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Versions: (draft-fluhrer-qr-ikev2) 00 01 02 03 04

Internet Engineering Task Force                               S. Fluhrer
Internet-Draft                                                 D. McGrew
Intended status: Standards Track                           P. Kampanakis
Expires: January 3, 2019                                   Cisco Systems
                                                              V. Smyslov
                                                              ELVIS-PLUS
                                                            July 2, 2018


                  Postquantum Preshared Keys for IKEv2
                     draft-ietf-ipsecme-qr-ikev2-04

Abstract

   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
   keys.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 January 3, 2019.

Copyright Notice

   Copyright (c) 2018 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
   (http://trustee.ietf.org/license-info) 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 . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Upgrade procedure . . . . . . . . . . . . . . . . . . . . . .  10
   5.  PPK . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  PPK_ID format . . . . . . . . . . . . . . . . . . . . . .  11
     5.2.  Operational Considerations  . . . . . . . . . . . . . . .  12
       5.2.1.  PPK Distribution  . . . . . . . . . . . . . . . . . .  12
       5.2.2.  Group PPK . . . . . . . . . . . . . . . . . . . . . .  12
       5.2.3.  PPK-only Authentication . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     8.2.  Informational References  . . . . . . . . . . . . . . . .  16
   Appendix A.  Discussion and Rationale . . . . . . . . . . . . . .  17
   Appendix B.  Acknowledgements . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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) 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, it is done as
   a parallel check.

1.1.  Changes

   RFC EDITOR PLEASE DELETE THIS SECTION.

   Changes in this draft in each version iterations.

   draft-ietf-ipsecme-qr-ikev2-04

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

   draft-ietf-ipsecme-qr-ikev2-03

   o  Editorial changes and minor text nit fixes.

   o  Integrated Tommy P. text suggestions.

   draft-ietf-ipsecme-qr-ikev2-02

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

   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



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   o  Added IANA assigned message types where necessary.

   o  fixed minor text nits

   draft-ietf-ipsecme-qr-ikev2-01

   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.

   draft-ietf-ipsecme-qr-ikev2-00

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

   draft-fluhrer-qr-ikev2-05

   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
      section.

   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.

   draft-fluhrer-qr-ikev2-03

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

   o  Modified how the use of PPKs is negotiated.

   draft-fluhrer-qr-ikev2-02

   o  Simplified the protocol by stirring in the preshared key into the
      child SAs; this avoids the problem of having the responder decide



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      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
      Computer.

   draft-fluhrer-qr-ikev2-01

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

   draft-fluhrer-qr-ikev2-00

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   tuple:



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   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 he will include a notification USE_PPK in the IKE_SA_INIT
   request message as follows:

   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
   (because the responder response included a COOKIE notification), then
   the resend would include the USE_PPK notification if the original
   message did.

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

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

   When the initiator receives this reply, he 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 he 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
   situation.

   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.




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   If the responder did include the USE_PPK notification, then the
   initiator selects a PPK, along with its identifier PPK_ID.  Then, she
   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.  Using 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 this document
   was written, 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
   ------------------------------------------------------------------
   HDR, SK {IDi, [CERT,] [CERTREQ,]
       [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, she cannot
   authenticate the initiator), but she could be able to continue with



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   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 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, she 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 )

   She 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 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 she 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.
   Note, that Authentication Method is still indicated in the AUTH
   payload.




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   This table summarizes the above logic for the responder:

    Received    Received    Have       PPK
    USE_PPK    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 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 he checks for the
   presence of the PPK_IDENTITY notification.  If he receives one, he
   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, he 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
   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



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   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 PPK_IDENTITY notification is received, the responder
   follows rules described above for non-EAP authentication case.

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

   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:

   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
      used.

   o  That the use of PPK is currently not mandatory.




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   With this configuration, the node will continue to operate with nodes
   that have not yet been upgraded.  This is due to the USE_PPK notify
   and the NO_PPK_AUTH notify; if the initiator has not been upgraded,
   he will not send the USE_PPK notify (and so the responder will know
   that we will not use a PPK).  If the responder has not been upgraded,
   she will not send the USE_PPK 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 both the responder and initiator have
   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
   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



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      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
   parties.

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
   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 algorithm
   ("Algorithm=urn:ietf:params:xml:ns:keyprov:pskc:pin") as the PIN.

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
   members can record other members' traffic 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 got access to a group PPK, all
   communications inside the group are revealed.



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   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 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.  Addtionally, 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; 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 128-bit security
   level.

   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, 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 of high enough entropy, 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 from the postquantum



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   SK_d.  This would reduce the amount of data available to an attacker
   with a Quantum Computer.

   If 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.

   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.

   In addition, the policy SHOULD be set to negotiate only quantum-
   resistant 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 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.

   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 might not include
   the USE_PPK notification in the response.  In this situation when the
   initiator aborts negotiation he leaves 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 attack and take protection 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.





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   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
   created.

   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 initial request containing the USE_PPK
   notification and injects the forget 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, but instead waits some time for more
   responses (possibly retransmitting the request).  If all the received
   responses contain no USE_PPK, then the exchange is aborted.

7.  IANA Considerations

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

   16435       USE_PPK
   16436       PPK_IDENTITY
   16437       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
   [RFC5226].





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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, <https://www.rfc-
              editor.org/info/rfc2119>.

   [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, <https://www.rfc-editor.org/info/rfc7296>.

8.2.  Informational References

   [I-D.hoffman-c2pq]
              Hoffman, P., "The Transition from Classical to Post-
              Quantum Cryptography", draft-hoffman-c2pq-03 (work in
              progress), February 2018.

   [IKEV2-IANA-PRFS]
              "Internet Key Exchange Version 2 (IKEv2) Parameters,
              Transform Type 2 - Pseudorandom Function Transform IDs",
              <https://www.iana.org/assignments/ikev2-parameters/
              ikev2-parameters.xhtml#ikev2-parameters-6>.

   [RFC2409]  Harkins, D. and D. Carrel, "The Internet Key Exchange
              (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998,
              <https://www.rfc-editor.org/info/rfc2409>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", RFC 5226,
              DOI 10.17487/RFC5226, May 2008, <https://www.rfc-
              editor.org/info/rfc5226>.

   [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, <https://www.rfc-
              editor.org/info/rfc6023>.

   [RFC6030]  Hoyer, P., Pei, M., and S. Machani, "Portable Symmetric
              Key Container (PSKC)", RFC 6030, DOI 10.17487/RFC6030,
              October 2010, <https://www.rfc-editor.org/info/rfc6030>.






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   [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,
              <https://www.rfc-editor.org/info/rfc7619>.

   [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, <https://www.rfc-
              editor.org/info/rfc8019>.

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 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 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
   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

   Email: sfluhrer@cisco.com


   David McGrew
   Cisco Systems

   Email: mcgrew@cisco.com


   Panos Kampanakis
   Cisco Systems

   Email: pkampana@cisco.com


   Valery Smyslov
   ELVIS-PLUS

   Phone: +7 495 276 0211
   Email: svan@elvis.ru




















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