


Network Working Group                                           M. Myers
Internet-Draft                                   TraceRoute Security LLC
Expires: January 12, 2007                                  H. Tschofenig
                                                                 Siemens
                                                           July 11, 2006


                        OCSP Extensions to IKEv2
                     draft-myers-ikev2-ocsp-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   While IKEv2 supports public key based authentication (PKI), the
   corresponding use of in-band CRLs is problematic due to unbounded CRL
   size.  The size of an OCSP response is however well-bounded and
   small.  This document defines the "OCSP Content" extension to IKEv2.
   A CERTREQ payload with "OCSP Content" identifies one 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



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   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Extension Definition . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  OCSP Request . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  OCSP Response  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Extension Requirements . . . . . . . . . . . . . . . . . . . .  6
     4.1.  OCSP Request . . . . . . . . . . . . . . . . . . . . . . .  6
     4.2.  OCSP Response  . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Examples and Discussion  . . . . . . . . . . . . . . . . . . .  8
     5.1.  Peer to Peer . . . . . . . . . . . . . . . . . . . . . . .  8
     5.2.  Extended Authentication Protocol (EAP) . . . . . . . . . .  9
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
   9.  Normative References . . . . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14



















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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 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 historic 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.

   CRLs can however 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 certificate revocation status in favor of blind
   trust.

   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 one 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.










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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].














































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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 one or more OCSP Responder
   certificate hashes in the Certificate Authority field of the CERTREQ
   payload.

   The presence of OCSP Content (14) in a CERTREQ message:

   1.  identifies one 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.

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.














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4.  Extension Requirements

4.1.  OCSP Request

   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 relates to the certificate of a trusted OCSP responder.

   Therefore an OCSP Request as defined in Section 3.1 above SHALL be
   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].

   The sender of an OCSP Request CERTREQ MAY abort an IKEv2 exchange if
   either:

   1.  the corresponding OCSP Response CERT payload indicates that the
       subject certificate is revoked; OR

   2.  the corresponding OCSP Response CERT payload indicates an OCSP
       error (e.g., malformedRequest, internalError, tryLater,
       sigRequired, unauthorized, etc.).

   The sender of an OCSP Request CERTREQ SHOULD accept an IKEv2 exchange
   if a corresponding OCSP Response CERT payload is not received.  This
   might be an indication that this OCSP extension is not supported.

4.2.  OCSP Response

   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 SHALL be transmitted separate from any



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   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 structure and encoding of the Certificate Data field of an OCSP
   Response CERT payload SHALL be identical to that defined in
   [RFC2560].











































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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}

   In (2) Responder sends an OCSP Request CERTREQ payload identifying
   one or more OCSP responders trusted by 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) 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.  [RFC2560] allows for pre-produced responses.
   It is thus easily inferred that OCSP responses can be produced in the
   absence of a corresponding request (OCSP nonces notwithstanding).  In
   such instances OCSP Requests are simply index values into these data.

   It is also important in extending IKEv2 towards OCSP in this scenario
   that the Initiator has certain knowledge that the Responder is



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   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.  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.


        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 }

   In the EAP scenario, messages (5) through (8) are not relevant to
   this document.  Note that while [IKEv2] allows for the optional
   inclusion of a CERTREQ in (2), this document asserts no need of its
   use.  It is assumed that environments including this optional payload
   and yet wishing to implement the OCSP extension to IKEv2 are
   sufficiently robust as to accommodate this redundant payload.










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6.  Security Considerations

   For the reasons noted above, OCSP request as defined in Section 3.1
   is used in place of 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] needs to be acquired from IANA.

               Certificate Encoding               Value
               --------------------               -----
               OCSP Content                        14









































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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.

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
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@siemens.com
   URI:   http://www.tschofenig.com


































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