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Postquantum Preshared Keys for IKEv2

The information below is for an old version of the document.
Document Type This is an older version of an Internet-Draft whose latest revision is Replaced
Authors Scott Fluhrer , David McGrew , Panos Kampanakis
Last updated 2017-01-30 (Latest revision 2016-10-28)
Replaced by draft-ietf-ipsecme-qr-ikev2, draft-ietf-ipsecme-qr-ikev2, draft-ietf-ipsecme-qr-ikev2, RFC 8784
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Send notices to "David Waltermire" <>
Internet Engineering Task Force                               S. Fluhrer
Internet-Draft                                                 D. McGrew
Intended status: Informational                             P. Kampanakis
Expires: May 1, 2017                                       Cisco Systems
                                                        October 28, 2016

                  Postquantum Preshared Keys for IKEv2


   This document describes an extension of IKEv2 to allow it to be
   resistant to a Quantum Computer, by using preshared keys

Status of This Memo

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

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 1, 2017.

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   Copyright (c) 2016 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Changes . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Creating Child SA Keying Material . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informational References  . . . . . . . . . . . . . . . .   7
   Appendix A.  Discussion and Rationale . . . . . . . . . . . . . .   7
   Appendix B.  Acknowledgement  . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   It is an open question whether or not it is feasible to build a
   quantum computer, but if it is, many of the cryptographic algorithms
   and protocols currently in use would be insecure.  A quantum computer
   would be able to solve DH and ECDH problems, and this would imply
   that the security of existing IKEv2 systems would be compromised.
   IKEv1 when used with preshared keys does not share this
   vulnerability, because those keys are one of the inputs to the key
   derivation function.  If the preshared key have sufficient entropy
   and the PRF and encryption and authentication transforms are
   postquantum secure, then the resulting system is believed to be
   quantum resistant, that is, believed to be invulnerable to an
   attacker with a Quantum Computer.

   This document describes a way to extend IKEv2 to have a similar
   property; assuming that the two end systems share a long secret key,
   then the resulting exchange is quantum resistant.  By bringing
   postquantum security to IKEv2, this note removes the need to use an
   obsolete version of the Internet Key Exchange in order to achieve
   that security goal.

   The general idea is that we add an additional secret that is shared
   between the initiator and the responder; this secret is in addition
   to the authentication method that is already provided within IKEv2.
   We stir in this secret when generating the key material (KEYMAT) keys
   for the child SAs (along with the parameters that IKEv2 normally
   uses); this secret provides quantum resistance to the IPsec SAs.

   It was considered important to minimize the changes to IKEv2.  The
   existing mechanisms to do authentication and key exchange remain in
   place (that is, we continue to do (EC)DH, and potentially a PKI

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   authentication if configured).  This does not replace the
   authentication checks that the protocol does; instead, it is done as
   a parallel check.

1.1.  Changes

   Changes in this draft from the previous versions


   - Simplified the protocol by stirring in the preshared key into the
   child SAs; this avoids the problem of having the responder decide
   which preshared key to use (as it knows the initiator identity at
   that point); it does mean that someone with a Quantum Computer can
   recover the initial IKE negotation.

   - Removed positive endorsements of various algorithms.  Retained
   warnings about algorithms known to be weak against a Quantum Computer


   - Added explicit guidance as to what IKE and IPsec algorithms are
   Quantum Resistant


   - We switched from using vendor ID's to transmit the additional data
   to notifications

   - We added a mandatory cookie exchange to allow the server to
   communicate to the client before the initial exchange

   - We added algorithm agility by having the server tell the client
   what algorithm to use in the cookie exchange

   - We have the server specify the PPK Indicator Input, which allows
   the server to make a trade-off between the efficiency for the search
   of the clients PPK, and the anonymity of the client.

   - We now use the negotiated PRF (rather than a fixed HMAC-SHA256) to
   transform the nonces during the KDF

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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2.  Assumptions

   We assume that each IKE peer (both the initiator and the responder)
   has an optional Postquantum Preshared Key (PPK) (potentially on a
   per-peer basis, selected by peer identity), and also has a
   configurable flag that determines whether this postquantum preshared
   key is mandatory.  This preshared key is independent of the preshared
   key (if any that the IKEv2 protocol uses to perform authentication.

3.  Exchanges

   If the initiator has a configured postquantum preshared key (whether
   or not it is optional), then it will include a notify payload in its
   initial encrypted exchange as follows:

   Initiator                       Responder
       [IDr,] AUTH, SAi2,
       TS, TSr, N(PPK_NOTIFY)}  --->

   N(PPK_NOTIFY) is a status notification payload with the type [TBA];
   it has a protocol ID of 0, and no SPI and no notification data
   associated with it.

   When the responder receives the initial encrypted exchange, it checks
   to see if it received a notify within that exchange, is configured to
   support PPK with the initiator's identity, and whether that use is
   mandatory.  If the notify was received, and the responder does have a
   PPK for that identity, then it responds with the standard IKE
   response with the PPK_NOTIFY notify message included, namely:

   Initiator                       Responder
                             <--- HDR, SK {IDr, [CERT,] AUTH,
                                      SAr2, TSi, TSr, N(PPK_NOTIFY)}

   If the responder is not configured to support PPK with that identity,
   it continues with the standard IKE protocol, not including the

   If the responder is configured to support PPK with that identity, and
   it does not receive the notification, then if the PPK usage is
   configured as mandatory, it MUST abort the exchange.  If the PPK
   usage is configured as optional, it continues with the standard IKE
   protocol, not including the notification.

   This table summarizes the above logic by the responder

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 Received Nonce     Have PPK   PPK Mandatory    Action
      No               No          *            Standard IKE protocol
      No              Yes         No            Standard IKE protocol
      No              Yes        Yes            Abort negotiation
     Yes               No          *            Standard IKE protocol
     Yes              Yes          *            Include PPK_NOTIFY Nonce

   When the initiator receives the response, then (if it is configured
   to use a PPK with the responder), then it checks for the presense of
   the notification.  If it receives one, it marks the SA as using the
   configured PPK; if it does not receive one, it MUST either abort the
   exchange (if the PPK was configured as mandatory), or it MUST
   continue without using the PPK (if the PPK was configured as

   The protocol continues as standard until it comes time to compute the
   child SA keying material.

4.  Creating Child SA Keying Material

   When it comes time to generate the keying material for a child SA,
   the implementation (both the initiator and the responder) checks to
   see if they agreed to use a PPK.  If they did, then they look up
   (based on the peer's identity) the configured PPK, and then both
   sides use one of these alternative formula (based on whether an
   optional Diffie-Hellman was included):

    Ni&apos; = prf(PPK, Ni)
    Nr&apos; = prf(PPK, Nr)
    KEYMAT = prf+(SK_d, Ni&apos; | Nr&apos;)


    Ni&apos; = prf(PPK, Ni)
    Nr&apos; = prf(PPK, Nr)
    KEYMAT = prf+(SK_d, g^ir (new) | Ni&apos; | Nr&apos;)

   where PPK is the configured postquantum preshared key, Ni, Nr are the
   nonces from the IKE_SA_INIT exchange if this require is the first
   Child SA created or the fresh Ni and Nr from the CREATE_CHILD_SA
   exchange if this is a subsequent creation, and prf is the
   pseudorandom function that was negotiated for this SA.

   This is the standard IKE KEYMAT generation, except that the nonces
   are transformed (via the negotiated PRF function) using the preshared
   PPK value

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   We use this negotiated PRF, rather than negotiating a separate one,
   because this PRF is agreed by both sides to have sufficient security
   properties (otherwise, they would have negotiated something else),
   and so that we don't need to specify a separate negotiation

   When you rekey an IKE SA (generating a fresh SKEYSEED), the initiator
   and the responder will transform the nonces using the same PPK as
   they used during the original IKE SA negotiation.  That is, they will
   use the alternate derivation:

    Ni&apos; = prf(PPK, Ni)
    Nr&apos; = prf(PPK, Nr)
    SKEYSEED = prf( SK_d (old), g^ir (new) | Ni&apos; | Nr&apos; )
    (SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr) =
          prf+(SKEYSEED, Ni&apos; | Nr&apos; | SPIi | SPIr)

   An implementation MAY rekey the initial IKE SA immediately after
   negotiating it; this would reduce the amount of data available to an
   attacker with a Quantum Computer

5.  Security Considerations

   Quantum computers are able to perform Grover's algorithm; that
   effectively halves the size of a symmetric key.  Because of this, the
   user SHOULD ensure that the postquantum preshared key used has at
   least 256 bits of entropy, in order to provide a 128 bit security

   Although this protocol preserves all the security properties of IKE
   against adversaries with conventional computers, this protocol allows
   an adversary with a Quantum Computer to decrypt all traffic encrypted
   with the initial IKE SA.  In particular, it allows the adversary to
   recover the identities of both sides.  If there is IKE traffic other
   than the identities that need to be protected against such an
   adversary, one suggestion would be to form an initial IKE SA (which
   is used to exchange identities), perhaps by using the protocol
   documented in RFC6023.  Then, you would immediately create a child
   IKE SA (which is used to exchange everything else).  Because the
   child IKE SA keys are a function of the PPK (among other things),
   traffic protected by that SA is secure against Quantum capable

   In addition, the policy SHOULD be set to negotiate only quantum-
   resistant symmetric algorithms; while this RFC doesn't claim to give
   advise as to what algorithms are secure (as that may change based on
   future cryptographical results), here is a list of defined IKEv2 and

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   IPsec algorithms that should NOT be used, as they are known not to be
   Quantum Resistant

   Any IKE Encryption algorithm, PRF or Integrity algorithm with key
   size <256 bits

   Any ESP Transform with key size <256 bits

   PRF_AES128_XCBC and PRF_AES128_CBC; even though they are defined to
   be able to use an arbitrary key size, they convert it into a 128 bit
   key internally

6.  References

6.1.  Normative References

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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, <>.

6.2.  Informational References

   [RFC6023]  Nir, Y., Tschofenig, H., Deng, H., and R. Singh, "A
              Childless Initiation of the Internet Key Exchange Version
              2 (IKEv2) Security Association (SA)", RFC 6023,
              DOI 10.17487/RFC6023, October 2010,

   [SPDP]     McGrew, D., "A Secure Peer Discovery Protocol (SPDP)",
              2001, <>.

Appendix A.  Discussion and Rationale

   The idea behind this is that while a Quantum Computer can easily
   reconstruct the shared secret of an (EC)DH exchange, they cannot as
   easily recover a secret from a symmetric exchange this makes the
   IPsec KEYMAT and any child SA's SKEYSEED depend on both the symmetric

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   PPK, and also the Diffie-Hellman exchange.  If we assume that the
   attacker knows everything except the PPK during the key exchange, and
   there are 2**n plausible PPK's, then a Quantum Computer (using
   Grover's algorithm) would take O(2**(n/2)) time to recover the PPK.
   So, even if the (EC)DH can be trivially solved, the attacker still
   can't recover any key material (except for the SK values for the
   initial IKE exchange) unless they can find the PPK, and that's too
   difficult if the PPK has enough entropy (say, 256 bits).  Note that
   we do allow an attacker with a Quantum Computer to rederive the
   keying material for the initial IKE SA; this was a compromise to
   allow the responder to select the correct PPK quickly.

   Another goal of this protocol is to minimize the number of changes
   within the IKEv2 protocol, and in particular, within the cryptography
   of IKEv2.  By limiting our changes to notifications, and translating
   the nonces, it is hoped that this would be implementable, even on
   systems that perform much of the IKEv2 processing is in hardware.

   A third goal was to be friendly to incremental deployment in
   operational networks, for which we might not want to have a global
   shared key, and also if we're rolling this out incrementally.  This
   is why we specifically try to allow the PPK to be dependent on the
   peer, and why we allow the PPK to be configured as optional.

   A fourth goal was to avoid violating any of the security goals of

   The third and fourth goals are in partial conflict.  In order to
   achieve postquantum security, we need to stir in the PPK when the
   keys are computed, however the keys are computed before we know who
   we're talking to (and so which PPK we should use).  And, we can't
   just tell the other side which PPK to use, as we might use different
   PPK's for different peers, and so that would violate the anonymity
   goal.  If we just (for example) included a hash of the PPK, someone
   listening in could easily tell when we're using the same PPK for
   different exchanges, and thus deduce that the systems are related.
   The compromise we selected was to stir in the PPK in all the derived
   keys except the initial IKE SA keys, While this allows an attacker
   with a Quantum Computer to recover the identities, a poll on the
   IPsecME mailing list indicated that the majority of the people on the
   list did not think anonymity was an important property within IKE.
   We stir in the shared secret within the Child SA keying material;
   this allows an implementation that wants to protect the other IKE-
   based traffic to create an initial IKE SA to exchange identities, and
   then immediately create a Child SA, and use that Child SA to exchange
   the rest of the negotiation.

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   In addition, when we stir in the PPK, we always use it to modify a
   nonce (using the negotiated PRF).  We modify the nonce (rather than,
   say, including the PPK in with the prf or prf+ computation directly)
   so that this would be easier to implement on an hardware-based IKE
   implementation; the prf computations might be built-in, but the
   nonces would be external inputs, and so modifying those would
   minimize the changes.

Appendix B.  Acknowledgement

   The idea of stirring in the PPK into the IPsec key generation process
   was originally suggested on the list by Tero Kivinen.

Authors' Addresses

   Scott Fluhrer
   Cisco Systems


   David McGrew
   Cisco Systems


   Panos Kampanakis
   Cisco Systems


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