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Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                        February 3, 2021
Expires: August 7, 2021

Alternative Approach for Mixing Preshared Keys in IKEv2 for Post-quantum


   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) 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 the same protection against quantum computers,
   which allows to extend it on the initial IKEv2 SA.

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 August 7, 2021.

Copyright Notice

   Copyright (c) 2021 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
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   to this document.  Code Components extracted from this document must

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

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
   it), the needed sequence of actions introduces an additional delay
   and adds unnecessary complexity to the protocol.

   Since [RFC8784] was written, a new IKE_INTERMEDIATE exchange for
   IKEv2 was defined in [I-D.ietf-ipsecme-ikev2-intermediate].  While
   the primary motivation for developing this exchange was to allow

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   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",
   "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

   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 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(USE_PPK)              --->
                           <---    HDR, SAr1, KEr, Nr, [CERTREQ,]

   If the responder returned both these notifications, 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].

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

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   the IKE_INTERMEDIATE request.  If a series of the IKE_INTERMEDIATE
   exchanges takes place, the PPK_IDENTITY 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.

   Initiator                       Responder
              [, N(PPK_IDENTITY, PPK_ID_2)] ...
              [, N(PPK_IDENTITY, PPK_ID_n)]}   --->

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

   If the responder doesn't support the alternative approach, it will
   ignore the received PPK_IDENTITY notification(s) and won't include
   any additional notifications in the response.  If the responder
   doesn't have any of the PPKs which IDs were sent by the initiator,
   then it MUST behave as if it doesn't support the alternative
   approach, i.e. include no 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 and tries to use PPK
   as described in [RFC8784].

   If the responder supports this extension and is configured with one
   of the PPKs which IDs were sent by the initiator, then the responder
   chooses one of these PPKs 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.  The keys for
   the IKE SA are computed using PPK, as described in Section 4.

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   If the responder returns PPK identity that was not suggested by the
   initiator, then the initiator must treat this as a fatal error and
   MUST abort the IKE SA establishment.

   Since the responder selects PPK before it knows 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"

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.

   The IKE SA keys are computed as follows.  A new SKEYSEED' value is
   computed using the negotiated PPK and the most recently computed SK_d
   key.  Note, that PPK is applied to SK_d exactly how 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.

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

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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

   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.

   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 specification makes no request to IANA.

8.  Acknowledgements

   The author would like to thank Paul Wouters for valuable comments.

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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,

   [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,

              Smyslov, V., "Intermediate Exchange in the IKEv2
              Protocol", draft-ietf-ipsecme-ikev2-intermediate-05 (work
              in progress), September 2020.

9.2.  Informative References

              Smyslov, V. and B. Weis, "Group Key Management using
              IKEv2", draft-ietf-ipsecme-g-ikev2-02 (work in progress),
              January 2021.

              Tjhai, C., Tomlinson, M., Bartlett, G., Fluhrer, S.,
              Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
              Key Exchanges in IKEv2", draft-ietf-ipsecme-ikev2-
              multiple-ke-02 (work in progress), January 2021.

Author's Address

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   Valery Smyslov
   PO Box 81
   Moscow (Zelenograd)  124460

   Phone: +7 495 276 0211
   Email: svan@elvis.ru

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