Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum Security
draft-smyslov-ipsecme-ikev2-qr-alt-08

Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                            19 June 2023
Expires: 21 December 2023

Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum
                                Security
                 draft-smyslov-ipsecme-ikev2-qr-alt-08

Abstract

   An IKEv2 extension defined in [RFC8784] allows IPsec traffic to be
   protected against someone storing VPN communications today and
   decrypting it later, when (and if) cryptographically relevant quantum
   computers are available.  However, this protection doesn't cover an
   initial IKEv2 SA, which might be unacceptable in some scenarios.
   This specification defines an alternative way get protection against
   quantum computers, but unlike the [RFC8784] solution it covers the
   initial IKEv2 SA too.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 21 December 2023.

Copyright Notice

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   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   3
   3.  Alternative Approach Description  . . . . . . . . . . . . . .   3
   4.  Computing IKE SA Keys . . . . . . . . . . . . . . . . . . . .   6
   5.  Comparison of the Conventional and the Alternative
           Approaches  . . . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Internet Key Exchange Protocol version 2, defined in [RFC7296],
   is used in the IPsec architecture to perform authenticated key
   exchange.  [RFC8784] defines an extension of IKEv2 for protecting
   today's VPN traffic against future quantum computers.  At the time
   this extension was being developed, it was a consensus in the IPSECME
   WG that only IPsec traffic needs to have such a protection.  It was
   believed that no sensitive information is transferred over IKE SA and
   extending the protection to also cover IKE SA traffic would require
   serious modifications to core IKEv2 protocol, that contradicted to
   one of the goals to minimize such changes.  For the cases when this
   protection is needed it was suggested to immediately rekey IKE SA
   once it is created.

   In some situations it is desirable to have this protection for IKE SA
   from the very beginning, when an initial IKE SA is created.  An
   example of such situation is Group Key Management protocol using
   IKEv2, defined in [I-D.ietf-ipsecme-g-ikev2].  In this protocol
   session keys are transferred from Group Controller/Key Server (GCKS)
   to Group Members (GM) immediately once an initial IKE SA is created.
   While it is possible to postpone transfer of the keys until the IKE
   SA is rekeyed (and [I-D.ietf-ipsecme-g-ikev2] specifies how to do
   this), the needed sequence of actions introduces an additional delay
   and adds unnecessary complexity to the protocol.

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   Since [RFC8784] was written, a new IKE_INTERMEDIATE exchange for
   IKEv2 was defined in [RFC9242].  While the primary motivation for
   developing this exchange was to allow multiple key exchanges to be
   used in IKEv2 (which is defined in
   [I-D.ietf-ipsecme-ikev2-multiple-ke]), the IKE_INTERMEDIATE exchange
   itself can be used for other purposes too.

   This specification makes use of the IKE_INTERMEDIATE exchange to
   define an alternative approach to [RFC8784], which allows getting
   protection against quantum computers for initial IKE SA.

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.

   We will use a term Conventional Approach in the content of using PPK
   to refer to the [RFC8784] and a term Alternative Approach to refer to
   this specification.

3.  Alternative Approach Description

   The IKE initiator who supports the IKE_INTERMEDIATE exchange and
   wants to use PPK includes both the INTERMEDIATE_EXCHANGE_SUPPORTED
   and the USE_PPK notifications in the IKE_SA_INIT request.  If the
   responder supports the IKE_INTERMEDIATE exchange and is willing to
   use PPK, it includes both these notifications in the response.

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

   If this is the case, then the initiator MAY choose to use the
   IKE_INTERMEDIATE exchange to negotiate PPK identity with the
   responder.  Note, that it is up to the initiator whether to use the
   alternative or conventional approaches, i.e.  whether to send PPK
   identity in the IKE_INTERMEDIATE exchange or in the IKE_AUTH
   exchange, as defined in the [RFC8784].

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   If the initiator decides to use alternative approach, it includes one
   or more PPK_IDENTITY_KEY notification containing PPK identities the
   initiator believes are appropriate for the IKE SA being created, into
   the IKE_INTERMEDIATE request.

   The PPK_IDENTITY_KEY is a Status Type IKEv2 notification.  Its Notify
   Message Type is <TBA by IANA>, Protocol ID and SPI Size fields are
   both set to 0.  The format of the notification data is shown below on
   Figure 1.

                        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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                             PPK_ID                            ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        PPK Confirmation                       +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 1: PPK_IDENTITY_KEY Notification Data Format

   Where:

   *  PPK_ID (variable) -- PPK_ID as defined in Section 5.1 of
      [RFC8784].

   *  PPK Confirmation (8 octest) -- value, which allows the responder
      to check whether it has the same PPK as the initiator for a given
      PPK_ID.  This field contains the first 8 octets of a string
      computed as prf( PPK, Ni | Nr | SPIi | SPIr ), where prf is the
      negotiated PRF; PPK is the key value for a specified PPK_ID; Ni,
      Nr, SPIi, SPIr -- nonces and IKE SPIs for the SA being
      established.

   If a series of the IKE_INTERMEDIATE exchanges takes place, the
   PPK_IDENTITY_KEY notification(s) MUST be sent in the last one, i.e.
   in the IKE_INTERMEDIATE exchange immediately preceding the IKE_AUTH
   exchange.  If the last IKE_INTERMEDIATE exchange contains other
   payloads aimed for some other purpose, then the notification(s) MAY
   be piggybacked with these payloads.

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   Initiator                         Responder
   ------------------------------------------------------------------
   HDR, SK { ... N(PPK_IDENTITY_KEY, PPK_ID_1)
              [, N(PPK_IDENTITY_KEY, PPK_ID_2)] ...
              [, N(PPK_IDENTITY_KEY, PPK_ID_n)]}   --->

   Depending on the responder's capabilities and policy the following
   situations are possible.

   a.  If the responder doesn't support the alternative approach, it
       will ignore the received PPK_IDENTITY_KEY notification(s) and
       won't include any additional notifications in the response.

      Initiator                       Responder
      ---------------------------------------------------------------
                              <---    HDR, SK { ... }

       In this case the initiator cannot make an initial IKE SA to be a
       quantum computer resistant, so if this is a requirement for the
       initiator, then it MUST abort creating IKE SA.  Otherwise, the
       initiator continues with the IKE_AUTH exchange as described in
       [RFC8784].

   b.  If the responder supports this extension, but doesn't have any of
       the PPKs which IDs were sent by the initiator or it has some of
       proposed PPKs, but their values mismatch the initiator's ones
       (based on the information from the PPK Confirmation field), then
       it MUST return the PPK_IDENTITY notification containing no data.

      Initiator                       Responder
      ---------------------------------------------------------------
                              <---    HDR, SK { N(PPK_IDENTITY) }

       In this case the initiator cannot achieve quantum computer
       resistance using the proposed PPKs.  If this is a requirement for
       the initiator, then it MUST abort creating IKE SA.  Otherwise,
       the initiator continues with the IKE_AUTH exchange as defined in
       [RFC7296].  The initiator SHOULD NOT fall back to the IKE_AUTH
       exchange defined in [RFC8784] in this case, since the initiator
       already knows that no one of the proposed PPKs is suitable for
       the responder.  If using PPK is mandatory for the responder, then
       it will send back the AUTHENTICATION_FAILED notification in the
       IKE_AUTH response (as specified in Table 1 of [RFC8784]).
       Otherwise the IKE SA will be created as per [RFC7296].

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   c.  If the responder supports this extension and is configured with
       one of the PPKs which IDs were sent by the initiator and this PPK
       matches the initiator's one (based on the information from the
       PPK Confirmation field), then the responder selects this PPK and
       returns back its identity in the PPK_IDENTITY notification.

      Initiator                       Responder
      ---------------------------------------------------------------
                     <---    HDR, SK { ... N(PPK_IDENTITY, PPK_ID_i)}

       In this case the IKE_AUTH exchange is performed as defined in
       [RFC7296], so that neither PPK_IDENTITY nor NO_PPK_AUTH
       notifications are sent, since it's already known which PPK to
       use.  However, the keys for the IKE SA are computed using PPK, as
       described in Section 4.  If the responder returns PPK identity
       that was not proposed by the initiator, then the initiator must
       treat this as a fatal error and MUST abort the IKE SA
       establishment.

   Note, that in the last two cases the IKE_AUTH exchange is performed
   as per [RFC7296], despite the fact that peers exchanged the USE_PPK
   notifications in the IKE_SA_INIT exchange, and, according to the
   [RFC8784], they were required to use the modified IKE_AUTH exchnage
   defined there.  This requirement is relaxed if peers follow this
   specification.

   Since the responder selects PPK before it knows the identity of the
   initiator, a situation may occur, when the responder agrees to use
   some PPK in the IKE_INTERMEDIATE exchange, but later discovers during
   the IKE_AUTH exchange that this particular PPK is not associated with
   the initiator's identity in its local policy.  Note, that the
   responder does have this PPK, but it is just not listed among the
   PPKs for using with this initiator.  In this case the responder
   SHOULD abort negotiation and return back the AUTHENTICATION_FAILED
   notification to be consistent with its policy.  However, if using PPK
   with this initiator is marked optional in the local policy, then the
   responder MAY continue creating IKE SA using the negotiated "wrong"
   PPK.

4.  Computing IKE SA Keys

   Once the PPK is negotiated in the last IKE_INTERMEDIATE exchange, the
   IKE SA keys are recalculated.  Note that if the IKE SA keys are also
   recalculated as the result of the other actions performed in the
   IKE_INTERMEDIATE exchange (for example, as defined in
   [I-D.ietf-ipsecme-ikev2-multiple-ke], then applying PPK MUST be done
   after all of them, so that recalculating IKE SA keys with PPK is the
   last action before they are used in the IKE_AUTH exchange.

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   The IKE SA keys are computed differently compared to [RFC8784].  A
   new SKEYSEED' value is computed using the negotiated PPK and the most
   recently computed SK_d key.  Note, that the PPK is applied to SK_d
   exactly how it is specified in [RFC8784], and the result is used as
   SKEYSEED'.

   SKEYSEED' = prf+ (PPK, SK_d)

   Then the SKEYSEED' is used to recalculate all SK_* keys as defined in
   Section 2.14 of [RFC7296].

   {SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr}
                              = prf+ (SKEYSEED', Ni | Nr | SPIi | SPIr )

   In the formula above Ni and Nr are nonces from the IKE_SA_INIT
   exchange and SPIi, SPIr - SPIs of the IKE SA being created.  Note,
   that SK_d, SK_pi, and SK_pr are not individually recalculated using
   PPK, as it is defined in [RFC8784].

   The resulting keys are then used in the IKE_AUTH exchange and in the
   created IKE SA.

5.  Comparison of the Conventional and the Alternative Approaches

   This specification isn't intended to be a replacement for [RFC8784].
   Instead, it is supposed to be used in situations where the
   conventional approach has a significant shortcomings.  However, if
   the partners support both approaches, then the alternative approach
   MAY also be used in situations where convenient approach suffices.

   The alternative approach has the following advantages:

   1.  The main advantage of the alternative approach is that it allows
       an initial IKE SA to be protected against quantum computers.
       This is important for those IKE extensions which transfer
       sensitive information, e.g. cryptographic keys, over initial IKE
       SA.  The prominent example of such extensions is
       [I-D.ietf-ipsecme-g-ikev2].

   2.  Using the alternative approach allows the initiator to specify
       several appropriate PPKs and the responder to choose one of them.
       This feature could simplify PPK rollover.

   3.  With the alternative approach there is no need for the initiator
       to calculate the content of the AUTH payload twice (with and
       without PPK) to support a situation when using PPK is optional
       for both sides.

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   The main disadvantage of the alternative approach is that it requires
   an additional round trip (the IKE_INTERMEDIATE exchange) to set up
   IKE SA.  However, if the IKE_INTERMEDIATE exchange has to be used for
   some other purposes in any case, then PPK stuff can be piggybacked
   with other payloads, thus eliminating this penalty.

6.  Security Considerations

   Security considerations of using Post-quantum Preshared Keys in the
   IKEv2 protocol are discussed in [RFC8784].  This specification
   defines an alternative way of exchanging PPK identity information.

7.  IANA Considerations

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

   <TBA>       PPK_IDENTITY_KEY

8.  Acknowledgements

   The author would like to thank Paul Wouters for valuable comments and
   Tero Kivinen for pointing out to the problem of mismatched preshared
   keys.

9.  References

9.1.  Normative References

   [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>.

   [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>.

   [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>.

   [RFC8784]  Fluhrer, S., Kampanakis, P., McGrew, D., and V. Smyslov,
              "Mixing Preshared Keys in the Internet Key Exchange
              Protocol Version 2 (IKEv2) for Post-quantum Security",
              RFC 8784, DOI 10.17487/RFC8784, June 2020,
              <https://www.rfc-editor.org/info/rfc8784>.

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   [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>.

9.2.  Informative References

   [I-D.ietf-ipsecme-g-ikev2]
              Smyslov, V. and B. Weis, "Group Key Management using
              IKEv2", Work in Progress, Internet-Draft, draft-ietf-
              ipsecme-g-ikev2-09, 19 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ipsecme-
              g-ikev2-09>.

   [I-D.ietf-ipsecme-ikev2-multiple-ke]
              Tjhai, C., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
              Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
              Key Exchanges in IKEv2", Work in Progress, Internet-Draft,
              draft-ietf-ipsecme-ikev2-multiple-ke-12, 1 December 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ipsecme-
              ikev2-multiple-ke-12>.

Author's Address

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

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