NETWORK WORKING GROUP L. Zhu Internet-Draft P. Leach Updates: 4120 (if approved) K. Jaganathan
Expires: January 17, 2007Intended status: Standards Track Microsoft Corporation July 16,Expires: April 14, 2007 October 11, 2006 Anonymity Support for Kerberos draft-ietf-krb-wg-anon-01draft-ietf-krb-wg-anon-02 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 January 17,April 14, 2007. Copyright Notice Copyright (C) The Internet Society (2006). Abstract This document defines extensions to the use of anonymousKerberos ticketsprotocol for the purpose of authenticatingKerberos client to authenticate the serversKerberos Key Distribution Center and enablingthe Kerberos server, without revealing the client's identity. These extensions can be used to secure communication between a client and a server, without identifyingthe anonymous client toand 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 . . . . . . . . . . . . . . . . . 7 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 89 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 89 9. Normative References . . . . . . . . . . . . . . . . . . . . . 89 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 11 1. Introduction In certain situations or environments,situations, 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 isExtensions to [RFC4120] are specified in this document by which a client requestscan authenticate the KDC and request an anonymous ticket andticket. The client can use thatthe anonymous ticket to authenticate the server and secureprotect subsequent client-server communications. This providesThese extensions provide Kerberos with functional equivalence to TLS [RFC2246]Transport Layer Security (TLS) [RFC4346]. By using the extensions defined in environments where Kerberos is a more attractive authentication mechanism. Usingthis mechanism,specification, the client has toMAY reveal its identity in its initial request to its own Key Distribution Center (KDC) [RFC4120], and thenKDC, but it can remain anonymous thereafter to KDCs on the cross- realmcross-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 The anonymous Kerberos realm name is a reserved realm name as defined in [KRBNAM] and itsbased on [KRBNAM]. The value is the literal "RESERVED:ANONYMOUS". The anonymous Kerberos principal name is a reserved Kerberos principal name as defined in [KRBNAM], itsbased on [KRBNAM]. The value of the name-type [RFC4120]field is KRB_NT_RESRVED [KRBNAM], and itsthe value of the name-string [RFC4120]field is a sequence of two KerberosString components: "RESERVED", "ANONYMOUS". InNote that in this specification, onlythe clientanonymous principal name or the clientand realm can be anonymous;are only applicable to the server name orclient in Kerberos messages, the server realm can notMUST NOT be anonymous.anonymous in any Kerberos message. The transited field [RFC4120] of a ticket is an anonymous authentication path if the tr-type field of the TransitedEncoding type [RFC4120] is NO-TRANSITED-INFO and the contents field is an empty OCTET STRING. NO-TRANSITED-INFO TBA This transited encoding type indicatesmeans that there isno information available aboutof the authentication path.path is disclosed. The anonymous ticket flag is defined as 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 ticket is a ticket that has all of the following properties: o The cname field [RFC4120] contains the anonymous Kerberos principal name. o The crealm field [RFC4120] contains either the client's realm name of the client who made the requestor the anonymous kerberosrealm name, based on the local policy of the KDC.name. o The transited field [RFC4120] can contain either the client's "normal"authentication path according toas described in Section 184.108.40.206 of [RFC4120] or the anonymous authentication path. o ItThe anonymous ticket contains no information that can reveal the client's identity. However the ticket canMAY contain the client realm and the realms on the authentication path, and theauthorization data maythat MAY provide additionalinformation ofrelated to the client.client's identity. For example, an anonymous principal that is only identifiable within a particular group of users can be implemented byusing authorization data.data and such authorization data, if included in the anonymous ticket, shall disclose the client's membership of that group. o The anonymous ticket flag is set. Notes:The anonymous ticket flag MUST NOT be set by implementations of this specification if the ticketrequest-anonymous KDC option is not an anonymous ticket. 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. The request-anonymous KDC option is defined as bit TBA (withdefined as bit TBA (with the first bit being bit 0) in the KDCOptions: KDCOptions ::= KerberosFlags -- request-anonymous(TBA) -- KDCOptions and KerberosFlags are defined in [RFC4120] 4. Protocol Description In order to request an anonymous ticket, the client sets the request- anonymous KDC option in an Authentication Exchange (AS) or Ticket Granting Service (TGS) request [RFC4120]. The client can request an anonymous TGT based on a normal TGT. Note that ifIf the client wishes to authenticate the KDC anonymously, it sets the client name as anonymous in the AS exchange and provides a PA_PK_AS_REQ pre- authentication data [RFC4556] where both the signerInfos field and the certificates field of the SignedData [RFC3852] of PA_PK_AS_REQ are empty. Because the anonymous client does not have an associated asymmetric key pair, the client MUST use the Diffie-Hellman key agreement method by filling in the Diffie-Hellman domain parameters in the clientPublicValue [RFC4556]. If the ticket in the PA-TGS-REQ [RFC4120] of the TGS request is anonymous, or if the client in the AS request is anonymous, the request-anonymous KDC option MUST be set in the request. When propagating authorization data, care MUST be taken byUpon receiving the TGS to ensure thatAS request with a PA_PK_AS_REQ from the client confidentiality is not violated:anonymous client, the TGS MUST either failKDC skips the request or remove authorization data that may revealchecks for the client's identity. An optional authorization element unknown bysignature and the TGS MUST be removed if it can be ignoredclient's public key (such as ones enclosed inthe AD-IF-RELEVANT orverification of the AD-KDCIssued containers [RFC4120]). The TGS can strip critical unknown authorization data if such data do not convey any rights based onbinding between the requestingclient's identity. Here is a table ofpublic key and the known authorization-data elements, flagged with whether they interfere withclient anonymityname), but performs otherwise- applicable checks, and recommendations for howproceeds as normal according 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[RFC4556]. For example, the requestAS MUST check if unknown If it is inappropriate to remove an authorization element fromthe TGSclient's Diffie-Hellman domain parameters are acceptable. The Diffie-Hellman key agreement method MUST be used and the reply key is derived according to Section 220.127.116.11 of [RFC4556]. If the clientPublicValue is not present in the request, the KDC MUST return a KRB-ERROR [RFC4120] with the code KDC_ERR_PUBLIC_KEY_ENCRYPTION_NOT_SUPPORTED [RFC4556] and there is no accompanying e-data. The client that made the anonymous request can authenticate the KDC based on the KDC's signature in the reply. If the KDC does not have an asymmetric key pair, it MAY reply anonymously. In which case, both the signerInfos field and the certificates field of the SignedData [RFC3852] of PA_PK_AS_REP in the reply are empty. The server name in order to producean anonymous ticket,reply contains the name of the TGS. Upon receipt of an anonymous KDC reply, the client MUST return an error messagereject the returned ticket if it can not authenticate the KDC otherwise. The client can use its keys to mutually authenticate with the code KDC_ERR_POLICYKDC, and request an anonymous TGT in the AS request. And in that case, the reply key is selected as normal according to Section 3.1.3 of [RFC4120]. For the TGS exchange, the reply key is selected as normal according to Section 3.3.3 of [RFC4120]. When policy allows, the KDC issues an anonymous ticket. TheBased on local policy, the client realm in the anonymous ticket can be the anonymous realm name based on local policy. Theor the realm of the KDC. However, in all cases, the client name and the client realm in the EncKDCRepPart of the reply [RFC4120] MUST match with the corresponding client name and the client realm of the anonymous reply ticket.ticket in the reply. The client thenMUST use the client name and the client realm returned in the EncKDCRepPart in subsequent message exchanges when using thatthe obtained anonymous ticket. If there is a key known by both the client and the KDC for encrypting the KDC reply, the cname field inDuring the request [RFC4120] canTGS request, when propagating authorization data, care MUST be anonymous. Iftaken by the TGS to ensure that the client confidentiality is anonymous and the KDC doesnot have a key to encrypt the reply, the KDCviolated. The TGS MUST return an error message witheither fail the code KDC_ERR_NULL_KEY [RFC4120]. For AS exchange, ifrequest or remove authorization data that may reveal the reply key is selected fromclient's identity. An optional authorization element unknown by the client keys (for example,TGS MUST be removed if it can be ignored (such as describedones enclosed in Section 3.1.3 of [RFC4120]), thenthe client principal MUST NOT be anonymous.AD-IF-RELEVANT structure). The clientTGS can useonly strip critical unknown authorization data if the client keysticket does not convey any rights such as those conveyed by a KDCIssued authorization data element. If a ticket contains a KDCIssued authorization data element, then no other authorization data elements may be removed if they could serve to request an anonymous TGT inlimit the AS request. The anonymous client name, for example, can be used in conjunctionrights conveyed by the KDCIssued element. Here is a table of the known authorization-data elements, tagged with whether they interfere with PKINIT [RFC4556]. An anonymous PKINITclient can authenticate the KDC based onanonymity 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, fail the KDC certificate. For TGS exchange,request if unknown AD-AND-OR RFC4120 Yes, remove if unknown AD-MANDATORY-FOR-KDC RFC4120 Yes, fail the reply key is selected according to Section 3.3.3 of [RFC4120] as normal.request if unknown 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: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 thelocal policy of the KDC. Thus a "normal" authentication path in an anonymous ticket can be a partial path: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 authtime field contains the timeauthtime of the initial authentication for the principal who has madeticket in the request.PA-TGS-REQ [RFC4120]. An anonymous ticket can be renewed, and the authtime field of a renewed ticket is the authtime in the anonymous ticket thaton which the renewed ticket was based on.based. If it is inappropriate to remove an authorization element 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 the client is anonymous and the KDC does not have a key to encrypt the reply, the KDC MUST return an error message with the code KDC_ERR_NULL_KEY [RFC4120] and there is no accompanying e-data. If a client requires anonymous communication then the client MUST check to make sure that the ticket in the reply is actually anonymous by checking the presence of the anonymous ticket flag. BecauseThis is because KDCs ignore unknown KDC options, aoptions. 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]. No transited policy checking is needed for the anonymous authentication path. However, transited policy checks defined in Section 2.7 of [RFC4120] would apply to an anonymous ticket that contains a "normal" authentication path. A server accepting 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 ofthe initiator/ client software (toinitiator/client 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.initiator/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 the anonymous identity. In addition, Section 2.1.1 of [RFC1964] defines the single string representation of a Kerberos principal name with the name_type GSS_KRB5_NT_PRINCIPAL_NAME. For 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]. InNote that in this specification only the client/initiator can be the anonymous identity.anonymous. 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. AThis is because 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 is indeed anonymous before communicating with the cross-realm KDC or theserver, otherwise the client's identity may be revealed to the serverunintentionally. 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 implementationimplementations of most (for now all) KDC's respond to requests at the time that they are received. Traffic analasysanalysis on the connection to the KDC will allow an attacketattacker 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.straightforward. 7. Acknowledgements The authors would like to thankClifford Neuman contributed the following individualscore notions of this document. Martin Rex wrote the text for their insightful commentsGSS-API considerations. Nicolas Williams reviewed the GSS-API considerations section and fruitful discussions:suggested ideas for improvements. Sam Hartman, Clifford Neuman, Martin Rex,Hartman and Nicolas Williams,Williams were great champions of this work. In addition, the following individuals made significant contributions: Jeffery Altman, Tom Yu, Chaskiel M Grundman, Love Hoernquist Aestrand, and Jeffery Hutzelman. 8. IANA Considerations NoSection 3 defines the anonymous Kerberos name and the anonymous Kerberos realm based on [KRBNAM]. The IANA actions are requiredregistry for [KRBNAM] need to be updated to add references to 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. [RFC3852] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.1", RFC 4346, April 2006. [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 Full Copyright Statement Copyright (C) The Internet Society (2006). 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