NETWORK WORKING GROUP L. Zhu Internet-Draft P. Leach Updates: 4120 (if approved) K. Jaganathan Expires:
December 5, 2006January 17, 2007 Microsoft Corporation June 3,July 16, 2006 Anonymity Support for Kerberos draft-ietf-krb-wg-anon-00draft-ietf-krb-wg-anon-01 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.January 17, 2007. 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. 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 . . . . . . . . . . . . . . . . . 67 6. Security Considerations . . . . . . . . . . . . . . . . . . . 78 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 78 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 78 9. Normative References . . . . . . . . . . . . . . . . . . . . . 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 910 Intellectual Property and Copyright Statements . . . . . . . . . . 1011 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 AnThe anonymous ticketKerberos realm name is a ticket that has all of the following properties: o The client's principalreserved realm name as defined in [KRBNAM] and its value is the anonymous Kerberos principal name.literal "RESERVED:ANONYMOUS". The anonymous Kerberos principal name is defined as follows: it isa reserved Kerberos principal name as defined in [KRBNAM], theits name-type [RFC4120] is KRB_NT_RESRVED [KRBNAM], and the name- stringits name-string [RFC4120] is a sequence of two KerberosString components: "RESERVED", "ANONYMOUS". o The client's realm name isIn this specification, only the anonymous kerberos realm name. The anonymous Kerberos realm name is defined as follows: it is a reserved realmclient name as defined in [KRBNAM] andor the client realm name is the literal "RESERVED:ANONYMOUS". o The authtime field in the ticket is set tocan be anonymous; the time ofserver name or the ticket request,server realm can not the time of the initial authentication for the principal who has made the request. obe anonymous. The transited field [RFC4120] can either contain the client's authentication path or contain theof a ticket is an anonymous authentication path defined as follows:if the tr-type field of the transited fieldTransitedEncoding type [RFC4120] 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. theNO-TRANSITED-INFO TBA This transited field does not contain the anonymous authentication path as defined above), then the reply ticket, if any, MUST NOT containencoding type indicates that there is no information available about the anonymousauthentication path. For application servers, no transited policyThe anonymous ticket flag is defined foras bit TBA (with the first bit being bit 0) in the TicketFlags: TicketFlags ::= KerberosFlags -- anonymous(TBA) -- TicketFlags and KerberosFlags are defined in [RFC4120] An anonymous authentication path, butticket is a ticket that has all of the transited checks would still apply if anfollowing properties: o The cname field [RFC4120] contains the anonymous ticketKerberos principal name. o The crealm field [RFC4120] contains a "normal" authentication path. Note thateither the realm name of the client who made the request or the anonymous kerberos realm name, based on the local policy of the KDC. o The transited field [RFC4120] can contain either the client's "normal" authentication path in an anonymous ticket can be a partial path, thus it may not be sufficientaccording to identifySection 220.127.116.11 of [RFC4120] or the originating client realm.anonymous authentication path. o It contains no information that can reveal the client's identity other than, at most,identity. However the client'sticket can contain the client realm orand the realm(s)realms on the authentication path. o Thepath, and the authorization data may provide additional information of the client. For example, an anonymous ticket flag (as defined later in this section)principal that is set.only identifiable within a particular group of users can be implemented by using authorization data. o 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]set. Notes: The anonymous ticket flag MUST NOT be set by implementations of this specification if the ticket is not an anonymous ticket as definedticket. The server principal name and the server realm in this section.a cross-realm referral TGT are not dependent on whether the client is the anonymous principal or not. The request-anonymous KDC option is defined as bit 14TBA (with the first bit being bit 0) in the KDCOptions: KDCOptions ::= KerberosFlags -- request-anonymous(14)request-anonymous(TBA) -- KDCOptions and KerberosFlags are defined in [RFC4120] The anonymous transited encoding type is defined as follows: NO-TRANSITED-INFO 0 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 ASAuthentication Exchange (AS) or TGSTicket Granting Service (TGS) request [RFC4120]. The client can request an anonymous TGT based on a normal TGT. Note that if the serviceticket 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 KDCpropagating authorization data, care MUST be taken by the TGS to ensure that implements this specificationthe client confidentiality is not violated: the TGS MUST NOT carry informationeither fail the request or remove authorization data that canmay reveal the client's identity,identity. An optional authorization element unknown by the TGS MUST be removed if it can be ignored (such as ones enclosed in the AD-IF-RELEVANT or the AD-KDCIssued containers [RFC4120]). The TGS can strip critical unknown authorization data if such data do not convey any rights based on the requesting client's identity. Here is a table of the known authorization-data elements, flagged with whether they interfere with client anonymity and recommendations for how to process them. ad-type References Can Breach Confidentiality? ------------------------------------------------------------------ AD-IF-RELEVANT RFC4120 Yes, remove if unknown AD-KDCIssued RFC4120 Yes, remove if unknown AD-AND-OR RFC4120 Yes, remove if unknown AD-MANDATORY-FOR-KDC RFC4120 Yes, fail the request if unknown If it is inappropriate to remove an authorization element from the TGS request intoin order to produce an anonymous ticket, the returnedKDC MUST return an error message with the code KDC_ERR_POLICY [RFC4120]. When policy allows, the KDC issues an anonymous ticket. It shouldThe client realm in the anonymous ticket can be noted that unless otherwise specified by this documentthe anonymous realm name based on local policy. The client principalname and the client realm inthe Kerberos messagesEncKDCRepPart of the reply [RFC4120] should beMUST match with the corresponding client name and the client realm that can uniquely identifyof the anonymous reply ticket. The client principal to the KDC, notthen MUST use the anonymousclient principalname and the emptyclient realm name. For example,returned in the EncKDCRepPart in subsequent message exchanges when using that anonymous ticket. If there is a key known by both the client nameand realmthe KDC for encrypting the KDC reply, the cname field in the request body[RFC4120] can be anonymous. If the client is anonymous and the EncKDCRepPart ofKDC does not have a key to encrypt the reply, the KDC MUST return an error message with the code KDC_ERR_NULL_KEY [RFC4120]. For AS exchange, if the reply [RFC4120] are identifierskey is selected from the client keys (for example, as described in Section 3.1.3 of [RFC4120]), then the client principal. In other words,principal MUST NOT be anonymous. The client can use the client name andkeys to request an anonymous TGT in the AS request. The anonymous client realmname, for example, can be used in conjunction with PKINIT [RFC4556]. An anonymous PKINIT client can authenticate the KDC based on the KDC certificate. For TGS exchange, the reply key is selected according to Section 3.3.3 of [RFC4120] as normal. The KDC fills out the transited field of the anonymous ticket in the reply as follows: If the service ticket in a TGS request is an anonymous ticket with a "normal" authentication path, then the authentication path in the reply ticket MUST also contain a "normal" authentication path: the TGS MUST add the name of the previous realm. However, if the service ticket in a TGS request is an anonymous ticket with an anonymous authentication path, then the reply ticket can contain either an anonymous authentication path or a "normal" authentication path, based on the local policy of the KDC. Thus a "normal" authentication path in an anonymous ticket can be a partial path: it may not include all the intermediate realms on the authentication path. The KDC fills out the authtime field of the anonymous ticket in the reply as follows: If the anonymous ticket is returned in an AS exchange, the authtime field of the ticket contains the request time. If the anonymous ticket is returned in a TGS exchange, the EncKDCRepPart does not match with thatauthtime field contains the time of the returnedinitial authentication for the principal who has made the request. An anonymous ticket. If either local policy prohibits issuingticket can be renewed, and the authtime field of anonymous tickets or ita renewed ticket is inappropriate to remove information (such as restrictions) fromthe TGS requestauthtime in order to produce an anonymous ticket,the KDC MUST return an error message withanonymous ticket that the code KDC_ERR_POLICY [RFC4120].renewed ticket was based on. If a client requires anonymous communication then the client shouldMUST check to make sure that the resultingticket in the reply 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 beNo transited policy checking is needed for the anonymous client principal name and the client realmauthentication path. However, transited policy checks defined in Section 2.7 of the Authenticator MUST be[RFC4120] would apply to an empty KerberosString [RFC4120].anonymous ticket that contains a "normal" authentication path. A server accepting suchan 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 the name_type GSS_C_NT_ANONYMOUS [RFC2743] to represent anthe anonymous identity. In addition, according toSection 2.1.1 of [RFC1964] defines the single string representation of the anonymous clienta Kerberos principal name can be "RESERVED/ANONYMOUS" or "RESERVED/ ANONYMOUS@RESERVED:ANONYMOUS"with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. Implementations conformingFor the anonymous principals, the name component within the exportable name as defined in Section 2.1.3 of [RFC1964] MUST signify the realm name according to Section 2.1.1 of [RFC1964]. In this specification MUST be able to acceptonly the GSS_C_NT_ANONYMOUS name form andclient/initiator can be the GSS_KRB5_NT_PRINCIPAL_NAME name forms, and consider them equivalent.anonymous identity. Portable initiators are RECOMMENDED to use default credentials whenever possible, and request anonymity only through the input anon_req_flag [RFC2743] to GSS_Init_Sec_Context(). 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 areis 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. If the KDC that issued an anonymous ticket were to maintain records of the association of identities to an anonymous ticket, then someone obtaining such records could breach the anonymity. Additionally, the implementation of most (for now all) KDC's respond to requests at the time that they are received. Traffic analasys on the connection to the KDC will allow an attacket to match client identities to anonymous tickets issued. Because there are plaintext parts of the tickets that are exposed on the wire, such matching by a third party observer is relatively straigtforward. 7. Acknowledgements The authors would like to thank the following individuals for their insightful comments and fruitful discussions: Sam Hartman, Clifford Neuman, Martin Rex, Nicolas Williams, Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love Hoernquist Aestrand, Jeffery Hutzelman,and Clifford Neuman.Jeffery Hutzelman. 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 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. [RFC4556] Zhu, L. and B. Tung, "Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)", RFC 4556, June 2006. Authors' Addresses Larry Zhu Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: firstname.lastname@example.org Paul Leach Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: email@example.com Karthik Jaganathan Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: firstname.lastname@example.org 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. 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