An Alternative Approach for Postquantum Preshared Keys in IKEv2
draft-smyslov-ipsecme-ikev2-qr-alt-00

Network Working Group                                         V. Smyslov
Internet-Draft                                                ELVIS-PLUS
Intended status: Standards Track                        October 17, 2019
Expires: April 19, 2020

    An Alternative Approach for Postquantum Preshared Keys in IKEv2
                 draft-smyslov-ipsecme-ikev2-qr-alt-00

Abstract

   An IKEv2 extension defined in [I-D.ietf-ipsecme-qr-ikev2] 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

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   This Internet-Draft will expire on April 19, 2020.

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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 . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   The Internet Key Exchange Protocol version 2, defined in [RFC7296],
   is used in the IPsec architecture to perform authenticated key
   exchange.  [I-D.ietf-ipsecme-qr-ikev2] 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.yeung-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.yeung-g-ikev2] specifies how to do it), the needed
   sequence of actions introduces an additional delay and adds
   unnecessary complexity to the protocol.

   Since [I-D.ietf-ipsecme-qr-ikev2] 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 Post-Quantum Key Exchanges

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   to be used in IKEv2 (which is another long-term approach to protect
   against Quantum Computers and is defined in
   [I-D.tjhai-ipsecme-hybrid-qske-ikev2]), 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 [I-D.ietf-ipsecme-qr-ikev2], 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 [I-D.ietf-ipsecme-qr-ikev2] 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, she
   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 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 to send PPK identity in the
   IKE_INTERMEDIATE exchange or in the IKE_AUTH exchange, as defined in
   the [I-D.ietf-ipsecme-qr-ikev2].

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   If the initiator decides to use alternative approach, he includes one
   or more PPK_IDENTITY notification containing PPK identities, which
   the initiator believes can be used for the IKE SA being created, into
   the IKE_INTERMEDIATE request.  If a series of the IKE_INTERMEDIATE
   exchanges take 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 exchange contains other
   payloads aimed for some other purpose, then the notification(s) MAY
   be piggybacked with these payloads.

   Initiator                       Responder
   ------------------------------------------------------------------
   HDR, SK { ... N(PPK_IDENTITY, PPK_ID_1)
              [, 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.

   First, if the responder doesn't support the alternative approach, she
   will ignore the received PPK_IDENTITY 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.  Depending on his policy, the initiator
   may abort negotiation or may continue with the IKE_AUTH exchange.  In
   the latter case depending on the policy the initiator may try to
   negotiate the use of PPK with conventional approach, as described in
   [I-D.ietf-ipsecme-qr-ikev2], or may proceed with the standard
   IKE_AUTH exchange, thus giving up using PPK for this IKE SA.

   Another situation occurs when the responder supports this extension,
   but has no PPK with identity equal to any of the identities provided
   by the initiator.  Depending on responder's policy the following
   scenarios are possible.

   If using PPK is mandatory for the responder, then she returns the
   AUTHENTICATION_FAILED notification, thus informing the initiator that
   the SA cannot be created.  In this case the initiator MUST abort the
   process of IKE SA establishment.

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

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   If using PPK is optional for the responder, then she returns the
   empty PPK_IDENTITY notification, thus informing the initiator that
   the IKE SA can be created only without using PPK.

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

   In this case the initiator depending on whether using PPK is
   mandatory or not in his own policy may continue establishing IKE SA
   without PPK or abort it.

   Finally, if the responder supports this extension and is configured
   with one of the PPKs which identities were provided by the initiator,
   then the responder chooses an appropriate PPK and returns back the
   PPK_IDENTITY notification containing its identity.

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

   In this case the IKE_AUTH exchange is performed as defined in the
   core IKEv2 specification.  In particular, neither PPK_IDENTITY nor
   NO_PPK_AUTH notifications are included, 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.

   Note, that 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.

4.  Computing IKE SA Keys

   With alternative approach the keys are computed similarly to
   [I-D.ietf-ipsecme-qr-ikev2], with the difference, that all SK_* (and
   not only SK_d, SK_pi and SK_pr) keys are calculated using PPK:

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

   SK_d  = prf+ (PPK, SK_d')
   SK_ai = prf+ (PPK, SK_ai')
   SK_ar = prf+ (PPK, SK_ar')
   SK_ei = prf+ (PPK, SK_ei')
   SK_er = prf+ (PPK, SK_er')
   SK_pi = prf+ (PPK, SK_pi')
   SK_pr = prf+ (PPK, SK_pr')

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   If the last IKE_INTERMEDIATE exchange performs an update of the IKE
   SA keys (e.g. as a result of additional key exchange, as described in
   [I-D.tjhai-ipsecme-hybrid-qske-ikev2]), then applying PPK MUST be
   performed to the result of this update.  In other words, it must be
   the last action in calculating SK_* keys for the IKE SA being
   created.

5.  Comparison of the Conventional and the Alternative Approaches

   This specification isn't intended to be a replacement for
   [I-D.ietf-ipsecme-qr-ikev2].  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.yeung-g-ikev2].

   2.  Using 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 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 Postquantum Preshared Keys in the
   IKEv2 protocol are discussed in [I-D.ietf-ipsecme-qr-ikev2].  This
   specification defines an alternative way of exchanging PPK identity
   information.

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7.  IANA Considerations

   This specification makes no request to IANA.

8.  References

8.1.  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>.

   [I-D.ietf-ipsecme-qr-ikev2]
              Fluhrer, S., McGrew, D., Kampanakis, P., and V. Smyslov,
              "Postquantum Preshared Keys for IKEv2", draft-ietf-
              ipsecme-qr-ikev2-08 (work in progress), March 2019.

   [I-D.ietf-ipsecme-ikev2-intermediate]
              Smyslov, V., "Intermediate Exchange in the IKEv2
              Protocol", draft-ietf-ipsecme-ikev2-intermediate-02 (work
              in progress), July 2019.

8.2.  Informative References

   [I-D.yeung-g-ikev2]
              Weis, B. and V. Smyslov, "Group Key Management using
              IKEv2", draft-yeung-g-ikev2-16 (work in progress), July
              2019.

   [I-D.tjhai-ipsecme-hybrid-qske-ikev2]
              Tjhai, C., Tomlinson, M., grbartle@cisco.com, g., Fluhrer,
              S., Geest, D., Garcia-Morchon, O., and V. Smyslov,
              "Framework to Integrate Post-quantum Key Exchanges into
              Internet Key Exchange Protocol Version 2 (IKEv2)", draft-
              tjhai-ipsecme-hybrid-qske-ikev2-04 (work in progress),
              July 2019.

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Author's Address

   Valery Smyslov
   ELVIS-PLUS
   PO Box 81
   Moscow (Zelenograd)  124460
   RU

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

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