NETWORK WORKING GROUP L. Zhu Internet-Draft P. Leach Updates: 4120 (if approved) K. Jaganathan Expires: December 5, 2006 Microsoft Corporation June 3, 2006 Anonymity Support for Kerberos draft-ietf-krb-wg-anon-00 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 December 5, 2006. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document defines the use of anonymous Kerberos tickets for the purpose of authenticating the servers and enabling secure communication between a client and a server, without identifying the client to the server. Zhu, et al. Expires December 5, 2006 [Page 1] Internet-Draft Kerberos Anonymity Support June 2006 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 4. Protocol Description . . . . . . . . . . . . . . . . . . . . . 5 5. GSS-API Implementation Notes . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 9. Normative References . . . . . . . . . . . . . . . . . . . . . 7 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . . . 10 Zhu, et al. Expires December 5, 2006 [Page 2] Internet-Draft Kerberos Anonymity Support June 2006 1. Introduction In certain situations or environments, the Kerberos [RFC4120] client may wish to authenticate a server and/or protect communications without revealing its own identity. For example, consider an application which provides read access to a research database, and which permits queries by arbitrary requestors. A client of such a service might wish to authenticate the service, to establish trust in the information received from it, but might not wish to disclose its identity to the service for privacy reasons. To accomplish this, a Kerberos mechanism is specified in this document by which a client requests an anonymous ticket and use that to authenticate the server and secure subsequent client-server communications. This provides Kerberos with functional equivalence to TLS [RFC2246] in environments where Kerberos is a more attractive authentication mechanism. Using this mechanism, the client has to reveal its identity in its initial request to its own Key Distribution Center (KDC) [RFC4120], and then it can remain anonymous thereafter to KDCs on the cross- realm authentication path, if any, and to the server with which it communicates. 2. Conventions Used in This Document 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]. 3. Definitions An anonymous ticket is a ticket that has all of the following properties: o The client's principal name is the anonymous Kerberos principal name. The anonymous Kerberos principal name is defined as follows: it is a reserved Kerberos principal name as defined in [KRBNAM], the name-type is KRB_NT_RESRVED [KRBNAM], and the name- string is a sequence of two KerberosString components: "RESERVED", "ANONYMOUS". o The client's realm name is the anonymous kerberos realm name. The anonymous Kerberos realm name is defined as follows: it is a reserved realm name as defined in [KRBNAM] and the realm name is the literal "RESERVED:ANONYMOUS". Zhu, et al. Expires December 5, 2006 [Page 3] Internet-Draft Kerberos Anonymity Support June 2006 o The authtime field in the ticket is set to the time of the ticket request, not the time of the initial authentication for the principal who has made the request. o The transited field [RFC4120] can either contain the client's authentication path or contain the anonymous authentication path defined as follows: the tr-type field of the transited field is NO-TRANSITED-INFO (as defined later in this section) and the contents field is an empty OCTET STRING. If a TGS request contains an anonymous ticket with a "normal" authentication path (i.e. the transited field does not contain the anonymous authentication path as defined above), then the reply ticket, if any, MUST NOT contain the anonymous authentication path. For application servers, no transited policy is defined for the anonymous authentication path, but all of the transited checks would still apply if an anonymous ticket contains a "normal" authentication path. Note that the "normal" authentication path in an anonymous ticket can be a partial path, thus it may not be sufficient to identify the originating client realm. o It contains no information that can reveal the client's identity other than, at most, the client's realm or the realm(s) on the authentication path. o The anonymous ticket flag (as defined later in this section) is set. The anonymous ticket flag is defined as bit 14 (with the first bit being bit 0) in the TicketFlags: TicketFlags ::= KerberosFlags -- anonymous(14) -- TicketFlags and KerberosFlags are defined in [RFC4120] The anonymous ticket flag MUST NOT be set by implementations of this specification if the ticket is not an anonymous ticket as defined in this section. The request-anonymous KDC option is defined as bit 14 (with the first bit being bit 0) in the KDCOptions: KDCOptions ::= KerberosFlags -- request-anonymous(14) -- KDCOptions and KerberosFlags are defined in [RFC4120] The anonymous transited encoding type is defined as follows: NO-TRANSITED-INFO 0 Zhu, et al. Expires December 5, 2006 [Page 4] Internet-Draft Kerberos Anonymity Support June 2006 This transited encoding type indicates that there is no information available about the authentication path. Note that the server principal name and the server realm in a cross- realm referral TGT are not dependent on whether the client is the anonymous principal or not. 4. Protocol Description In order to request an anonymous ticket, the client sets the request- anonymous KDC option in an AS or TGS request [RFC4120]. Note that if the service ticket in the PA-TGS-REQ [RFC4120] is anonymous, the request-anonymous KDC option MUST be set in the request. When policy allows, the KDC issues an anonymous ticket. The KDC that implements this specification MUST NOT carry information that can reveal the client's identity, from the TGS request into the returned anonymous ticket. It should be noted that unless otherwise specified by this document the client principal name and the client realm in the Kerberos messages [RFC4120] should be the client name and client realm that can uniquely identify the client principal to the KDC, not the anonymous client principal name and the empty realm name. For example, the client name and realm in the request body and the EncKDCRepPart of the reply [RFC4120] are identifiers of the client principal. In other words, the client name and client realm in the EncKDCRepPart does not match with that of the returned anonymous ticket. If either local policy prohibits issuing of anonymous tickets or it is inappropriate to remove information (such as restrictions) from the TGS request in order to produce an anonymous ticket, the KDC MUST return an error message with the code KDC_ERR_POLICY [RFC4120]. If a client requires anonymous communication then the client should check to make sure that the resulting ticket is actually anonymous by checking the presence of the anonymous ticket flag. Because KDCs ignore unknown KDC options, a KDC that does not understand the request-anonymous KDC option will not return an error, but will instead return a normal ticket. The subsequent client and server communications then proceed as described in [RFC4120]. The client principal name in the Authenticator of the KRB_AP_REQ MUST be the anonymous client principal name and the client realm of the Authenticator MUST be an empty KerberosString [RFC4120]. Zhu, et al. Expires December 5, 2006 [Page 5] Internet-Draft Kerberos Anonymity Support June 2006 A server accepting such an anonymous service ticket may assume that subsequent requests using the same ticket originate from the same client. Requests with different tickets are likely to originate from different clients. Interoperability and backward-compatibility notes: the KDC is given the task of rejecting a request for an anonymous ticket when the anonymous ticket is not acceptable by the server. 5. GSS-API Implementation Notes At the GSS-API [RFC2743] level, the use of an anonymous principal by the initiator/client requires a software change of the initiator/ client software (to assert the "anonymous" flag when calling GSS_Init_Sec_Context(). GSS-API does not know or define "anonymous credentials", so the (printable) name of the anonymous principal will rarely be used by or relevant for the initator/client. The printable name is relevant for the acceptor/server when performing an authorization decision based on the name that pops up from GSS_Accept_Sec_Context() upon successful security context establishment. A GSS-API initiator MUST carefully check the resulting context attributes from the initial call to GSS_Init_Sec_Context() when requesting anonymity, because (as in the GSS-API tradition and for backwards compatibility) anonymity is just another optional context attribute. It could be that the mechanism doesn't recognize the attribute at all or that anonymity is not available for some other reasons -- and in that case the initiator must NOT send the initial security context token to the acceptor, because it will likely reveal the initiators identity to the acceptor, something that can rarely be "un-done". GSS-API defines name_type GSS_C_NT_ANONYMOUS [RFC2743] to represent an anonymous identity. In addition, according to Section 2.1.1 of [RFC1964] the string representation of the anonymous client principal name can be "RESERVED/ANONYMOUS" or "RESERVED/ ANONYMOUS@RESERVED:ANONYMOUS" with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. Implementations conforming to this specification MUST be able to accept the GSS_C_NT_ANONYMOUS name form and the GSS_KRB5_NT_PRINCIPAL_NAME name forms, and consider them equivalent. Portable initiators are RECOMMENDED to use default credentials whenever possible, and request anonymity only through the input anon_req_flag to GSS_Init_Sec_Context(). Zhu, et al. Expires December 5, 2006 [Page 6] Internet-Draft Kerberos Anonymity Support June 2006 6. Security Considerations Since KDCs ignore unknown options [RFC4120], a client requiring anonymous communication needs to make sure that the ticket is actually anonymous. A KDC that that does not understand the anonymous option would not return an anonymous ticket. By using the mechanism defined in this specification, the client does not reveal its identity to the server but its identity may be revealed to the KDC of the server principal (when the server principal is in a different realm than that of the client), and any KDC on the cross-realm authentication path. The Kerberos client MUST verify the ticket being used are indeed anonymous before communicating with the cross-realm KDC or the server, otherwise the client's identity may be revealed to the server unintentionally. In cases where specific server principals must not have access to the client's identity (for example, an anonymous poll service), the KDC can define server principal specific policy that insure any normal service ticket can NEVER be issued to any of these server principals. 7. Acknowledgements The authors would like to thank the following individuals for their insightful comments and fruitful discussions: Sam Hartman, Martin Rex, Nicolas Williams, Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love Hoernquist Aestrand, Jeffery Hutzelman, and Clifford Neuman. 8. IANA Considerations No IANA actions are required for this document. 9. Normative References [KRBNAM] Zhu, L., "Additonal Kerberos Naming Contraints", draft-ietf-krb-wg-naming, work in progress. [RFC1964] Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC 1964, June 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. [RFC2743] Linn, J., "Generic Security Service Application Program Zhu, et al. Expires December 5, 2006 [Page 7] Internet-Draft Kerberos Anonymity Support June 2006 Interface Version 2, Update 1", RFC 2743, January 2000. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. Zhu, et al. Expires December 5, 2006 [Page 8] Internet-Draft Kerberos Anonymity Support June 2006 Authors' Addresses Larry Zhu Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: lzhu@microsoft.com Paul Leach Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: paulle@microsoft.com Karthik Jaganathan Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: karthikj@microsoft.com Zhu, et al. Expires December 5, 2006 [Page 9] Internet-Draft Kerberos Anonymity Support June 2006 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights 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; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. 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Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Zhu, et al. Expires December 5, 2006 [Page 10]