Internet Engineering Task Force                               S. Fluhrer
Internet-Draft                                                 D. McGrew
Intended status: Standards Track                           P. Kampanakis
Expires: April 18, 2018                                    Cisco Systems
                                                              V. Smyslov
                                                        October 15, 2017

                  Postquantum Preshared Keys for IKEv2


   The possibility of Quantum Computers pose a serious challenge to
   cryptography algorithms deployed 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

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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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 18, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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
     1.1.  Changes . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Upgrade procedure . . . . . . . . . . . . . . . . . . . . . .   9
   5.  PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  PPK_ID format . . . . . . . . . . . . . . . . . . . . . .   9
     5.2.  Operational Considerations  . . . . . . . . . . . . . . .  10
       5.2.1.  PPK Distribution  . . . . . . . . . . . . . . . . . .  10
       5.2.2.  Group PPK . . . . . . . . . . . . . . . . . . . . . .  11
       5.2.3.  PPK-only Authentication . . . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informational References  . . . . . . . . . . . . . . . .  14
   Appendix A.  Discussion and Rationale . . . . . . . . . . . . . .  15
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   It is an open question whether or not it is feasible to build a
   Quantum Computer (and if so, when one might 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 in polynomial time [I-D.hoffman-c2pq], and this
   would imply that the security of existing IKEv2 [RFC7296] systems
   would be compromised.  IKEv1 [RFC2409], 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, invulnerable to
   an attacker with a Quantum Computer.

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   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 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 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 document 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 in each version iterations.


   o  Migrated from draft-fluhrer-qr-ikev2-05 to draft-ietf-ipsecme-qr-
      ikev2-00 that is a WG item.


   o  Nits and editorial fixes.

   o  Made PPK_ID format and PPK Distributions subsection of the PPK
      section.  Also added an Operational Considerations section.

   o  Added comment about Child SA rekey in the Security Considerations

   o  Added NO_PPK_AUTH to solve the cases where a PPK_ID is not
      configured for a responder.

   o  Various text changes and clarifications.

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   o  Expanded Security Considerations section to describe some security
      concerns and how they should be addressed.


   o  Modified how we stir the PPK into the IKEv2 secret state.

   o  Modified how the use of PPKs is negotiated.


   o  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 negotiation.

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


   o  Added explicit guidance as to what IKE and IPsec algorithms are
      quantum resistant.


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

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

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

   o  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.

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

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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 any specific
   responder.  In addition, implementations have 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.  The PPK specific
   configuration that is assumed on each peer consists of the following

   Peer, PPK, PPK_ID, mandatory_or_not

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 mandatory),
   then it will include a notification PPK_SUPPORT in the initial
   exchange as follows:

   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, 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.

   If the responder does not support this specification or does not have
   any PPK configured, then it ignores the received notification and
   continues with the IKEv2 protocol as normal.  Otherwise the responder
   checks if it has a PPK configured, and if it does, then the responder
   replies with the IKEv2 initial exchange including a PPK_SUPPORT
   notification in the response:

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

   When the initiator receives this reply, it checks whether the
   responder included the PPK_SUPPORT notification.  If the responder
   did not and the flag mandatory_or_not indicates that using PPKs is
   mandatory for communication with this responder, then the initiator
   MUST abort the exchange.  This situation may happen in case of
   misconfiguration, when the initiator believes it has a mandatory to
   use PPK for the responder, while the responder either doesn't support
   PPKs at all or doesn't have any PPK configured for the initiator.
   See Section 6 for discussion of the possible impacts of this

   If the responder did not include the PPK_SUPPORT notification and
   using PPKs for this responder is optional, then the initiator
   continues with the IKEv2 protocol as normal, without using PPKs.

   If the responder did include the PPK_SUPPORT notification, then the
   initiator selects a PPK, along with its identifier PPK_ID.  Then, it
   computes this modification of the standard IKEv2 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 IKEv2 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(NO_PPK_AUTH)]}  --->

   PPK_IDENTITY is a status notification with the type [TBA]; it has a
   protocol ID of 0, no SPI and a notification data that consists of the
   identifier PPK_ID.

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   A situation may happen when the responder has some PPKs, but doesn't
   have a PPK with the PPK_ID received from the initiator.  In this case
   the responder cannot continue with PPK (in particular, it cannot
   authenticate the initiator), but it could be able to continue with
   normal IKEv2 protocol if the initiator provided its authentication
   data computed as in normal IKEv2, without using PPKs.  For this
   purpose, if using PPKs for communication with this responder is
   optional for the initiator, then the initiator MAY include a
   notification NO_PPK_AUTH in the above message.

   NO_PPK_AUTH is a status notification with the type [TBA]; it has a
   protocol ID of 0 and no SPI.  A notification data consists of the
   initiator's authentication data computed using SK_pi' (i.e. the data
   that computed without using PPKs and would normally be placed in the
   AUTH payload).  Authentication Method for computing the
   authentication data MUST be the same as indicated in the AUTH payload
   and is not included in the notification.  Note that if the initiator
   decides to include NO_PPK_AUTH notification, then it means that the
   initiator needs to perform authentication data computation twice that
   may consume substantial computation power (e.g. if digital signatures
   are involved).

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

    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/SK_ai values to decrypt/check the message and
   then scans through the payloads for the PPK_ID attached to the
   PPK_IDENTITY notification.  If no PPK_IDENTITY notification is found
   and the peers successfully exchanged PPK_SUPPORT notifications in the
   initial exchange, then the responder MUST send back
   AUTHENTICATION_FAILED notification and then fail the negotiation.

   If the PPK_IDENTITY notification contains PPK_ID that is not known to
   the responder or is not configured for use for the identity from IDi
   payload, then the responder checks whether using PPKs for this
   initiator is mandatory and whether the initiator included NO_PPK_AUTH
   notification in the message.  If using PPKs is mandatory or no
   NO_PPK_AUTH notification found, then then the responder MUST send
   back AUTHENTICATION_FAILED notification and then fail the
   negotiation.  Otherwise (when PPK is optional and the initiator
   included NO_PPK_AUTH notification) the responder MAY continue regular
   IKEv2 protocol, except that it uses the data from the NO_PPK_AUTH
   notification as the authentication data (which usually resides in the
   AUTH payload), for the purpose of the initiator authentication.

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   Note, that Authentication Method is still indicated in the AUTH

   This table summarizes the above logic by the responder:

   Received     Received    Have       PPK
  PPK_SUPPORT  NO_PPK_AUTH  PPK     Mandatory    Action
       No         *          No         *        Standard IKEv2 protocol
       No         *         Yes        No        Standard IKEv2 protocol
       No         *         Yes       Yes        Abort negotiation
      Yes        No          No         *        Abort negotiation
      Yes       Yes          No       Yes        Abort negotiation
      Yes       Yes          No        No        Standard IKEv2 protocol
      Yes         *         Yes         *        Use PPK

   If PPK is in use, then the responder extracts corresponding PPK and
   computes the following values:

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

   The responder then continues with the exchange (validating the AUTH
   payload that the initiator included) as usual and sends back a
   response, which includes the PPK_IDENTITY notification with no data
   to indicate that the PPK is used in the exchange:

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

   When the initiator receives the response, then it checks for the
   presence of the PPK_IDENTITY 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
   fail the IKE SA negotiation sending the AUTHENTICATION_FAILED
   notification in the Informational exchange (if the PPK was configured
   as mandatory), or continue without using the PPK (if the PPK was not
   configured as mandatory and the initiator included the NO_PPK_AUTH
   notification in the request).

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4.  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:

   o  The set of PPKs (and corresponding PPK_IDs) that this node would
      need to know.

   o  For each peer that this node would initiate to, which PPK will be

   o  That the use of PPK is currently not mandatory.

   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 and the NO_PPK_AUTH 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 to not use a PPK).  If both peers have
   been upgraded, but the responder isn't yet configured with the PPK
   for the initiator, then the responder could do standard IKEv2
   protocol if the initiator sent NO_PPK_AUTH notification.  If the
   responder has not been upgraded and properly configured, 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).

5.  PPK

5.1.  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 standard, both the
   initiator and the responder are configured with fixed PPK and PPK_ID
   values, and do the look up based on PPK_ID value.  It is anticipated
   that later standards will extend this technique to allow dynamically
   changing PPK values.  To facilitate such an extension, we specify

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   that the PPK_ID the initiator sends will have its first octet be the
   PPK_ID Type value.  This document defines two values for PPK_ID Type:

   o  PPK_ID_OPAQUE (1) - for this type 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.

   o  PPK_ID_FIXED (2) - in this case 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 value 0 is reserved; values 3-127 are reserved for
   IANA; values 128-255 are for private use among mutually consenting

5.2.  Operational Considerations

   The need to maintain several independent sets of security credentials
   can significantly complicate security administrators job, and can
   potentially slow down widespread adoption of this solution.  It is
   anticipated, that administrators will try to simplify their job by
   decreasing the number of credentials they need to maintain.  This
   section describes some of the considerations for PPK management.

5.2.1.  PPK Distribution

   PPK_IDs 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 and out
   of scope of this document or IKEv2, [RFC6030] describes a format for
   symmetric key exchange.  That format could be reused with the Key Id
   field being the PPK_ID (without the PPK_ID Type octet for a
   PPK_ID_FIXED), the PPK being the secret, and the algorithm
   ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin") as PIN.

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5.2.2.  Group PPK

   This document doesn't explicitly require that PPK is unique for each
   pair of peers.  If it is the case, then this solution provides full
   peer authentication, but it also means that each host must have that
   many independent PPKs, how many peers it is going to communicate
   with.  As the number of hosts grows this will scale badly.

   Even though it is NOT RECOMMENDED, it is possible to use a single PPK
   for a group of users.  Since each peer uses classical public key
   cryptography in addition to PPK for key exchange and authentication,
   members of the group can neither impersonate each other nor read
   other's traffic, unless they use Quantum Computers to break public
   key operations.

   Although it's probably safe to use group PPK in short term, the fact,
   that the PPK is known to a (potentially large) group of users makes
   it more susceptible to theft.  If an attacker equipped with a Quantum
   Computer got access to a group PPK, then all the communications
   inside the group are revealed.

5.2.3.  PPK-only Authentication

   If Quantum Computers become a reality, classical public key
   cryptography will provide little security, so administrators may find
   it attractive not to use it at all for authentication.  This will
   reduce the number of credentials they need to maintain to PPKs only.
   Combining group PPK and PPK-only authentication is NOT RECOMMENDED,
   since in this case any member of the group can impersonate any other
   member even without help of Quantum Computers.

   PPK-only authentication can be achieved in IKEv2 if NULL
   Authentication method [RFC7619] is employed.  Without PPK the NULL
   Authentication method provides no authentication of the peers,
   however since a PPK is stirred into the SK_pi and the SK_pr, the
   peers become authenticated if a PPK is in use.  Using PPKs MUST be
   mandatory for the peers if they advertise support for PPK in initial
   exchange and use NULL Authentication.  Addtionally, since the peers
   are authenticated via PPK, the ID Type in the IDi/IDr payloads SHOULD
   NOT be ID_NULL, despite using NULL Authentication method.

6.  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

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   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 was 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).

   Although this protocol preserves all the security properties of IKEv2
   against adversaries with conventional computers, it 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, implementations MAY rekey the initial IKE SA immediately
   after negotiating it to generate a new SKEYSEED with from the
   postquantum SK_d.  This would reduce the amount of data available to
   an attacker with a Quantum Computer.

   Alternatively, an initial IKE SA (which is used to exchange
   identities) can take place, perhaps by using the protocol documented
   in [RFC6023].  After the childless IKE SA is created, implementations
   would immediately create a new IKE SA (which is used to exchange
   everything else) by using a rekey mechanism for IKE SAs.  Because the
   rekeyed IKE SA keys are a function of SK_d, which is a function of
   the PPK (among other things), traffic protected by that IKE SA is
   secure against Quantum capable adversaries.

   If some sensitive information (like keys) is to be transferred over
   IKE SA, then implementations MUST rekey the initial IKE SA before
   sending this information to get protection against Quantum Computers.

   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), below is a list of defined IKEv2 and
   IPsec algorithms that should NOT be used, as they are known not to be
   quantum resistant

   o  Any IKEv2 Encryption algorithm, PRF or Integrity algorithm with
      key size less than 256 bits.

   o  Any ESP Transform with key size less than 256 bits.

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   o  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.

   Section 3 requires the initiator to abort the initial exchange if
   using PPKs is mandatory for it, but the responder didn't include the
   PPK_SUPPORT notification in the response.  In this situation when the
   initiator aborts negotiation it leaves half-open IKE SA on the
   responder (because the initial exchange completes successfully from
   responder's point of view).  This half-open SA will eventually expire
   and be deleted, but if the initiator continues its attempts to create
   IKE SA with a high enough rate, then the responder may consider it as
   a Denial-of-Service attack and take some measures (see [RFC8019] for
   more detail).  It is RECOMMENDED that implementations in this
   situation cache the negative result of negotiation for some time and
   don't make attempts to create it again for some time, because this is
   a result of misconfiguration and probably some re-configuration of
   the peers is needed.

   If using PPKs is optional for both peers and they authenticate
   themselves using digital signatures, then an attacker in between,
   equipped with a Quantum Computer capable of breaking public key
   operations in real time, is able to mount downgrade attack by
   removing PPK_SUPPORT notification from the initial exchange and
   forging digital signatures in the subsequent exchange.  If using PPKs
   is mandatory for at least one of the peers or PSK is used for
   authentication, then the attack will be detected and the SA won't be

   If using PPKs is mandatory for the initiator, then an attacker
   capable to eavesdrop and to inject packets into the network can
   prevent creating IKE SA by mounting the following attack.  The
   attacker intercepts the the initial request containing the
   PPK_SUPPORT notification and injects the forget response containing
   no PPK_SUPPORT.  If the attacker manages to inject this packet before
   the responder sends a genuine response, then the initiator would
   abort the exchange.  To thwart this kind of attack it is RECOMMENDED,
   that if using PPKs is mandatory for the initiator and the received
   response doesn't contain the PPK_SUPPORT notification, then the
   initiator doesn't abort exchange immediately, but instead waits some
   time for more responses (possibly retransmitting the request).  If
   all the received responses contain no PPK_SUPPORT, then the exchange
   is aborted.

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7.  IANA Considerations

   This document defines three new Notify Message Types in the "Notify
   Message Types - Status Types" registry:

   <TBA>       PPK_SUPPORT
   <TBA>       NO_PPK_AUTH

   This document also creates a new IANA registry for the PPK_ID types.
   The initial values of this registry are:

       PPK_ID Type               Value
       -----------               -----
       Reserved                  0
       PPK_ID_OPAQUE             1
       PPK_ID_FIXED              2
       Unassigned                3-127
       Reserved for private use  128-255

   Changes and additions to this registry are by Expert Review

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,

   [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

              Hoffman, P., "The Transition from Classical to Post-
              Quantum Cryptography", draft-hoffman-c2pq-01 (work in
              progress), July 2017.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,

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   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [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, <>.

   [RFC7619]  Smyslov, V. and P. Wouters, "The NULL Authentication
              Method in the Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015,

   [RFC8019]  Nir, Y. and V. Smyslov, "Protecting Internet Key Exchange
              Protocol Version 2 (IKEv2) Implementations from
              Distributed Denial-of-Service Attacks", RFC 8019,
              DOI 10.17487/RFC8019, November 2016,

Appendix A.  Discussion and Rationale

   The idea behind this document 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
   PPKs, 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, which is 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

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   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 or quantum resistant IKEv2 is rolled 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

Appendix B.  Acknowledgements

   We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett
   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


   Valery Smyslov

   Phone: +7 495 276 0211

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