Internet Engineering Task Force                               S. Fluhrer
Internet-Draft                                                 D. McGrew
Intended status: Informational                             P. Kampanakis
Expires: October 21, 2017                                  Cisco Systems
                                                          April 19, 2017

                  Postquantum Preshared Keys for IKEv2


   The possibility of quantum computers pose a serious challenge to
   cryptography algorithms widely today.  IKEv2 is one example of a
   cryptosystem that could be broken; someone storing VPN communications
   today could decrypt them at a later time when a quantum computer is
   available.  It is anticipated that IKEv2 will be extended to support
   quantum secure key exchange algorithms; however that is not likely to
   happen in the near term.  To address this problem before then, 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

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   This Internet-Draft will expire on October 21, 2017.

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   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Changes . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  PPK ID format . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  PPK Distribution  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Upgrade procedure . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informational References  . . . . . . . . . . . . . . . .  10
   Appendix A.  Discussion and Rationale . . . . . . . . . . . . . .  10
   Appendix B.  Acknowledgement  . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   It is an open question whether or not it is feasible to build a
   quantum computer (and if so, when might one be implemented), 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
   strong preshared keys is not vulnerable to quantum attacks, because
   those keys are one of the inputs to the key derivation function.  If
   the preshared key has sufficient entropy and the PRF, 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

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   to the authentication method that is already provided within IKEv2.
   We stir in this secret into the SK_d value, which is used to generate
   the key material (KEYMAT) keys and the SKEYSEED for the child SAs;
   this secret provides quantum resistance to the IPsec SAs (and any
   child IKE SAs).  We also stir in the secret into the SK_pi, SK_pr
   values; this allows both sides to detect a secret mismatch cleanly.

   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
   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


   - Modified how we stir the PPK into the IKEv2 secret state

   - Modified how the use of PPKs is negotiated


   - 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

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   - 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].

2.  Assumptions

   We assume that each IKE peer has a list of Postquantum Preshared Keys
   (PPK) along with their identifiers (PPK_id), and any potential IKE
   initiator has a selection of which PPK to use with with any specific
   responder.  In addition, the implementation has a configurable flag
   that determines whether this postquantum preshared key is mandatory.
   This PPK is independent of the preshared key (if any) that the IKEv2
   protocol uses to perform authentication.

3.  Exchanges

   If the initiator is configured to use a postquantum preshared key
   with the responder (whether or not the use of the PPK is optional),
   then it will include a notify payload in the initial exchange as

   Initiator                       Responder
   HDR, SAi1, KEi, Ni, N(PPK_SUPPORT)  --->

   N(PPK_SUPPORT) 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.

   If the initiator needs to resend this initial message with a cookie
   (because the responder response included a cookie notification), then
   the resend would include the PPK_SUPPORT notification if the original
   message did.

   When the responder receives this initial exchange with the notify,
   then it MUST check if has a PPK configured.  If it does, it MUST

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   reply with the IKE initial exchange including a notification in

   Initiator                       Responder
                   <--- HDR, SAr1, KEr, Nr, [CERTREQ], N(PPK_SUPPORT)

   If the responder does not have a PPK configured, then it continues
   with the IKE protocol as normal, not including the notify.

   When the initiator receives this reply, it checks whether the
   responder included the PPK_SUPPORT notify.  If the responder did not,
   then the initiator MUST either proceed with the standard IKE
   negotiation (without using a PPK), or abort the exchange (for
   example, because the initiator has the PPK marked as mandatory).  If
   the responder did include the PPK_SUPPORT notify, then it selects a
   PPK, along with its identifier PPK_id.  Then, it computes this
   modification of the standard IKE key derivation:

    SKEYSEED = prf(Ni | Nr, g^ir)
    {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_pi = prf(PPK, SK_pi')
    SK_pr = prf(PPK, SK_pr')

   That is, we use the standard IKE key derivation process except that
   the three subkeys SK_d, SK_pi, SK_pr are run through the prf again,
   this time using the PPK as the key.

   The initiator then sends the initial encrypted message, including the
   PPK_id value as follows:

   Initiator                       Responder
       [IDr,] AUTH, SAi2,
       TSi, TSr, N(PPK_IDENTITY)(PPK_id)}  --->

   N(PPK_IDENITY) is a status notification payload with the type [TBA];
   it has a protocol ID of 0, and no SPI and has a notification data
   that consists of the identifier PPK_id.

   When the responder receives this encrypted exchange, it first
   computes the values:

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

   It then uses the SK_ei value to decrypt the message; and then finds
   the PPK_id value attached to the notify.  It then scans through the
   payload for the PPK_id attached to the N(PPK_IDENTITY); if it has no
   such PPK, it fails the negotiation.  If it does have a PPK with that
   identity, it further computes:

    SK_d = prf(PPK, SK_d')
    SK_pi = prf(PPK, SK_pi')
    SK_pr = prf(PPK, SK_pr')

   And computes the enchange (validating the AUTH payload that the
   initiator included) as standard.

   This table summarizes the above logic by the responder

 Received PPK_SUPPORT  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_SUPPORT

   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 to generate SK_d, SK_pi, SK_pr (as shown above); 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 optional).

   If the initial exchange had PPK_SUPPORT sent by both the initiator
   and the responder, and the initiator does not include a PPK_NOTIFY
   notification, then the responder SHOULD fail the exchange.

   With this protocol, the computed SK_d is a function of the PPK, and
   assuming that the PPK has sufficient entropy (for example, at least
   2**256 possible values), then even if an attacker were able to
   recover the rest of the inputs to the prf function, it would be
   infeasible to use Grover's algorithm with a Quantum Computer to
   recover the SK_d value.  Similarly, every child SA key is a function
   of SK_d, hence all the keys for all the child SAs are also quantum
   resistant (assuming that the PPK was high entropy and secret, and
   that all the subkeys are sufficiently long).  However, this quantum

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   resistance does not extend to the initial SK_ei, SK_er keys; 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.

4.  PPK ID format

   This standard requires that both the initiator and the responder have
   a secret PPK value, with the responder selecting the PPK based on the
   PPK_ID that the initiator sends.  In this initial standard, both the
   initator and the responder are configured with fixed PPK and PPK_ID
   values, and do the look up based on that.  It is anticipated that
   later standards will extend this technique to allow dynamically
   changing PPK values.  To facilitate such an extension, we specify
   that the PPK_ID that the initiator sends will have its first octet be
   the PPK ID Type value, which is encoded as follows:

       PPK ID Type               Value

       PPK_ID_OPAQUE             0
       PPK_ID_FIXED              1
       RESERVED TO IANA          2-127
       Reserved for private use  128-255

   For PPK_ID_OPAQUE, the format of the PPK ID (and the PPK itself) is
   not specified by this document; it is assumed to be mutually
   intelligible by both by initiator and the responder.  This PPK ID
   type is intended for those implementations that choose not to
   disclose the type of PPK to active attackers.

   For PPK_ID_FIXED, the format of the PPK ID and the PPK are fixed
   octet strings; the remaining bytes of the PPK_ID are a configured
   value.  We assume that there is a fixed mapping between PPK_ID and
   PPK, which is configured locally to both the initiator and the
   responder.  The responder can use to do a look up the passed PPK_id
   value to determine the corresponding PPK value.  Not all
   implementations are able to configure arbitrary octet strings; to
   improve the potential interoperability, it is recommended that, in
   the PPK_ID_FIXED case, both the PPK and the PPK_ID strings be limited
   to the base64 character set, namely the 64 characters 0-9, A-Z, a-z,
   + and /.

   The PPK ID type values 2-127 are reserved for IANA; values 128-255
   are for private use among mutually consenting parties.

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5.  PPK Distribution

   PPK_id's of the type PPK_ID_FIXED (and the corresponding PPKs) are
   assumed to be configured within the IKE device in an out-of-band
   fashion.  While the method of distribution is a local matter, one
   suggestion would be to reuse the format within [RFC6030], with the
   Key Id field being the PPK_ID (without the 0x01 prefix for a
   PPK_ID_FIXED), and with the PPK being the secret, and the algorithm
   as PIN ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin").

6.  Upgrade procedure

   This algorithm was designed so that someone can introduce PPKs into
   an existing IKE network without causing network disruption.

   In the initial phase of the network upgrade, the network
   administrator would visit each IKE node, and configure:

   - The set of PPKs (and corresponding PPK_id's) that this node would
   need to know

   - For each peer that this node would initiate to, which PPK that we
   would use

   - That the use of PPK is currently optional

   With this configuration, the node will continue to operate with nodes
   that have not yet been upgraded.  This is due to the PPK_SUPPORT
   notify; if the initiator has not been upgraded, it will not send the
   PPK_SUPPORT notify (and so the responder will know that we will not
   use a PPK); if the responder has not been upgraded, it will not send
   the PPK_SUPPORT notify (and so the initiator will know not to use a
   PPK).  And, if both peers have been upgraded, they will both realize
   it, and in that case, the link will be quantum secure

   As an optional second step, after all nodes have been upgraded, then
   the administrator may then go back through the nodes, and mark the
   use of PPK as mandatory.  This will not affect the strength against a
   passive attacker; it would mean that an attacker with a Quantum
   Computer (which is sufficiently fast to be able to break the (EC)DH
   in real time would not be able to perform a downgrade attack).

7.  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

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   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 SK_d, which is a function of the
   PPK (among other things), traffic protected by that SA is secure
   against Quantum capable adversaries.

   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
   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

8.  References

8.1.  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,

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   [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, <>.

8.2.  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,

   [RFC6030]  Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric
              Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030,
              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
   SK_d, and hence the IPsec KEYMAT and any child SA's SKEYSEED, depend
   on both the symmetric 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_ei,
   SK_er, SK_ai, SK_ar values for the initial IKE exchange) unless they
   can find the PPK, and that's too difficult if the PPK has enough
   entropy (for example, 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

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   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

Appendix B.  Acknowledgement

   We would like to thank Tero Kivine, Valery Smyslov, Paul Wouters and
   the rest of the ipsecme working group for their feedback and
   suggestions for the scheme

Authors' Addresses

   Scott Fluhrer
   Cisco Systems


   David McGrew
   Cisco Systems


   Panos Kampanakis
   Cisco Systems


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