path: root/doc/rfc/rfc9593.txt

Internet Engineering Task Force (IETF)                        V. Smyslov
Request for Comments: 9593                                    ELVIS-PLUS
Category: Standards Track                                      July 2024
ISSN: 2070-1721


Announcing Supported Authentication Methods in the Internet Key Exchange
                       Protocol Version 2 (IKEv2)

Abstract

   This specification defines a mechanism that allows implementations of
   the Internet Key Exchange Protocol Version 2 (IKEv2) to indicate the
   list of supported authentication methods to their peers while
   establishing IKEv2 Security Associations (SAs).  This mechanism
   improves interoperability when IKEv2 partners are configured with
   multiple credentials of different types for authenticating each
   other.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9593.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology and Notation
   3.  Protocol Details
     3.1.  Exchanges
     3.2.  SUPPORTED_AUTH_METHODS Notify Message Type
       3.2.1.  2-Octet Announcement
       3.2.2.  3-Octet Announcement
       3.2.3.  Multi-octet Announcement
   4.  Interaction with IKEv2 Extensions concerning Authentication
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Appendix A.  Examples of Announcing Supported Authentication
           Methods
     A.1.  No Need to Use the IKE_INTERMEDIATE Exchange
     A.2.  With Use of the IKE_INTERMEDIATE Exchange
   Acknowledgments
   Author's Address

1.  Introduction

   The Internet Key Exchange Protocol Version 2 (IKEv2), defined in
   [RFC7296], performs authenticated key exchange in IPsec.  IKEv2,
   unlike its predecessor IKEv1, defined in [RFC2409], doesn't include a
   mechanism to negotiate an authentication method that the peers would
   use to authenticate each other.  It is assumed that each peer selects
   whichever authentication method it thinks is appropriate, depending
   on authentication credentials it has.

   This approach generally works well when there is no ambiguity in
   selecting authentication credentials.  SA establishment failure
   between peers may occur when there are several credentials of
   different types configured on one peer, while only some of them are
   supported on the other peer.  Another problem situation is when a
   single credential may be used to produce different types of
   authentication tokens (e.g., signatures of different formats).  Since
   IKEv2 requires that each peer use exactly one authentication method,
   and it doesn't provide means for peers to indicate to the other side
   which authentication methods they support, the peer that supports a
   wider range of authentication methods (or authentication token
   formats) could improperly select a method (or format) that is not
   supported by the other side.

   Emerging post-quantum signature algorithms may bring additional
   challenges for implementations, especially if so-called hybrid
   schemes are used (e.g., see [COMPOSITE-SIGS]).

   This specification defines an extension to the IKEv2 protocol that
   allows peers to announce their supported authentication methods, thus
   decreasing risks of SA establishment failure in situations when there
   are several ways for the peers to authenticate themselves.

2.  Terminology and Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Protocol Details

   When establishing an IKE SA, each party may send to its peer a list
   of the authentication methods it supports and is configured to use.
   For this purpose, this specification introduces a new Notify Message
   Type SUPPORTED_AUTH_METHODS.  The Notify payload with this Notify
   Message Type is utilized to convey the supported authentication
   methods of the party sending it.  The sending party may additionally
   specify that some of the authentication methods are only for use with
   the particular trust anchors.  The receiving party may take this
   information into consideration when selecting an algorithm for its
   authentication (i.e., the algorithm used for calculation of the AUTH
   payload) if several alternatives are available.  To simplify the
   receiver's task of linking the announced authentication methods with
   the trust anchors, the protocol ensures that the
   SUPPORTED_AUTH_METHODS notification is always co-located with the
   CERTREQ payload in the same message.

3.1.  Exchanges

   The initiator starts the IKE_SA_INIT exchange as usual.  If the
   responder is willing to use this extension, it includes a new
   notification SUPPORTED_AUTH_METHODS in the IKE_SA_INIT response
   message.  This notification contains a list of authentication methods
   supported by the responder, ordered by their preference.

   Initiator                              Responder
   -----------                            -----------
   HDR, SAi1, KEi, Ni -->
                                      <-- HDR, SAr1, KEr, Nr, [CERTREQ,]
                                        [N(SUPPORTED_AUTH_METHODS)(...)]

                     Figure 1: The IKE_SA_INIT Exchange

   If the initiator doesn't support this extension, it ignores the
   received notification as an unknown status notify.

   Regardless of whether the notification is received, if the initiator
   supports and is willing to use this extension, it includes the
   SUPPORTED_AUTH_METHODS notification in the IKE_AUTH request message,
   with a list of authentication methods supported by the initiator,
   ordered by their preference.

   Initiator                              Responder
   -----------                            -----------
   HDR, SK {IDi, [CERT,] [CERTREQ,]
   [IDr,] AUTH, SAi2, TSi, TSr,
   [N(SUPPORTED_AUTH_METHODS)(...)] }  -->
                                      <-- HDR, SK {IDr, [CERT,]
                                               AUTH, SAr2, TSi, TSr }

                      Figure 2: The IKE_AUTH Exchange

   Because the responder sends the SUPPORTED_AUTH_METHODS notification
   in the IKE_SA_INIT exchange, it must take into account that the
   response message could grow so much that the IP fragmentation might
   take place.

   *  the SUPPORTED_AUTH_METHODS notification to be included is so
      large, that the responder suspects that IP fragmentation of the
      resulting IKE_SA_INIT response message may happen;

   *  both peers support the IKE_INTERMEDIATE exchange, defined in
      [RFC9242] (i.e., the responder has received and is going to send
      the INTERMEDIATE_EXCHANGE_SUPPORTED notification);

   then the responder MAY choose not to send an actual list of the
   supported authentication methods in the IKE_SA_INIT exchange and
   instead ask the initiator to start the IKE_INTERMEDIATE exchange for
   the list to be sent in.  This would allow using IKE fragmentation
   [RFC7383] for long messages (which cannot be used in the IKE_SA_INIT
   exchange), thus avoiding IP fragmentation.  In this case, the
   responder includes a SUPPORTED_AUTH_METHODS notification containing
   no data in the IKE_SA_INIT response.

   If the initiator receives the empty SUPPORTED_AUTH_METHODS
   notification in the IKE_SA_INIT exchange, it means that the responder
   is going to send the list of the supported authentication methods in
   the IKE_INTERMEDIATE exchange.  If this exchange is to be initiated
   anyway for some other reason, then the responder MAY use it to send
   the SUPPORTED_AUTH_METHODS notification.  Otherwise, the initiator
   MAY start the IKE_INTERMEDIATE exchange for this sole purpose by
   sending an empty IKE_INTERMEDIATE request.  The initiator MAY also
   indicate its identity (and possibly the perceived responder's
   identity too) by including the IDi payload (possibly along with the
   IDr payload) in the IKE_INTERMEDIATE request.  This information could
   help the responder to send back only those authentication methods
   that are configured to be used for authentication of this particular
   initiator.  If these payloads are sent, they MUST be identical to the
   IDi/IDr payloads sent later in the IKE_AUTH request.

   If the responder has sent any CERTREQ payload in the IKE_SA_INIT,
   then it SHOULD resend the same payload(s) in the IKE_INTERMEDIATE
   response containing the SUPPORTED_AUTH_METHODS notification if any of
   the included Announcements has a non-zero Cert Link field (see
   Sections 3.2.2 and 3.2.3).  This requirement allows peers to have a
   list of Announcements and a list of CAs in the same message, which
   simplifies their linking.  Note that this requirement is always
   fulfilled for the IKE_SA_INIT and IKE_AUTH exchanges.  However, if
   for any reason the responder doesn't resend CERTREQ payload(s) in the
   IKE_INTERMEDIATE exchange, then the initiator MUST NOT abort
   negotiation.  Instead, the initiator MAY either link the
   Announcements to the CAs received in the IKE_SA_INIT response, or it
   MAY ignore the Announcements containing links to CAs.

   If multiple IKE_INTERMEDIATE exchanges take place during IKE SA
   establishments, it is RECOMMENDED that the responder use the last
   IKE_INTERMEDIATE exchange (the one just before IKE_AUTH) to send the
   list of supported authentication methods.  However, it is not always
   possible for the responder to know how many IKE_INTERMEDIATE
   exchanges the initiator will use.  In this case the responder MAY
   send the list in any IKE_INTERMEDIATE exchange.  If the initiator
   sends IDi/IDr in an IKE_INTERMEDIATE request, then it is RECOMMENDED
   that the responder sends back the list of authentication methods in
   the response.

   Initiator                              Responder
   -----------                            -----------
   HDR, SAi1, KEi, Ni -->
                                      <-- HDR, SAr1, KEr, Nr, [CERTREQ,]
                                          [N(SUPPORTED_AUTH_METHODS)()]

   HDR, SK {..., [IDi, [IDr,]]}  -->
                                      <-- HDR, SK {..., [CERTREQ,]
                                      [N(SUPPORTED_AUTH_METHODS)(...)] }

   HDR, SK {IDi, [CERT,] [CERTREQ,]
   [IDr,] AUTH, SAi2, TSi, TSr,
   [N(SUPPORTED_AUTH_METHODS)(...)] }  -->
                                      <-- HDR, SK {IDr, [CERT,]
                                               AUTH, SAr2, TSi, TSr }

         Figure 3: Using the IKE_INTERMEDIATE Exchange for Sending
                           Authentication Methods

   Note that sending the SUPPORTED_AUTH_METHODS notification and using
   information obtained from it are optional for both the initiator and
   the responder.  If multiple SUPPORTED_AUTH_METHODS notifications are
   included in a message, all their announcements form a single ordered
   list, unless overridden by other extension (see Section 4).

3.2.  SUPPORTED_AUTH_METHODS Notify Message Type

   The format of the SUPPORTED_AUTH_METHODS Notify payload is shown
   below.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Next Payload  |C|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Protocol ID  |   SPI Size    |      Notify Message Type      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~          List of Supported Auth Methods Announcements         ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 4: SUPPORTED_AUTH_METHODS Notify Payload Format

   The Notify payload format is defined in Section 3.10 of [RFC7296].
   When a Notify payload of type SUPPORTED_AUTH_METHODS is sent, the
   Protocol ID field is set to 0, the SPI Size is set to 0 (meaning
   there is no SPI field), and the Notify Message Type is set to 16443.

   Notification data contains the list of supported authentication
   methods announcements.  Each individual announcement is a variable-
   size data blob whose format depends on the announced authentication
   method.  The blob always starts with an octet containing the length
   of the blob followed by an octet containing the authentication
   method.  Authentication methods are represented as values from the
   "IKEv2 Authentication Method" registry defined in [IKEV2-IANA].  The
   meaning of the remaining octets of the blob, if any, depends on the
   authentication method.  Note that, for the currently defined
   authentication methods, the length octet fully defines both the
   format and the semantics of the blob.

   If more authentication methods are defined in the future, the
   corresponding documents must describe the semantics of the
   announcements for these methods.  Implementations MUST ignore
   announcements whose semantics they don't understand.

3.2.1.  2-Octet Announcement

   If the announcement contains an authentication method that is not
   concerned with public key cryptography, then the following format is
   used.

                        1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Length (=2)  |  Auth Method  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: 2-Octet Announcement Format

   Length:  Length of the blob in octets; must be 2 for this case.

   Auth Method:  Announced authentication method.

   This format is applicable for the authentication methods "Shared Key
   Message Integrity Code" (2) and "NULL Authentication" (13).  Note
   that the authentication method "Generic Secure Password
   Authentication Method" (12) would also fall in this category;
   however, it is negotiated separately (see [RFC6467]), and for this
   reason there is no point to announce it via this mechanism.  See also
   Section 4.

3.2.2.  3-Octet Announcement

   If the announcement contains an authentication method that is
   concerned with public key cryptography, then the following format is
   used.  This format allows linking the announcement with a particular
   trust anchor from the Certificate Request payload.

                        1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Length (=3)  |  Auth Method  |   Cert Link   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 6: 3-Octet Announcement Format

   Length:  Length of the blob in octets; must be 3 for this case.

   Auth Method:  Announced authentication method.

   Cert Link:  Links this announcement with a particular CA.

   If the Cert Link field contains a non-zero value N, it means that the
   announced authentication method is intended to be used only with the
   N-th trust anchor (CA certificate) from the Certificate Request
   payload(s) sent by this peer.  If it is zero, then this
   authentication method may be used with any CA.  If multiple CERTREQ
   payloads were sent, the CAs from all of them are treated as a single
   list for the purpose of the linking.  If no Certificate Request
   payload were received, the content of this field MUST be ignored and
   treated as zero.

   This format is applicable for the authentication methods "RSA Digital
   Signature" (1), "DSS Digital Signature" (3), "ECDSA with SHA-256 on
   the P-256 curve" (9), "ECDSA with SHA-384 on the P-384 curve" (10)
   and "ECDSA with SHA-512 on the P-521 curve" (11).  Note, however,
   that these authentication methods are currently superseded by the
   "Digital Signature" (14) authentication method, which has a different
   announcement format, described below.

3.2.3.  Multi-octet Announcement

   The following format is currently used only with the "Digital
   Signature" (14) authentication method.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Length (>3)  |  Auth Method  |   Cert Link   |               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
   |                                                               |
   ~                      AlgorithmIdentifier                      ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 7: Multi-octet Announcement Format

   Length:  Length of the blob in octets; must be greater than 3 for
      this case.

   Auth Method:  Announced authentication method.  At the time of
      writing this document, only value 14 ("Digital Signature") is
      allowed.

   Cert Link:  Links this announcement with a particular CA; see
      Section 3.2.2 for details.

   AlgorithmIdentifier:  The variable-length ASN.1 object that is
      encoded using Distinguished Encoding Rules (DER) [X.690] and
      identifies the signature algorithm (see Section 4.1.1.2 of
      [RFC5280]).

   The "Digital Signature" authentication method, defined in [RFC7427],
   supersedes previously defined signature authentication methods.  In
   this case, the real authentication algorithm is identified via
   AlgorithmIdentifier ASN.1 object.  Appendix A of [RFC7427] contains
   examples of commonly used ASN.1 objects.

4.  Interaction with IKEv2 Extensions concerning Authentication

   Generally in IKEv2 each party independently determines the way it
   authenticates itself to the peer.  In other words, authentication
   methods selected by the peers need not be the same.  However, some
   IKEv2 extensions break this rule.

   The prominent example is "Secure Password Framework for Internet Key
   Exchange Version 2" [RFC6467], which defines a framework for using
   secure password authentication in IKEv2.  With this framework, peers
   negotiate using one of the secure password methods in the IKE_SA_INIT
   exchange -- the initiator sends a list of supported methods in the
   request, and the responder picks one of them and sends it back in the
   response.

   If peers negotiate secure password authentication, then the selected
   method is used by both initiator and responder, and no other
   authentication methods are involved.  For this reason, there is no
   point to announce supported authentication methods in this case.
   Thus, if the peers choose to go with secure password authentication,
   they MUST NOT send the SUPPORTED_AUTH_METHODS notification.

   In the situation when peers are going to use Multiple Authentication
   Exchanges [RFC4739], they MAY include multiple SUPPORTED_AUTH_METHODS
   notifications (instead of one), each containing authentication
   methods appropriate for each authentication round.  The notifications
   are included in the order of the preference of performing
   authentication rounds.

5.  IANA Considerations

   This document defines a new type in the "IKEv2 Notify Message Status
   Types" registry:

            +=======+============================+===========+
            | Value | Notify Message Status Type | Reference |
            +=======+============================+===========+
            | 16443 | SUPPORTED_AUTH_METHODS     | RFC 9593  |
            +-------+----------------------------+-----------+

                                 Table 1

6.  Security Considerations

   Security considerations for the IKEv2 protocol are discussed in
   [RFC7296].  Security properties of different authentication methods
   vary.  Refer to corresponding documents, listed in the "IKEv2
   Authentication Method" registry on [IKEV2-IANA] for discussion of
   security properties of each authentication method.

   Announcing authentication methods gives an eavesdropper additional
   information about peers' capabilities.  If a peer advertises "NULL
   Authentication" along with other methods, then an active on-path
   attacker can encourage peers to use NULL authentication by removing
   all other announcements.  Note that this is not a real "downgrade"
   attack, since authentication methods in IKEv2 are not negotiated, and
   in this case NULL authentication should be allowed by local security
   policy.

   Similarly, if an on-path attacker can break some of the announced
   authentication methods online, then the attacker can encourage peers
   to use one of these weaker methods by removing all other
   announcements, and if this succeeds, then perform a person-in-the-
   middle attack.

7.  References

7.1.  Normative References

   [IKEV2-IANA]
              IANA, "Internet Key Exchange Version 2 (IKEv2)
              Parameters",
              <https://www.iana.org/assignments/ikev2-parameters/>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7427]  Kivinen, T. and J. Snyder, "Signature Authentication in
              the Internet Key Exchange Version 2 (IKEv2)", RFC 7427,
              DOI 10.17487/RFC7427, January 2015,
              <https://www.rfc-editor.org/info/rfc7427>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC9242]  Smyslov, V., "Intermediate Exchange in the Internet Key
              Exchange Protocol Version 2 (IKEv2)", RFC 9242,
              DOI 10.17487/RFC9242, May 2022,
              <https://www.rfc-editor.org/info/rfc9242>.

   [X.690]    ITU-T, "Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ISO/IEC 8825-1:2021 (E), ITU-T
              Recommendation X.690, February 2021.

7.2.  Informative References

   [COMPOSITE-SIGS]
              Ounsworth, M., Gray, J., Pala, M., and J. Klaussner,
              "Composite Signatures For Use In Internet PKI", Work in
              Progress, Internet-Draft, draft-ietf-lamps-pq-composite-
              sigs-01, 24 May 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
              pq-composite-sigs-01>.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
              <https://www.rfc-editor.org/info/rfc2409>.

   [RFC4739]  Eronen, P. and J. Korhonen, "Multiple Authentication
              Exchanges in the Internet Key Exchange (IKEv2) Protocol",
              RFC 4739, DOI 10.17487/RFC4739, November 2006,
              <https://www.rfc-editor.org/info/rfc4739>.

   [RFC6467]  Kivinen, T., "Secure Password Framework for Internet Key
              Exchange Version 2 (IKEv2)", RFC 6467,
              DOI 10.17487/RFC6467, December 2011,
              <https://www.rfc-editor.org/info/rfc6467>.

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,
              <https://www.rfc-editor.org/info/rfc7383>.

Appendix A.  Examples of Announcing Supported Authentication Methods

   This appendix shows some examples of announcing authentication
   methods.  This appendix is purely informative; if it disagrees with
   the body of this document, the other text is considered correct.
   Note that some payloads that are not relevant to this specification
   may be omitted for brevity.

A.1.  No Need to Use the IKE_INTERMEDIATE Exchange

   This example illustrates the situation when the
   SUPPORTED_AUTH_METHODS Notify payload fits into the IKE_SA_INIT
   message, and thus the IKE_INTERMEDIATE exchange is not needed.  In
   this scenario, the responder announces that it supports the "Shared
   Key Message Integrity Code" and the "NULL Authentication"
   authentication methods.  The initiator informs the responder that it
   supports only the "Shared Key Message Integrity Code" authentication
   method.

   Initiator                              Responder
   -----------                            -----------
                        IKE_SA_INIT
   HDR, SAi1, KEi, Ni -->
                                      <-- HDR, SAr1, KEr, Nr,
                                          N(SUPPORTED_AUTH_METHODS(
                                          PSK, NULL))

                         IKE_AUTH
   HDR, SK {IDi,
   AUTH, SAi2, TSi, TSr,
   N(SUPPORTED_AUTH_METHODS(PSK))}  -->
                                      <-- HDR, SK {IDr,
                                          AUTH, SAr2, TSi, TSr}

A.2.  With Use of the IKE_INTERMEDIATE Exchange

   This example illustrates the situation when the IKE_INTERMEDIATE
   exchange is used.  In this scenario, the responder announces that it
   supports the "Digital signature" authentication method using the
   RSASSA-PSS algorithm with CA1 and CA2 and the same method using the
   ECDSA algorithm with CA3.  The initiator supports only the "Digital
   signature" authentication method using the RSASSA-PSS algorithm with
   no link to a particular CA.

   Initiator                              Responder
   -----------                            -----------
                        IKE_SA_INIT
   HDR, SAi1, KEi, Ni,
   N(SIGNATURE_HASH_ALGORITHMS) -->
                                      <-- HDR, SAr1, KEr, Nr,
                                          CERTREQ(CA1, CA2, CA3),
                                          N(SIGNATURE_HASH_ALGORITHMS),
                                          N(SUPPORTED_AUTH_METHODS())

                      IKE_INTERMEDIATE
   HDR, SK {..., IDi]}  -->
                                      <-- HDR, SK {...,
                                          CERTREQ(CA1, CA2, CA3),
                                          N(SUPPORTED_AUTH_METHODS(
                                          SIGNATURE(RSASSA-PSS:1),
                                          SIGNATURE(RSASSA-PSS:2),
                                          SIGNATURE(ECDSA:3)))}

                         IKE_AUTH
   HDR, SK {IDi, CERT, CERTREQ(CA2),
   AUTH, SAi2, TSi, TSr,
   N(SUPPORTED_AUTH_METHODS(
   SIGNATURE(RSASSA-PSS:0)))}  -->
                                      <-- HDR, SK {IDr, CERT,
                                          AUTH, SAr2, TSi, TSr}

Acknowledgments

   The author would like to thank Paul Wouters for his valuable comments
   and proposals.  The author is also grateful to Daniel Van Geest, who
   made proposals for protocol improvement.  Reese Enghardt and Rifaat
   Shekh-Yusef contributed to the clarity of the document.

Author's Address

   Valery Smyslov
   ELVIS-PLUS
   PO Box 81
   Moscow (Zelenograd)
   124460
   Russian Federation
   Phone: +7 495 276 0211
   Email: svan@elvis.ru