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Network Working Group                                           M. MyersRequest for Comments: 4806                       TraceRoute Security LLCCategory: Standards Track                                  H. Tschofenig                                           Siemens Networks GmbH & Co KG                                                           February 2007Online Certificate Status Protocol (OCSP) Extensions to IKEv2Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The IETF Trust (2006).Abstract   While the Internet Key Exchange Protocol version 2 (IKEv2) supports   public key based authentication, the corresponding use of in-band   Certificate Revocation Lists (CRL) is problematic due to unbounded   CRL size.  The size of an Online Certificate Status Protocol (OCSP)   response is however well-bounded and small.  This document defines   the "OCSP Content" extension to IKEv2.  A CERTREQ payload with "OCSP   Content" identifies zero or more trusted OCSP responders and is a   request for inclusion of an OCSP response in the IKEv2 handshake.  A   cooperative recipient of such a request responds with a CERT payload   containing the appropriate OCSP response.  This content is   recognizable via the same "OCSP Content" identifier.   When certificates are used with IKEv2, the communicating peers need a   mechanism to determine the revocation status of the peer's   certificate.  OCSP is one such mechanism.  This document applies when   OCSP is desired and security policy prevents one of the IKEv2 peers   from accessing the relevant OCSP responder directly.  Firewalls are   often deployed in a manner that prevents such access by IKEv2 peers   outside of an enterprise network.Myers & Tschofenig          Standards Track                     [Page 1]

RFC 4806                OCSP Extensions to IKEv2           February 2007Table of Contents1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .22.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .33.  Extension Definition . . . . . . . . . . . . . . . . . . . . .43.1.  OCSP Request . . . . . . . . . . . . . . . . . . . . . . .43.2.  OCSP Response  . . . . . . . . . . . . . . . . . . . . . .54.  Extension Requirements . . . . . . . . . . . . . . . . . . . .54.1.  Request for OCSP Support . . . . . . . . . . . . . . . . .54.2.  Response to OCSP Support . . . . . . . . . . . . . . . . .65.  Examples and Discussion  . . . . . . . . . . . . . . . . . . .65.1.  Peer to Peer . . . . . . . . . . . . . . . . . . . . . . .65.2.  Extended Authentication Protocol (EAP) . . . . . . . . . .76.  Security Considerations  . . . . . . . . . . . . . . . . . . .87.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .98.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .99.  Normative References . . . . . . . . . . . . . . . . . . . . .91.  Introduction   Version 2 of the Internet Key Exchange (IKE) protocol [IKEv2]   supports a range of authentication mechanisms, including the use of   public key based authentication.  Confirmation of certificate   reliability is essential in order to achieve the security assurances   public key cryptography provides.  One fundamental element of such   confirmation is reference to certificate revocation status (see   [RFC3280] for additional detail).   The traditional means of determining certificate revocation status is   through the use of Certificate Revocation Lists (CRLs).  IKEv2 allows   CRLs to be exchanged in-band via the CERT payload.   However, CRLs can grow unbounded in size.  Many real-world examples   exist to demonstrate the impracticality of including a multi-megabyte   file in an IKE exchange.  This constraint is particularly acute in   bandwidth-limited environments (e.g., mobile communications).  The   net effect is exclusion of in-band CRLs in favor of out-of-band (OOB)   acquisition of these data, should they even be used at all.   Reliance on OOB methods can be further complicated if access to   revocation data requires use of IPsec (and therefore IKE) to   establish secure and authorized access to the CRLs of an IKE   participant.  Such network access deadlock further contributes to a   reduced reliance on the status of certificate revocations in favor of   blind trust.Myers & Tschofenig          Standards Track                     [Page 2]

RFC 4806                OCSP Extensions to IKEv2           February 2007   OCSP [RFC2560] offers a useful alternative.  The size of an OCSP   response is bounded and small and therefore suitable for in-band   IKEv2 signaling of a certificate's revocation status.   This document defines an extension to IKEv2 that enables the use of   OCSP for in-band signaling of certificate revocation status.  A new   content encoding is defined for use in the CERTREQ and CERT payloads.   A CERTREQ payload with "OCSP Content" identifies zero or more trusted   OCSP responders and is a request for inclusion of an OCSP response in   the IKEv2 handshake.  A cooperative recipient of such a request   responds with a CERT payload containing the appropriate OCSP   response.  This content is recognizable via the same "OCSP Content"   identifier.2.  Terminology   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 inRFC 2119 [RFC2119].   This document defines the following terms:   OCSP request:      An OCSP request refers to the CERTREQ payload that contains a new      content encoding, referred to as OCSP Content, that conforms to      the definition and behavior specified inSection 3.1.   OCSP response:      An OCSP response refers to the CERT payload that contains a new      content encoding, referred to as OCSP Content, that conforms to      the definition and behavior specified inSection 3.2.   OCSP responder:      The term OCSP responder refers to the entity that accepts requests      from an OCSP client and returns responses as defined in [RFC2560].      Note that the OCSP responder does not refer to the party that      sends the CERT message.Myers & Tschofenig          Standards Track                     [Page 3]

RFC 4806                OCSP Extensions to IKEv2           February 20073.  Extension Definition   With reference to Section 3.6 of [IKEv2], the values for the Cert   Encoding field of the CERT payload are extended as follows (see also   the IANA Considerations section of this document):               Certificate Encoding               Value               --------------------               -----               OCSP Content                        143.1.  OCSP Request   A value of OCSP Content (14) in the Cert Encoding field of a CERTREQ   Payload indicates the presence of zero or more OCSP responder   certificate hashes in the Certificate Authority field of the CERTREQ   payload.Section 2.2 of [RFC2560] defines responses, which belong to   one of the following three groups:   (a) the CA who issued the certificate   (b) a Trusted Responder whose public key is trusted by the requester   (c) a CA Designated Responder (Authorized Responder) who holds a       specially marked certificate issued directly by the CA,       indicating that the responder may issue OCSP responses for that       CA   In case of (a), the use of hashes in the CERTREQ message is not   needed since the OCSP response is signed by the CA who issued the   certificate.  In case of (c), the OCSP response is signed by the CA   Designated Responder whereby the sender of the CERTREQ message does   not know the public key in advance.  The presence of OCSP Content in   a CERTREQ message will identify one or more OCSP responders trusted   by the sender in case of (b).   The presence of OCSP Content (14) in a CERTREQ message:   1.  identifies zero or more OCSP responders trusted by the sender;   2.  notifies the recipient of sender's support for the OCSP extension       to IKEv2; and   3.  notifies the recipient of sender's desire to receive OCSP       confirmation in a subsequent CERT payload.Myers & Tschofenig          Standards Track                     [Page 4]

RFC 4806                OCSP Extensions to IKEv2           February 20073.2.  OCSP Response   A value of OCSP Content (14) in the Cert Encoding field of a CERT   Payload indicates the presence of an OCSP response in the Certificate   Data field of the CERT payload.   Correlation between an OCSP response CERT payload and a corresponding   CERT payload carrying a certificate can be achieved by matching the   OCSP response CertID field to the certificate.  See [RFC2560] for the   definition of OCSP response content.4.  Extension Requirements4.1.  Request for OCSP Support   Section 3.7 of [IKEv2] allows for the concatenation of trust anchor   hashes as the Certification Authority value of a single CERTREQ   message.  There is no means however to indicate which among those   hashes, if present, relates to the certificate of a trusted OCSP   responder.   Therefore, an OCSP request, as defined inSection 3.1 above, is   transmitted separate from any other CERTREQ payloads in an IKEv2   exchange.   Where it is useful to identify more than one trusted OCSP responder,   each such identification SHALL be concatenated in a manner identical   to the method documented in Section 3.7 of [IKEv2] regarding the   assembly of multiple trust anchor hashes.   The Certification Authority value in an OCSP request CERTREQ SHALL be   computed and produced in a manner identical to that of trust anchor   hashes as documented in Section 3.7 of [IKEv2].   Upon receipt of an OCSP response CERT payload corresponding to a   prior OCSP request CERTREQ, the CERTREQ sender SHALL incorporate the   OCSP response into path validation logic defined by [RFC3280].   Note that the lack of an OCSP response CERT payload after sending an   OCSP request CERT payload might be an indication that this OCSP   extension is not supported.  As a result, it is recommended that   nodes be configured to require a response only if it is known that   all peers do in fact support this extension.  Otherwise, it is   recommended that the nodes be configured to try OCSP and, if there is   no response, attempt to determine certificate revocation status by   some other means.Myers & Tschofenig          Standards Track                     [Page 5]

RFC 4806                OCSP Extensions to IKEv2           February 20074.2.  Response to OCSP Support   Upon receipt of an OCSP request CERTREQ payload, the recipient SHOULD   acquire the related OCSP-based assertion and produce and transmit an   OCSP response CERT payload corresponding to the certificate needed to   verify its signature on IKEv2 payloads.   An OCSP response CERT payload is transmitted separate from any other   CERT payload in an IKEv2 exchange.   The means by which an OCSP response may be acquired for production of   an OCSP response CERT payload is out of scope of this document.   The Certificate Data field of an OCSP response CERT payload SHALL   contain a DER-encoded OCSPResponse structure as defined in [RFC2560].5.  Examples and Discussion   This section shows the standard IKEv2 message examples with both   peers, the initiator and the responder, using public key based   authentication, CERTREQ and CERT payloads.  The first instance   corresponds to Section 1.2 of [IKEv2], the illustrations of which are   reproduced below for reference.5.1.  Peer to Peer   Application of the IKEv2 extensions defined in this document to the   peer-to-peer exchange defined in Section 1.2 of [IKEv2] is as   follows.  Messages are numbered for ease of reference.        Initiator                             Responder        -----------                           -----------   (1)  HDR, SAi1, KEi, Ni              -->   (2)                                  <-- HDR, SAr1, KEr, Nr,                                            CERTREQ(OCSP Request)   (3)  HDR, SK {IDi, CERT(certificate),-->        CERT(OCSP Response),        CERTREQ(OCSP Request),        [IDr,] AUTH, SAi2, TSi, TSr}   (4)                                  <-- HDR, SK {IDr,                                            CERT(certificate),                                            CERT(OCSP Response),                                            AUTH, SAr2, TSi, TSr}                     OCSP Extensions to Baseline IKEv2Myers & Tschofenig          Standards Track                     [Page 6]

RFC 4806                OCSP Extensions to IKEv2           February 2007   In (2), Responder sends an OCSP request CERTREQ payload identifying   zero or more OCSP responders trusted by the Responder.  In response,   Initiator sends in (3) both a CERT payload carrying its certificate   and an OCSP response CERT payload covering that certificate.  In (3),   Initiator also requests an OCSP response via the OCSP request CERTREQ   payload.  In (4), the Responder returns its certificate and a   separate OCSP response CERT payload covering that certificate.   It is important to note that in this scenario, the Responder in (2)   does not yet possess the Initiator's certificate and therefore cannot   form an OCSP request as defined in [RFC2560].  To bypass this   problem, hashes are used as defined inSection 4.1.  In such   instances, OCSP Requests are simply index values into these data.   Thus, it is easily inferred that OCSP responses can be produced in   the absence of a corresponding request (provided that OCSP nonces are   not used, seeSection 6).   It is also important in extending IKEv2 toward OCSP in this scenario   that the Initiator has certain knowledge that the Responder is   capable of and willing to participate in the extension.  Yet the   Responder will only trust one or more OCSP responder signatures.   These factors motivate the definition of OCSP responder hash   extension.5.2.  Extended Authentication Protocol (EAP)   Another scenario of pressing interest is the use of EAP to   accommodate multiple end users seeking enterprise access to an IPsec   gateway.  Note that OCSP is used for the certificate status check of   the server side IKEv2 certificate and not for certificates that may   be used within EAP methods (either by the EAP peer or the EAP   server).  As with the preceding section, the following illustration   is extracted from [IKEv2].  In the event of a conflict between this   document and [IKEv2] regarding these illustrations, [IKEv2] SHALL   dominate.Myers & Tschofenig          Standards Track                     [Page 7]

RFC 4806                OCSP Extensions to IKEv2           February 2007        Initiator                            Responder        -----------                          -----------   (1)  HDR, SAi1, KEi, Ni              -->   (2)                                  <-- HDR, SAr1, KEr, Nr   (3)  HDR, SK {IDi,                   -->        CERTREQ(OCSP Request),        [IDr,] AUTH, SAi2, TSi, TSr}   (4)                                  <-- HDR, SK {IDr,                                            CERT(certificate),                                            CERT(OCSP Response),                                            AUTH, EAP}   (5)       HDR, SK {EAP}              -->   (6)                                  <-- HDR, SK {EAP (success)}   (7)       HDR, SK {AUTH}             -->   (8)                                  <-- HDR, SK {AUTH, SAr2, TSi,                                            TSr }                      OCSP Extensions to EAP in IKEv2   In the EAP scenario, messages (5) through (8) are not relevant to   this document.6.  Security Considerations   For the reasons noted above, an OCSP request, as defined inSection3.1, is used in place of an OCSP request syntax to trigger production   and transmission of an OCSP response.  OCSP, as defined in [RFC2560],   may contain a nonce request extension to improve security against   replay attacks (seeSection 4.4.1 of [RFC2560] for further details).   The OCSP request defined by this document cannot accommodate nonces.   [RFC2560] deals with this aspect by allowing pre-produced responses.   [RFC2560] points to this replay vulnerability and indicates: "The use   of precomputed responses allows replay attacks in which an old (good)   response is replayed prior to its expiration date but after the   certificate has been revoked.  Deployments of OCSP should carefully   evaluate the benefit of precomputed responses against the probability   of a replay attack and the costs associated with its successful   execution."  Nodes SHOULD make the required freshness of an OCSP   response configurable.Myers & Tschofenig          Standards Track                     [Page 8]

RFC 4806                OCSP Extensions to IKEv2           February 20077.  IANA Considerations   This document defines one new field type for use in the IKEv2 Cert   Encoding field of the Certificate Payload format.  Official   assignment of the "OCSP Content" extension to the Cert Encoding table   of Section 3.6 of [IKEv2] has been acquired from IANA.               Certificate Encoding               Value               --------------------               -----               OCSP Content                        148.  Acknowledgements   The authors would like to thank Russ Housley for his support.   Additionally, we would like to thank Pasi Eronen, Nicolas Williams,   Liqiang (Larry) Zhu, Lakshminath Dondeti, and Paul Hoffman for their   review.  Pasi gave us invaluable last-call comments.  We would also   like to thank Tom Taylor for his Gen-ART review.  Jari Arkko gave us   IESG review comments.9.  Normative References   [IKEv2]    Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",RFC 4306, December 2005.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2560]  Myers, M., Ankney, R., Malpani, A., Galperin, S., and C.              Adams, "X.509 Internet Public Key Infrastructure Online              Certificate Status Protocol - OCSP",RFC 2560, June 1999.   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet              X.509 Public Key Infrastructure Certificate and              Certificate Revocation List (CRL) Profile",RFC 3280,              April 2002.Myers & Tschofenig          Standards Track                     [Page 9]

RFC 4806                OCSP Extensions to IKEv2           February 2007Authors' Addresses   Michael Myers   TraceRoute Security LLC   EMail: mmyers@fastq.com   Hannes Tschofenig   Siemens Networks GmbH & Co KG   Otto-Hahn-Ring 6   Munich, Bavaria  81739   Germany   EMail: Hannes.Tschofenig@siemens.com   URI:http://www.tschofenig.comMyers & Tschofenig          Standards Track                    [Page 10]

RFC 4806                OCSP Extensions to IKEv2           February 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   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 inBCP 78 andBCP 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 athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Myers & Tschofenig          Standards Track                    [Page 11]