this date. The syntax of the attribute MUST be semantically equivalent with ISO date format. 4.1.1.3 principalNotUsedAfter The principal may not be used after this date. The syntax of the attribute MUST be semantically equivalent to standard ISO date format. 4.1.1.4 principalIsDisabled A boolean attribute used to temporarily disable a principal. 4.1.1.5 principalAliases This multivalued attribute contains a set of aliases for the principal. 4.1.2 Principal: Associations Each principal MUST be associated with exactly one 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). 4.1.3 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 Johansson Expires April 8, 2004 [Page 6] Internet-Draft An information model for Kerberos version 5 October 2003 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. 4.2 KeySet A KeySet is a set of keys associated with exactly one principal. This object and the associated associations MAY be supported by implementations. Implementations (eg and LDAP schema) MUST NOT mandate the representation of keys. 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 includes these objects for completeness. 4.2.1 KeySet: Attributes 4.2.1.1 keySetVersionNumber This is traditionally called the key version number (kvno). 4.2.2 KeySet: Associations To each KeySet MUST be associated a set of 1 or more Keys. 4.2.3 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. 4.3 Key 4.3.1 Key: Attributes Johansson Expires April 8, 2004 [Page 7] Internet-Draft An information model for Kerberos version 5 October 2003 4.3.1.1 keyEncryptionType TODO 4.3.1.2 keyValue TODO 4.3.1.3 keySaltValue TODO 4.3.1.4 keyStringToKeyParameter The syntax of this opaque object is defined by the encryption type used. 4.3.1.5 keyNotUsedAfter This key MUST NOT be used after this date. 4.3.1.6 keyNotUsedBefore This key MUST NOT be used before this date. 4.3.1.7 keyIsDisabled If this attribute is true the key MUST NOT be used 4.3.2 Key: Associations None 4.3.3 Key: Remarks The security of the keys is a requirement for the operation of Kerberos 5. 4.4 Policy 4.4.1 Policy: Attributes 4.4.1.1 policyIdentifier The policyIdentifier MUST be unique within the local administrative context and SHOULD be globally unique. It may be valuable to use a mechanism for policy identification which is likely to be representable in may possible implementations of this standard. A few Johansson Expires April 8, 2004 [Page 8] Internet-Draft An information model for Kerberos version 5 October 2003 possibilities are An Object Identifier (OID) A URN A UUID 4.4.1.2 policyIsMandatory This attribute indicates that the KDC MUST be able to correctly interpret and apply this policy for the key to be used. 4.4.1.3 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 attribute is optional in the sense that an implementation MAY use it to store an opaque value for those policy-types which are not representable in that implementation. When used the attribute MUST be implemented as a mandatory attribute if the implementation supports mandatory/optional attributes. 4.4.2 Password Quality Policy 4.4.2.1 Password Quality Policy: Attributes Password quality policy controls the requirements placed by the KDC on new passwords. TODO: update with information from Nico 4.4.3 Password Management Policy 4.4.3.1 Password Management Policy: Attributes Password management policy controls how passwords are changed. TODO: update with information from Nico and Ludovic 4.4.4 Keying Policy 4.4.4.1 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 Touring-complete language. Johansson Expires April 8, 2004 [Page 9] Internet-Draft An information model for Kerberos version 5 October 2003 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. 5.1 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]. 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 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. 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 demonstrates the value of creating an abstract information model which is mappable to multiple schema representations. Johansson Expires April 8, 2004 [Page 10] Internet-Draft An information model for Kerberos version 5 October 2003 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 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 April 8, 2004 [Page 11] Internet-Draft An information model for Kerberos version 5 October 2003 7. 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. Need to specify how extensions to the model happens. Comparison for equality is implied by specifying syntax, but what about ordering? Do we need/want to compare objects/ attribute-values for order? 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? Johansson Expires April 8, 2004 [Page 12] Internet-Draft An information model for Kerberos version 5 October 2003 References [I-D.ietf-krb-wg-kerberos-clarifications] Neuman, C., "The Kerberos Network Authentication Service (V5)", draft-ietf-krb-wg-kerberos-clarifications-04 (work in progress), June 2003. [I-D.ietf-krb-wg-kerberos-set-passwd] Trostle, J., "Kerberos Set/Change Password: Version 2", draft-ietf-krb-wg-kerberos-set-passwd-00 (work in progress), May 2003. [RFC1510] Kohl, J. and B. Neuman, "The Kerberos Network Authentication Service (V5)", RFC 1510, September 1993. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. Author's Address Leif Johansson Stockholm university Enheten f÷r IT och Media Stockholm SE-106 91 EMail: leifj@it.su.se URI: http://people.su.se/~leifj/ Johansson Expires April 8, 2004 [Page 13] Internet-Draft An information model for Kerberos version 5 October 2003 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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