Internet Engineering Task Force                               S. Fluhrer
Internet-Draft                                             P. Kampanakis
Intended status: Standards Track                               D. McGrew
Expires: July 17, 2020                                     Cisco Systems
                                                              V. Smyslov
                                                        January 14, 2020

        Mixing Preshared Keys in IKEv2 for Post-quantum Security


   The possibility of quantum computers poses a serious challenge to
   cryptographic 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
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 July 17, 2020.

Copyright Notice

   Copyright (c) 2020 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 . . . . . . . . . . . . . . . . . .   6
   2.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Exchanges . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Upgrade procedure . . . . . . . . . . . . . . . . . . . . . .  11
   5.  PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     5.1.  PPK_ID format . . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  Operational Considerations  . . . . . . . . . . . . . . .  13
       5.2.1.  PPK Distribution  . . . . . . . . . . . . . . . . . .  13
       5.2.2.  Group PPK . . . . . . . . . . . . . . . . . . . . . .  13
       5.2.3.  PPK-only Authentication . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     8.2.  Informational References  . . . . . . . . . . . . . . . .  18
   Appendix A.  Discussion and Rationale . . . . . . . . . . . . . .  19
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   Recent achievements in developing quantum computers demonstrate that
   it is probably feasible to build a cryptographically significant one.
   If such a computer is implemented, 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 quantum-secure, then the resulting system is believed

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   to be quantum-secure, that is, secure against classical attackers of
   today or future attackers 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-secure.  By bringing post-
   quantum security to IKEv2, this document 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) 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 PKI
   authentication if configured).  This document does not replace the
   authentication checks that the protocol does; instead, they are
   strengthened by using an additional secret key.

1.1.  Changes


   Changes in this draft in each version iterations.


   o  Updates the IANA section based on Eric V.'s IESG Review.

   o  Updates based on IESG Reviews (Alissa, Adam, Barry, Alexey, Mijra,
      Roman, Martin.


   o  Addresses issues raised during IETF LC.


   o  Addresses issues raised in AD review.


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   o  Editorial changes.


   o  Editorial changes.


   o  Editorial changes.


   o  Addressed comments received during WGLC.


   o  Using Group PPK is clarified based on comment from Quynh Dang.


   o  Editorial changes and minor text nit fixes.

   o  Integrated Tommy P. text suggestions.


   o  Added note that the PPK is stirred in the initial IKE SA setup

   o  Added note about the initiator ignoring any content in the
      PPK_IDENTITY notification from the responder.

   o  fixed Tero's suggestions from 2/6/1028

   o  Added IANA assigned message types where necessary.

   o  fixed minor text nits


   o  Nits and minor fixes.

   o  prf is replaced with prf+ for the SK_d and SK_pi/r calculations.

   o  Clarified using PPK in case of EAP authentication.

   o  PPK_SUPPORT notification is changed to USE_PPK to better reflect
      its purpose.

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   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.

   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.


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   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.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Assumptions

   We assume that each IKE peer has a list of Post-quantum Preshared
   Keys (PPK) along with their identifiers (PPK_ID), and any potential
   IKE initiator selects which PPK to use with any specific responder.
   In addition, implementations have a configurable flag that determines
   whether this post-quantum preshared key is mandatory.  This PPK is
   independent of the preshared key (if any) that the IKEv2 protocol
   uses to perform authentication (because the preshared key in IKEv2 is
   not used for any key derivation, and thus doesn't protect against
   quantum computers).  The PPK specific configuration that is assumed
   to be on each node consists of the following tuple:

   Peer, PPK, PPK_ID, mandatory_or_not

3.  Exchanges

   If the initiator is configured to use a post-quantum preshared key
   with the responder (whether or not the use of the PPK is mandatory),
   then it MUST include a notification USE_PPK in the IKE_SA_INIT
   request message as follows:

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

   N(USE_PPK) is a status notification payload with the type 16435; 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
   notification, then the resend would include the USE_PPK notification
   if the original message did (see Section 2.6 of [RFC7296]).

   If the responder does not support this specification or does not have
   any PPK configured, then it ignores the received notification (as
   defined in [RFC7296] for unknown status notifications) and continues
   with the IKEv2 protocol as normal.  Otherwise the responder replies
   with the IKE_SA_INIT message including a USE_PPK notification in the

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

   When the initiator receives this reply, it checks whether the
   responder included the USE_PPK 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 USE_PPK notification and using a
   PPK for this particular responder is optional, then the initiator
   continues with the IKEv2 protocol as normal, without using PPKs.

   If the responder did include the USE_PPK notification, then the
   initiator selects a PPK, along with its identifier PPK_ID.  Then, it
   computes this modification of the standard IKEv2 key derivation from
   Section 2.14 of [RFC7296]:

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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 }

    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 resulting subkeys SK_d, SK_pi, SK_pr (marked with primes in
   the formula above) are then run through the prf+ again, this time
   using the PPK as the key.  The result is the unprimed versions of
   these keys which are then used as inputs to subsequent steps of the
   IKEv2 exchange.

   Using a prf+ construction ensures that it is always possible to get
   the resulting keys of the same size as the initial ones, even if the
   underlying PRF has output size different from its key size.  Note,
   that at the time of this writing, all PRFs defined for use in IKEv2
   [IKEV2-IANA-PRFS] had output size equal to the (preferred) key size.
   For such PRFs only the first iteration of prf+ is needed:

    SK_d  = prf (PPK, SK_d'  | 0x01)
    SK_pi = prf (PPK, SK_pi' | 0x01)
    SK_pr = prf (PPK, SK_pr' | 0x01)

   Note that the PPK is used in SK_d, SK_pi and SK_pr calculation only
   during the initial IKE SA setup.  It MUST NOT be used when these
   subkeys are calculated as result of IKE SA rekey, resumption or other
   similar operation.

   The initiator then sends the IKE_AUTH request 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 16436; it has a
   protocol ID of 0, no SPI and a notification data that consists of the
   identifier PPK_ID.

   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 the responder could be able to

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   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 (based on the mandatory_or_not flag),
   then the initiator MUST include a NO_PPK_AUTH notification in the
   above message.  This notification informs the responder that PPK is
   optional and allows for authenticating the initiator without using

   NO_PPK_AUTH is a status notification with the type 16437; it has a
   protocol ID of 0 and no SPI.  The Notification Data field contains
   the initiator's authentication data computed using SK_pi', which has
   been computed without using PPKs.  This is the same data that would
   normally be placed in the Authentication Data field of an AUTH
   payload.  Since the Auth Method field is not present in the
   notification, the authentication method used for computing the
   authentication data MUST be the same as method indicated in the AUTH
   payload.  Note that if the initiator decides to include the
   NO_PPK_AUTH notification, the initiator needs to perform
   authentication data computation twice, which may consume 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 )

   The responder 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 USE_PPK notifications in
   the IKE_SA_INIT exchange, then the responder MUST send back
   AUTHENTICATION_FAILED notification and then fail the negotiation.

   If the PPK_IDENTITY notification contains a 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 is 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 for the responder:

    Received  Received   Configured  PPK is
    USE_PPK  NO_PPK_AUTH  with 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 the 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 IKE_AUTH 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); the content of the received PPK_IDENTITY (if
   any) MUST be ignored.  If the initiator does not receive the
   PPK_IDENTITY, 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).

   If EAP is used in the IKE_AUTH exchange, then the initiator doesn't
   include AUTH payload in the first request message, however the
   responder sends back AUTH payload in the first reply.  The peers then

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   exchange AUTH payloads after EAP is successfully completed.  As a
   result, the responder sends AUTH payload twice - in the first
   IKE_AUTH reply message and in the last one, while the initiator sends
   AUTH payload only in the last IKE_AUTH request.  See more details
   about EAP authentication in IKEv2 in Section 2.16 of [RFC7296].

   The general rule for using PPK in the IKE_AUTH exchange, which covers
   EAP authentication case too, is that the initiator includes
   PPK_IDENTITY (and optionally NO_PPK_AUTH) notification in the request
   message containing AUTH payload.  Therefore, in case of EAP the
   responder always computes the AUTH payload in the first IKE_AUTH
   reply message without using PPK (by means of SK_pr'), since PPK_ID is
   not yet known to the responder.  Once the IKE_AUTH request message
   containing the PPK_IDENTITY notification is received, the responder
   follows the rules described above for the non-EAP authentication

      Initiator                         Responder
      HDR, SK {IDi, [CERTREQ,]
          [IDr,] SAi2,
          TSi, TSr}  -->
                                   <--  HDR, SK {IDr, [CERT,] AUTH,
      HDR, SK {EAP}  -->
                                   <--  HDR, SK {EAP (success)}
      HDR, SK {AUTH,
          [, N(NO_PPK_AUTH)]}  -->
                                   <--  HDR, SK {AUTH, SAr2, TSi, TSr
                                        [, N(PPK_IDENTITY)]}

   Note that the diagram above shows both the cases when the responder
   uses PPK and when it chooses not to use it (provided the initiator
   has included NO_PPK_AUTH notification), and thus the responder's
   PPK_IDENTITY notification is marked as optional.  Also, note that the
   IKE_SA_INIT exchange in case of PPK is as described above (including
   exchange of the USE_PPK notifications), regardless whether EAP is
   employed in the IKE_AUTH or not.

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:

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   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 USE_PPK
   notification and the NO_PPK_AUTH notification; if the initiator has
   not been upgraded, it will not send the USE_PPK notification (and so
   the responder will know that the peers will not use a PPK).  If the
   responder has not been upgraded, it will not send the USE_PPK
   notification (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
   both the responder and initiator have been upgraded and properly
   configured, they will both realize it, and the Child SAs will be

   As an optional second step, after all nodes have been upgraded, then
   the administrator should then go back through the nodes, and mark the
   use of PPK as mandatory.  This will not affect the strength against a
   passive attacker, but it would mean that an active 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 specifications will extend this technique to allow
   dynamically changing PPK values.  To facilitate such an extension, we
   specify 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

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      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 the PPK_ID to
      look up 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 [RFC4648].

5.2.  Operational Considerations

   The need to maintain several independent sets of security credentials
   can significantly complicate a security administrator's job, and can
   potentially slow down widespread adoption of this specification.  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
   for the transport and provisioning of symmetric keys.  That format
   could be reused using the PIN profile (defined in Section 10.2 of
   [RFC6030]) with the "Id" attribute of the <Key> element being the
   PPK_ID (without the PPK_ID Type octet for a PPK_ID_FIXED) and the
   <Secret> element containing the PPK.

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 as
   many independent PPKs as the peers it is going to communicate with.
   As the number of peers grows the PPKs will not scale.

   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.  However group

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   members can record any traffic they have access to that comes from
   other group members and decrypt it later, when they get access to a
   quantum computer.

   In addition, the fact that the PPK is known to a (potentially large)
   group of users makes it more susceptible to theft.  When an attacker
   equipped with a quantum computer gets access to a group PPK, all
   communications inside the group are revealed.

   For these reasons using group PPK is NOT RECOMMENDED.

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 the 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
   IKE_SA_INIT and use NULL Authentication.  Additionally, since the
   peers are authenticated via PPK, the ID Type in the IDi/IDr payloads
   SHOULD NOT be ID_NULL, despite using the NULL Authentication method.

6.  Security Considerations

   Quantum computers are able to perform Grover's algorithm [GROVER];
   that effectively halves the size of a symmetric key.  Because of
   this, the user SHOULD ensure that the post-quantum preshared key used
   has at least 256 bits of entropy, in order to provide 128 bits of
   post-quantum security.  That provides security equivalent to Level 5
   as defined in the NIST PQ Project Call For Proposals [NISTPQCFP].

   With this protocol, the computed SK_d is a function of the PPK.
   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, all keys that are a function of SK_d, which
   include all Child SAs keys and all keys for subsequent IKE SAs
   (created when the initial IKE SA is rekeyed), are also quantum-secure

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   (assuming that the PPK was of high enough entropy, and that all the
   subkeys are sufficiently long).

   An attacker with a quantum computer that can decrypt the initial IKE
   SA has access to all the information exchanged over it, such as
   identities of the peers, configuration parameters and all negotiated
   IPsec SAs information (including traffic selectors), with the
   exception of the cryptographic keys used by the IPsec SAs which are
   protected by the PPK.

   Deployments that treat this information as sensitive or that send
   other sensitive data (like cryptographic keys) over IKE SA MUST rekey
   the IKE SA before the sensitive information is sent to ensure this
   information is protected by the PPK.  It is possible to create a
   childless IKE SA as specified in [RFC6023].  This prevents Child SA
   configuration information from being transmitted in the original IKE
   SA that is not protected by a PPK.  Some information related to IKE
   SA, that is sent in the IKE_AUTH exchange, such as peer identities,
   feature notifications, Vendor ID's etc. cannot be hidden from the
   attack described above, even if the additional IKE SA rekey is

   In addition, the policy SHOULD be set to negotiate only quantum-
   secure symmetric algorithms; while this RFC doesn't claim to give
   advice 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 to
   provide less than 128 bits of post-quantum security

   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.

   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 does not include
   the USE_PPK notification in the response.  In this situation, when
   the initiator aborts negotiation it leaves a half-open IKE SA on the
   responder (because IKE_SA_INIT completes successfully from the
   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 (DoS) attack and take protection measures (see
   [RFC8019] for more detail).  In this situation, it is RECOMMENDED

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   that the initiator caches the negative result of the negotiation and
   doesn't make attempts to create it again for some time.  This period
   of time may vary, but it is believed that waiting for at least few
   minutes will not cause the responder to treat it as DoS attack.
   Note, that this situation would most likely be a result of
   misconfiguration and some re-configuration of the peers would
   probably be 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 USE_PPK notification from the IKE_SA_INIT 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 able
   to eavesdrop and to inject packets into the network can prevent
   creating an IKE SA by mounting the following attack.  The attacker
   intercepts the initial request containing the USE_PPK notification
   and injects a forged response containing no USE_PPK.  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 USE_PPK notification, then the initiator doesn't abort the
   exchange immediately.  Instead it waits for more response messages
   retransmitting the request as if no responses were received at all,
   until either the received message contains the USE_PPK or the
   exchange times out (see section 2.4 of [RFC7296] for more details
   about retransmission timers in IKEv2).  If neither of the received
   responses contains USE_PPK, then the exchange is aborted.

   If using PPK is optional for both peers, then in case of
   misconfiguration (e.g., mismatched PPK_ID) the IKE SA will be created
   without protection against quantum computers.  It is advised that if
   PPK was configured, but was not used for a particular IKE SA, then
   implementations SHOULD audit this event.

7.  IANA Considerations

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

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   16435       USE_PPK           [THIS RFC]
   16436       PPK_IDENTITY      [THIS RFC]
   16437       NO_PPK_AUTH       [THIS RFC]

   This document also creates a new IANA registry "IKEv2 Post-quantum
   Preshared Key ID Types" in IKEv2 IANA registry
   ( for the PPK_ID
   types used in the PPK_IDENTITY notification defined in this
   specification.  The initial values of the new registry are:

   PPK_ID Type               Value      Reference
   -----------               -----      ---------
   Reserved                  0          [THIS RFC]
   PPK_ID_OPAQUE             1          [THIS RFC]
   PPK_ID_FIXED              2          [THIS RFC]
   Unassigned                3-127      [THIS RFC]
   Private Use               128-255    [THIS RFC]

   The PPK_ID type value 0 is reserved; values 3-127 are to be assigned
   by IANA; values 128-255 are for private use among mutually consenting
   parties.  To register new PPK_IDs in the unassigned range, a Type
   name, a Value between 3 and 127 and a Reference specification need to
   be defined.  Changes and additions to the unassigned range of this
   registry are by the Expert Review Policy [RFC8126].  Changes and
   additions to the private use range of this registry are by the
   Private Use Policy [RFC8126].

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, <>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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8.2.  Informational References

   [GROVER]   Grover, L., "A Fast Quantum Mechanical Algorithm for
              Database Search", Proc. of the Twenty-Eighth Annual ACM
              Symposium on the Theory of Computing (STOC 1996), 1996.

              Hoffman, P., "The Transition from Classical to Post-
              Quantum Cryptography", draft-hoffman-c2pq-06 (work in
              progress), November 2019.

              "Internet Key Exchange Version 2 (IKEv2) Parameters,
              Transform Type 2 - Pseudorandom Function Transform IDs",

              NIST, "NIST Post-Quantum Cryptography Call for Proposals",
              2016, <

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

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,

   [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,

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   [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,

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

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
   document 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 and 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 only
   adjusting the SK_d, SK_pi, SK_pr, it is hoped that this would be
   implementable, even on systems that perform most of the IKEv2
   processing 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-secure 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 properties
   provided by IKEv2.

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Appendix B.  Acknowledgements

   We would like to thank Tero Kivinen, Paul Wouters, Graham Bartlett,
   Tommy Pauly, Quynh Dang and the rest of the IPSecME Working Group for
   their feedback and suggestions for the scheme.

Authors' Addresses

   Scott Fluhrer
   Cisco Systems


   Panos Kampanakis
   Cisco Systems


   David McGrew
   Cisco Systems


   Valery Smyslov

   Phone: +7 495 276 0211

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