Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum Security
draft-ietf-ipsecme-ikev2-qr-alt-02

Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                            25 July 2024
Expires: 26 January 2025

Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum
                                Security
                   draft-ietf-ipsecme-ikev2-qr-alt-02

Abstract

   An Internet Key Exchange protocol version 2 (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.  The
   protection is achieved by means of Post-quantum Preshared Key (PPK)
   which is mixed into the session keys calculation.  However, this
   protection doesn't cover an initial IKEv2 SA, which might be
   unacceptable in some scenarios.  This specification defines an
   alternative way to get protection against quantum computers, which is
   similar to the solution defined in RFC8784, but protects the initial
   IKEv2 SA too.

   Besides, RFC8784 assumes that PPKs are static and thus they are only
   used when an initial IKEv2 Security Association (SA) is created.  If
   a fresh PPK is available before the IKE SA is expired, then the only
   way to use it is to delete the current IKE SA and create a new one
   from scratch, which is inefficient.  This specification also defines
   a way to use PPKs in active IKEv2 SA for creating additional IPsec
   SAs and for rekeys operations.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 January 2025.

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Copyright Notice

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   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Description  . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Creating Initial IKE SA . . . . . . . . . . . . . . . . .   4
       3.1.1.  Computing IKE SA Keys . . . . . . . . . . . . . . . .   7
     3.2.  Using PPKs in the CREATE_CHILD_SA Exchange  . . . . . . .   8
       3.2.1.  Computing Keys  . . . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Comparison this Specification with RFC8784 . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The Internet Key Exchange protocol version 2, defined in [RFC7296],
   is used in the IPsec architecture for performing authenticated key
   exchange.  Extension to IKEv2 for mixing preshared keys for post-
   quantum security defined in [RFC8784] allows today's IPsec traffic to
   be protected against future quantum computers.  The protection is
   achieved by means of using a Post-quantum Preshared Key (PPK) which
   is mixed into the session keys calculation.  At the time this
   extension was being developed, it was a consensus in the IPSECME WG
   that it is the IPsec traffic that mostly needs to have such a
   protection.  It was believed that information transferred over IKE SA
   (including peers' identities) is less important and extending the
   protection to also cover initial IKE SA would require serious
   modifications to core IKEv2 protocol, that contradicted to one of the
   goals to minimize such changes.  It was also decided that immediate

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   rekey of initial IKE SA would add this protection to the new IKE SA
   (albeit it wouldn't provide identity protection of the peers).

   However, 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.

   Some time after the protocol extension for mixing preshared keys in
   IKEv2 for post-quantum security was defined in [RFC8784], a new
   IKE_INTERMEDIATE exchange for IKEv2 [RFC9242] was developed.  While
   the primary motivation for developing this exchange was to allow
   multiple key exchanges to be used in IKEv2 (which is defined in
   [RFC9370]), 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 the mixing preshared keys in IKEv2
   for post-quantum security, which allows getting protection against
   quantum computers for initial IKE SA.

   Another issue with use of PPKs as it is defined in [RFC8784] is that
   this approach assumes that PPKs are static entities, which are
   changed very infrequently.  For this reason PPKs are only used once -
   when an initial IKE SA is established.  This restriction makes it
   difficult to use PPKs as defined in [RFC8784] when they are changed
   relatively frequently, for example as a result of Quantum Key
   Distribution (QKD).  If a fresh PPK becomes available before the IKE
   SA is expired, there is no way to use it except for deleting this IKE
   SA and re-creating a new once from scratch using the fresh PPK.

   This specification defines the use of PPKs in the CREATE_CHILD_SA
   exchange for creating additional IPsec SAs and for rekey of IKE and
   IPsec SAs.  This allows to leverage fresh PPKs without the need to
   delete IKE SA and create it from scratch.

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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 Description

3.1.  Creating Initial IKE SA

   The IKE initiator which supports the IKE_INTERMEDIATE exchange and
   wants to use PPK to protect initial IKE SA includes the
   INTERMEDIATE_EXCHANGE_SUPPORTED notification and a notification of
   type USE_PPK_ALT in the IKE_SA_INIT request.  If the responder
   supports the IKE_INTERMEDIATE exchange and is willing to use PPK for
   initial IKE SA protection, it includes both these notifications in
   the IKE_SA_INIT response.

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

   The USE_PPK_ALT is a Status Type IKEv2 notification.  Its Notify
   Message Type is <TBA by IANA>, Protocol ID and SPI Size are both set
   to 0.  This specification doesn't define any data that this
   notification may contain, so the Notification Data is left empty.
   However, future extensions of this specification may make use of it.
   Implementations MUST ignore any data they don't understand.

   Note, that this negotiation is independent from negotiation of using
   PPK defined in [RFC8784].  The initiator that supports both RFC8784
   and this specification MAY include both the USE_PPK_ALT (along with
   the INTERMEDIATE_EXCHANGE_SUPPORTED) and the USE_PPK notifications if
   it is configured to use either specification.  However, the responder
   supporting both specifications have to choose one to use, thus it
   MUST return either USE_PPK_ALT or USE_PPK notification in the
   response, but not both.

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   If the negotiation was successful, the initiator 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].  The receiver can determine the length of PPK_ID by
      subtracting 8 (the length of PPK Confirmation) from the
      Notification Data length.

   *  PPK Confirmation (8 octets) -- 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 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.  The PPK_IDENTITY notification
       is defined in [RFC8784].

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

       In this case the IKE_AUTH exchange is performed as defined in
       IKEv2 [RFC7296].  However, the keys for the IKE SA are computed
       using PPK, as described in Section 3.1.1.  If the responder
       returns PPK identity that was not proposed by the initiator, then
       the initiator should treat this as a fatal error and MUST abort
       the IKE SA establishment.

   b.  If the responder 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), and using PPK is mandatory for
       the responder, then it MUST return AUTHENTICATION_FAILED
       notification and abort creating the IKE SA.

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

   c.  If the responder 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), and using PPK is optional for
       the responder, then it doesn't include any PPK_IDENTITY
       notification to the response.

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

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       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 described
       in IKEv2 [RFC7296].

   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 during the IKE_AUTH
   exchange discovers 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, the
   responder MAY continue creating IKE SA using the negotiated "wrong"
   PPK if this is acceptable according to its local policy.

3.1.1.  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 [RFC9370]),
   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.

   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 and SPIr are the 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].

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

3.2.  Using PPKs in the CREATE_CHILD_SA Exchange

   If a fresh PPK is available to both peers at the time when an IKE SA
   is active, peers MAY use this PPK without creating a new IKE SA from
   scratch.  In this case the PPK can be used for creating additional
   IPsec SAs and for rekeying both IKE and IPsec SAs regardless whether
   the current IKE SA was created with use of a PPK or not.  Since the
   content of the CREATE_CHILD_SA messages is similar in all these
   cases, all the payloads not relevant to this specifications are
   omitted from the diagrams below for brevity.  Refer to Section 1.3 of
   [RFC7296] for the content of the CREATE_CHILD_SA messages.

   If the initiator wants to use a PPK in the CREATE_CHILD_SA exchange,
   it includes one or more PPK_IDENTITY_KEY notification containing PPK
   identities the initiator believes are appropriate for the SA being
   created, into the CREATE_CHILD_SA request.  If the responder supports
   use PPKs in the CREATE_CHILD_SA exchange and is configured and ready
   to do it, then it sends back the PPK_IDENTITY notification containing
   the ID of the selected PPK.

   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)]}   --->
                     <---    HDR, SK { ... N(PPK_IDENTITY, PPK_ID_i)}

   In case the responder doesn't support (or is not configured for)
   using PPKs in the CREATE_CHILD_SA exchange, or 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 doesn't
   include any PPK_IDENTITY notification in the response and new SA is
   created as defined in IKEv2 [RFC7296].  If this is inappropriate for
   the initiator, it MAY immediately delete this SA.

   Otherwise the new SA is created using the selected PPK.

3.2.1.  Computing Keys

   For the purpose of calculation session keys for the new SA, the
   current SK_d key is first mixed with the selected PPK:

   SK_d' = prf+ (PPK, SK_d)

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   The resulted key SK_d' is then used instead of SK_d in all formulas
   for computing keys for the new SA (Sections 2.17 and 2.18 of
   [RFC7296], Section 2.2.4 of [RFC9370]).

   Note, that if the PPK that was used for the IKE SA establishment is
   not changed, then there is no point to use it in the CREATE_CHILD_SA
   exchange.

4.  Security Considerations

   Security considerations of using Post-quantum Preshared Keys in the
   IKEv2 protocol are discussed in [RFC8784].  Compared to [RFC8784]
   this specification makes even initial IKE SA quantum secure.  In
   addition, a PPK is mixed into the SK_* keys calculation before the
   IKE_AUTH exchange starts, and since PPK is used in authentication
   too, that gives this exchange a QR protection even against active
   attacker.

   This specification relies on the IKE_INTERMEDIATE exchange.  Refer to
   [RFC9242] for discussion of related security issues.

   Section 4 of [RFC9370] discusses the potential impact of appearing a
   CRQC to various cryptographic primitives used in IKEv2.  It is worth
   to repeat here that it is believed that security of symmetric key
   cryptographic primitives will not be affected by CRQC.

5.  IANA Considerations

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

   <TBA>       USE_PPK_ALT
   <TBA>       PPK_IDENTITY_KEY

6.  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.  Thanks to Rebecca Guthrie for providing comments and proposals
   for the document.

7.  References

7.1.  Normative References

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   [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>.

   [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>.

7.2.  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-ikev2-11, 26 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ipsecme-
              g-ikev2-11>.

   [RFC9370]  Tjhai, CJ., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
              Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
              Key Exchanges in the Internet Key Exchange Protocol
              Version 2 (IKEv2)", RFC 9370, DOI 10.17487/RFC9370, May
              2023, <https://www.rfc-editor.org/info/rfc9370>.

Appendix A.  Comparison this Specification with RFC8784

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

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   The approach defined in this document has the following advantages:

   1.  The main advantage of this specification compared to approach
       from [RFC8784] is that it allows an initial IKE SA to be fully
       protected against quantum computers, including information
       transferred in the IKE_AUTH exchange.  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.  This specification allows the initiator to specify several
       appropriate PPKs and the responder to choose one of them.  This
       feature could simplify PPK rollover.

   3.  With this specification 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.

   The main disadvantage of the approach defined in this document is
   that it always requires an additional round trip (the
   IKE_INTERMEDIATE exchange) to set up IKE SA and initial IPsec 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.

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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