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+Network Working Group M. Myers
+Request for Comments: 4806 TraceRoute Security LLC
+Category: Standards Track H. Tschofenig
+ Siemens Networks GmbH & Co KG
+ February 2007
+
+
+ Online Certificate Status Protocol (OCSP) Extensions to IKEv2
+
+Status 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.
+
+
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+Myers & Tschofenig Standards Track [Page 1]
+
+RFC 4806 OCSP Extensions to IKEv2 February 2007
+
+
+Table of Contents
+
+ 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
+ 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
+ 3. Extension Definition . . . . . . . . . . . . . . . . . . . . . 4
+ 3.1. OCSP Request . . . . . . . . . . . . . . . . . . . . . . . 4
+ 3.2. OCSP Response . . . . . . . . . . . . . . . . . . . . . . 5
+ 4. Extension Requirements . . . . . . . . . . . . . . . . . . . . 5
+ 4.1. Request for OCSP Support . . . . . . . . . . . . . . . . . 5
+ 4.2. Response to OCSP Support . . . . . . . . . . . . . . . . . 6
+ 5. Examples and Discussion . . . . . . . . . . . . . . . . . . . 6
+ 5.1. Peer to Peer . . . . . . . . . . . . . . . . . . . . . . . 6
+ 5.2. Extended Authentication Protocol (EAP) . . . . . . . . . . 7
+ 6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
+ 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
+ 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
+ 9. Normative References . . . . . . . . . . . . . . . . . . . . . 9
+
+1. 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]
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+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 in RFC 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 in Section 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 in Section 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.
+
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+Myers & Tschofenig Standards Track [Page 3]
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+RFC 4806 OCSP Extensions to IKEv2 February 2007
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+3. 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 14
+
+3.1. 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.
+
+
+
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+Myers & Tschofenig Standards Track [Page 4]
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+RFC 4806 OCSP Extensions to IKEv2 February 2007
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+3.2. 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 Requirements
+
+4.1. 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 in Section 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.
+
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+Myers & Tschofenig Standards Track [Page 5]
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+RFC 4806 OCSP Extensions to IKEv2 February 2007
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+4.2. 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 IKEv2
+
+
+
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+Myers & Tschofenig Standards Track [Page 6]
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+RFC 4806 OCSP Extensions to IKEv2 February 2007
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+ 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 in Section 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, see Section 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.
+
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+ 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 in Section
+ 3.1, 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 (see Section 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.
+
+
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+Myers & Tschofenig Standards Track [Page 8]
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+RFC 4806 OCSP Extensions to IKEv2 February 2007
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+7. 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 14
+
+8. 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.
+
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+Authors' 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.com
+
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+Full Copyright Statement
+
+ Copyright (C) The IETF Trust (2007).
+
+ This document is subject to the rights, licenses and restrictions
+ contained in BCP 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
+
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+
+Acknowledgement
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
+
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+Myers & Tschofenig Standards Track [Page 11]
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