





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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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.






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   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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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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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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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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   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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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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