Internet Engineering Task Force                               S. Fluhrer
Internet-Draft                                                 D. McGrew
Intended status: Informational                             P. Kampanakis
Expires: February 5, 2017                                  Cisco Systems
                                                          August 4, 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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Changes . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Computing SKEYSEED  . . . . . . . . . . . . . . . . . . .   6
     3.2.  Verifying preshared key . . . . . . . . . . . . . . . . .   7
     3.3.  Child SAs . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informational References  . . . . . . . . . . . . . . . .   9
   Appendix A.  Discussion and Rationale . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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 IKE keys (along with the
   parameters that IKEv2 normally uses); this secret adds quantum
   resistance to the exchange.

   It was considered important to minimize the changes to IKEv2.  The
   existing mechanisms to do authentication and key exchange remain in

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   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


   - 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].

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), 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.

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   In addition, we assume that the initiator knows which PPK to use with
   the peer it is initiating to (for instance, if it knows the peer,
   then it can determine which PPK will be used).

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 exchange as follows:

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

   N(PPK_REQUEST) 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 recieves the initial exchange with the notify
   payload, then (if it is configured to support PPK), it responds with:

   Initiator                       Responder
                             <--- HDR, N(COOKIE), N(PPK_ENCODE)

   If it is not configured to support PPK, the responder continues with
   the standard IKEv2 protocol.

   In other words, it asks for the responder to generate and send a
   cookie in its responses (as listed in section 2.6 of RFC7296), and in
   addition, include a notify that gives details of how the initiator
   should indicate what the PPK is.  This notification payload has the
   type [TBA}; it has a protocol ID of 0, and no SPI; the notification
   data is of the format:

                           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 Indicator Algorithm                  |
      |                   PPK Indicator Input (variable)              |

   The PPK Indicator Algorithm is a 4 byte word that states which PPK
   indicator to use.  That is, it gives the encoding format for the PPK
   that should be used is given to the responder.  At present, the only
   assigned encoding is 0x00000001, which indicates that AES256_SHA256
   will be used (as explained below).

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   PPK Indicator Input is a data input to the PPK indicator Algorithm;
   its length will depend on the PPK indicator; for the indicator
   AES256_SHA256, this PPK Indicator Input is 16 bytes.

   The contents of this PPK Indicator Input is selected by responder
   policy; below we give trade-offs of the various possibilities

   When the initiator receives this notification, it responds as

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

   This is the standard IKEv2 cookie response, with a PPK_REQUEST
   notification added

   N(PPK_REQUEST) is a status notification payload with the type [TBA];
   it has a protocol ID of 0, and no SPI; however this time, the
   notification data as as follows:

                           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 Indicator Algorithm                  |
      |                   PPK Indicator Input (variable)              |
      |                      PPK Indicator (variable)                 |

   The PPK Indicator Algorithm and PPK Indicator Input are precisely the
   same as was given in the PPK_ENCODE format (as is repeated in case
   the responder ran this cookie protocol in a stateless manner).  The
   PPK Indicator is the encoded version of the PPK that the initiator
   has.  The idea behind this is to allow the responder to select which
   PPK it should use when it derives the IKEv2 keys.

   For the AES256_SHA256 PPK indicator, the PPK Indicator is 16 bytes.
   To compute it, we use HMAC_SHA256(PPK, "A") as the 256 bit AES key to
   encrypt the 16 bytes on PPK Indicator Input (in ECB mode), where "A"
   is a string consisting of a single 0x41 octet.

   When the responder receives this notification payload, it verifies
   that the PPK Indicator Algorithm is as it has specified, and it MAY
   verify that the PPK Indicator Input is as it has specified.  If
   everything is on the level, it scans through its list of configured
   postquantum preshared keys, and determines which one it is (possibly

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   (assuming AES256_SHA256_PPK) by computing AES256(HMAC_SHA256(PPK,
   "A"), PPK_Indicator_Input) and comparing that value to the 16 bytes
   within the payload.  Alternatively, it may have preselected a PPK
   Indicator Input, and has precomputed (again assuming
   AES256_SHA256_PPK) AES256(HMAC_SHA256(PPK, "A"), PPK_Indicator_Input)
   for each PPK it knows about (in which case, this is a simple search).

   If the responder finds a value that matches the payload for a
   particular PPK, that indicates that the intiator and responder share
   a PPK and can make use of this extension.  Upon finding such a
   preshared key, the responder includes a notification payload with the

   Initiator                       Responder
                       <--- HDR, SAr1, Ker, Nr, [CERTREQ], N(PPK_ACK)

   N(PPK_ACK) 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.  This notification serves as a postquantum
   preshared key confirmation.

   If the responder does not find such a PPK, then it MAY continue with
   the protocol without including a notification ID (if it is configured
   to not have mandatory preshared keys), or it MAY abort the exchange
   (if it configured to make preshared keys mandatory).

   When the initiator receives the response, it MUST check for the
   presence of the notification.  If it receives one, it marks the SA as
   using the configured preshared key; if it does not receive one, it
   MAY either abort the exchange (if the preshared key was configured as
   mandatory), or it MAY continue without using the preshared key (if
   the preshared key was configured as optional).

3.1.  Computing SKEYSEED

   When it comes time to generate the keying material during the initial
   Exchange, the implementation (both the initiator and the responder)
   checks to see if there was an agreed-upon preshared key.  If there
   was, then both sides use this alternative formula:

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

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   where PPK is the postquantum preshared key, Ni, Nr are the nonces
   exchanged in the IKEv2 exchange, and prf is the pseudorandom function
   that was negotiated for this SA.

   We reuse the negotiated PRF to transform the received nonces.  We use
   this 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 procedure.

3.2.  Verifying preshared key

   Once both the initiator and the responder have exchanged identities,
   they both double-check with their policy database to verify that they
   were configured to use those preshared keys when negotiating with the
   peer.  If they are not, they MUST abort the exchange.

3.3.  Child SAs

   When you create a child SA, 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 one of the
   alternative derivations (depending on whether an optional Diffie-
   Hellman was included):

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


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

   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:

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

4.  Security Considerations

   The PPK Indicator Input within the PPK_ENCODE notification are there
   to prevent anyone from deducing whether two different exchanges use
   the same PPK values.  To prevent such a leakage, servers are

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   encouraged to vary them as much as possible (however, they may want
   to repeat values to speed up the search for the PPK).  Repeating
   these values places the anonymity at risk; however it has no other
   security implication.

   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

   In addition, the policy SHOULD be set to negotiate only quantum-
   resistant symmetric algorithms; here is a list of defined IKEv2 (and
   IPsec) algorithms which are believed to be Quantum Resistant

   IKE Encryption algorithm: assuming that the negotiated keysize is >=
   256, then all of: ENCR_AES_CBC, ENCR_AES_CTR, ENCR_AES_CCM_*,

   IKE PRF: PRF_HMAC_SHA2_256, PRF_HMAC_SHA2_384, PRF_SHA2_512.  Note
   that PRF_AES128_XCBC and PRF_AES128_CBC are not on this list, even
   though they can use larger keys, because they use a 128 bit key

   IKE Integrity algorithm: AUTH_HMAC_SHA2_256, AUTH_HMAC_SHA2_384,

   AH Transforms: AH-SHA2-256, AH-SHA2-384, AH-SHA2-512, AH-AES-256-GMAC

   ESP Transforms: assuming that the negotiated keysize is >= 256, then

   ESP Authentication algorithms: HMAC-SHA2-256, HMAC-SHA2-384, HMAC-
   SHA2-512, AES-256-GMAC

5.  References

5.1.  Normative References

   [AES]      National Institute of Technology, "Specification for the
              Advanced Encryption Standard (AES)", 2001, <FIPS 197>.

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

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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,

   [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, <>.

5.2.  Informational References

   [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
   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
   unless they can find the PPK, and that's too difficult if the PPK has
   enough entropy (say, 256 bits).

   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
   IKEv2.  One such goal is anonymity; that someone listening into the
   exchanges cannot easily determine who is negotiating with whom.

   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

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   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 allow the responder to make the
   trade-off between anonymity and efficiency (by including the PPK
   Indicator Input, which varies how the PPK is encoded, and allowing
   the responder to specify it).

   A responder who values anonymitity may select a random PPK Indicator
   Input each time; in this case, the responder needs to do a linear
   scan over all PPK's it has been configured with

   A responder who can't afford a linear scan could precompute a small
   (possibly rolling) set of the PPK Indicator Inputs; in this case, it
   would precompute how each PPK would be indicated.  If it reissues the
   same PPK Indicator Input to two different exchanges, someone would be
   able to verify whether the same PPK was used; this is some loss of
   anonymity; but is considerably more efficient.

   An alternative approach to solve this problem would be to do a normal
   (non-QR) IKEv2 exchange, and when the two sides obtain identities,
   see if they need to be QR, and if so, create an immediate IKEv2 child
   SA (using the PPK).  One issue with this is that someone with a
   quantum computer could deduce the identities used; another issue is
   the added complexity required by the IKE state machines.

   A slightly different approach to try to make this even more friendly
   to IKEv2-based cryptographic hardware might be to use invertible
   cryptography when we present the nonces to the kdf.  The idea here is
   in case we have IKEv2 hardware that insists on selecting its own
   nonces (and so we won't be able to give a difference nonce to the
   KDF); instead, we encrypt the nonce that we send (and decrypt the
   nonce that we get).  Of course, this means that the responder will
   need to figure out which PPK we're using up front (based on the
   notifications); we're not sure if this idea would be a net
   improvement (especially since the transform we're proposing now is
   cryptographically secure and simple).

   The reasoning behind the cryptography used: the values we use in the
   AES256_SHA256 PPK Indicator Algorithm are cryptographically
   independent of the values used during the SKEYSEED generation
   (because, even if we use HMAC_256 as our PRF, HMAC_SHA256(PPK, A) is
   independent of HMAC_SHA256(PPK, B) if A and B are different strings
   (and as any real nonce must be longer than a single byte, there is

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   never a collision between that and "A".  This independent stems from
   the assumption that HMAC_SHA256 is a secure MAC.

   The method of encoding the PPK within the notification (using AES-
   256) was chosen as it met two goals:

   o  Anonymity; given A, AES256_K1(A), B, AES256_K2(B), it's fairly
      obvious that gives someone (even if they have a quantum computer)
      no clue about whether K1==K2 (unless either A==B or AES256_K1(A)==
      AES256_K2(B); both highly unlikely events if A and B are chosen

   o  Performance during the linear search; a responder could preexpand
      the AES keys, and so comparing a potential PPK against a
      notification from the initiator would amount to performing a
      single AES block encryption and then doing a 16 byte comparison.

   The first goal is considered important; one of the goals of IKEv2 is
   to provide anonymity.  The second is considered important because the
   linear scan directly affects scalability.  While this draft allows
   the server to gain performance at the cost of anonymity, it was
   considered useful if we make the fully-anonymous method as attractive
   as possible.  This use of AES makes this linear scan as cheap as
   possible (while preserving security).

   We allow the responder to specify the PPK Indicator Algorithm; this
   was in response to requests for algorithm agility.  At present, it
   appears unlikely that there would be a need for an additional
   encoding (as the current one is extremely conservative
   cryptographically); however the option is there.

   The current draft forces a cookie exchange, and hence adds a round
   trip over the normal IKEv2 operation.  This was done to allow the
   server to specify the PPK Indicator algorithm.  While as additional
   round trip may seem costly, it does not invalidate this proposal, The
   reason for this proposal is to give an alternative to IKEv1 with
   preshared keys.  While this additional round trip may seem costly, it
   is important to note that, even with the additional round trip, this
   proposal is still cheaper than IKEv1.  Thus the mechanisms specified
   in this note meet the goal of providing a better alternative than
   relying on an obsolete version of the protocol for post quantum

   One issue that is currently open: what should happen if the initiator
   guesses at the PPK Indicator Algorithm, selects a random PPK
   Indicator Input, and includes that in the initial message?  After
   all, if the server follows the recommendation that the cookie
   exchange is stateless, and if the server chooses the PPK Indicator

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   Input In randomly, it has no way to know that the client isn't
   running this protocol as specified.  If the responder supports that
   PPK Indicator Algorithm, it could very well respond without forcing a
   cookie exchange (which would eliminate a message exchange round).
   It's not clear is whether we should endorse this mode of operation,
   and explicitly state that if the server recieves such an initial
   request, and it doesn't recognize the PPK Indicator Input, it should
   act like it recieved an iniital PPK_REQUEST.

Authors' Addresses

   Scott Fluhrer
   Cisco Systems


   David McGrew
   Cisco Systems


   Panos Kampanakis
   Cisco Systems


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