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Versions: (draft-tjhai-ipsecme-hybrid-qske-ikev2) 00 01

Internet Engineering Task Force (IETF)                          C. Tjhai
Internet-Draft                                              M. Tomlinson
Updates: 7296 (if approved)                                 Post-Quantum
Intended status: Standards Track                             G. Bartlett
Expires: January 8, 2021                                  Quantum Secret
                                                              S. Fluhrer
                                                           Cisco Systems
                                                            D. Van Geest
                                                       ISARA Corporation
                                                       O. Garcia-Morchon
                                                              V. Smyslov
                                                            July 7, 2020

                    Multiple Key Exchanges in IKEv2


   This document describes how to extend the Internet Key Exchange
   Protocol Version 2 (IKEv2) to allow multiple key exchanges to take
   place while computing a shared secret during a Security Association
   (SA) setup.  The primary application of this feature in IKEv2 is the
   ability to perform one or more post-quantum key exchanges in
   conjunction with the classical (Elliptic Curve) Diffie-Hellman key
   exchange, so that the resulting shared key is resistant against
   quantum computer attacks.  Another possible application is the
   ability to combine several key exchanges in situations when no single
   key exchange algorithm is trusted by both initiator and responder.

   This document updates RFC7296 by renaming a transform type 4 from
   "Diffie-Hellman Group (D-H)" to "Key Exchange Method (KE)" and
   renaming a field in the Key Exchange Payload from "Diffie-Hellman
   Group Num" to "Key Exchange Method".  It also renames an IANA
   registry for this transform type from "Transform Type 4 - Diffie-
   Hellman Group Transform IDs" to "Transform Type 4 - Key Exchange
   Method Transform IDs".  These changes generalize key exchange
   algorithms that can be used in IKEv2.

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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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://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 8, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Problem Description . . . . . . . . . . . . . . . . . . .   3
     1.2.  Proposed Extension  . . . . . . . . . . . . . . . . . . .   3
     1.3.  Changes . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.4.  Document Organization . . . . . . . . . . . . . . . . . .   5
   2.  Design Criteria . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Multiple Key Exchanges  . . . . . . . . . . . . . . . . . . .   8
     3.1.  Overall Design  . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Overall Protocol  . . . . . . . . . . . . . . . . . . . .   9
       3.2.1.  IKE_SA_INIT Round: Negotiation  . . . . . . . . . . .  10
       3.2.2.  IKE_INTERMEDIATE Round: Additional Key Exchanges  . .  11
       3.2.3.  IKE_AUTH Exchange . . . . . . . . . . . . . . . . . .  12
       3.2.4.  CREATE_CHILD_SA Exchange  . . . . . . . . . . . . . .  12
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Alternative Design . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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1.  Introduction

1.1.  Problem Description

   Internet Key Exchange Protocol (IKEv2) as specified in [RFC7296] uses
   the Diffie-Hellman (DH) or Elliptic Curve Diffie-Hellman (ECDH)
   algorithm to establish a shared secret between an initiator and a
   responder.  The security of the DH and ECDH algorithms relies on the
   difficulty to solve a discrete logarithm problem in multiplicative
   and elliptic curve groups respectively when the order of the group
   parameter is large enough.  While solving such a problem remains
   difficult with current computing power, it is believed that general
   purpose quantum computers will be able to solve this problem,
   implying that the security of IKEv2 is compromised.  There are,
   however, a number of cryptosystems that are conjectured to be
   resistant against quantum computer attack.  This family of
   cryptosystems is known as post-quantum cryptography (PQC).  It is
   sometimes also referred to as quantum-safe cryptography (QSC) or
   quantum-resistant cryptography (QRC).

1.2.  Proposed Extension

   This document describes a method to perform multiple successive key
   exchanges in IKEv2.  It allows integration of QSC in IKEv2, while
   maintaining backwards compatibility, to derive a set of IKE keys that
   is resistant to quantum computer attacks.  This extension allows the
   negotiation of one or more QSC algorithm to exchange data, in
   addition to the existing DH or ECDH key exchange data.  We believe
   that the feature of using more than one post-quantum algorithms is
   important as many of these algorithms are relatively new and there
   may be a need to hedge the security risk with multiple key exchange
   data from several distinct QSC algorithms.

   The secrets established from each key exchange are combined in a way
   such that should the post-quantum secrets not be present, the derived
   shared secret is equivalent to that of the standard IKEv2; on the
   other hand, a post-quantum shared secret is obtained if both
   classical and post-quantum key exchange data are present.  This
   extension also applies to key exchanges in IKE Security Associations
   (SAs) for Encapsulating Security Payload (ESP) [RFC4303] or
   Authentication Header (AH) [RFC4302], i.e. Child SAs, in order to
   provide a stronger guarantee of forward security.

   Some post-quantum key exchange payloads may have sizes larger than
   the standard maximum transmission unit (MTU) size, and therefore
   there could be issues with fragmentation at the IP layer.  IKE does
   allow transmission over TCP where fragmentation is not an issue
   [RFC8229]; however, we believe that a UDP-based solution will be

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   required too.  IKE does have a mechanism to handle fragmentation
   within UDP [RFC7383], however that is only applicable to messages
   exchanged after the IKE_SA_INIT.  To use this mechanism, this
   specification relies on the IKE_INTERMEDIATE exchange as outlined in
   [I-D.ietf-ipsecme-ikev2-intermediate].  With this mechanism, we do an
   initial key exchange, using a smaller, possibly non-quantum resistant
   primitive, such as ECDH.  Then, before we do the IKE_AUTH exchange,
   we perform one or more IKE_INTERMEDIATE exchanges, each of which
   contains an additional key exchange.  As the IKE_INTERMEDIATE
   exchange is encrypted, the IKE fragmentation protocol [RFC7383] can
   be used.  The IKE SK_* values are updated after each exchange, and so
   the final IKE SA keys depend on all the key exchanges, hence they are
   secure if any of the key exchanges are secure.

   Note that readers should consider the approach defined in this
   document as providing a long term solution in upgrading the IKEv2
   protocol to support post-quantum algorithms.  A short term solution
   to make IKEv2 key exchange quantum secure is to use post-quantum pre-
   shared keys as discussed in [RFC8784].

   Note also, that the proposed approach of performing multiple
   successive key exchanges in such a way that resulting session keys
   depend on all of them is not limited to achieving quantum resistance
   only.  It can also be used when all the performed key exchanges are
   classical (EC)DH ones, where for some reasons (e.g. policy
   requirements) it is essential to perform multiple of them.

1.3.  Changes


   Changes in this draft in each version iterations.


   o  References are updated.


   o  Draft name changed as result of WG adoption and generalization of
      the approach.

   o  New exchange IKE_FOLLOWUP_KE is defined for additional key
      exchanges performed after CREATE_CHILD_SA.

   o  Nonces are removed from all additional key exchanges.

   o  Clarification that IKE_INTERMEDIATE must be negotiated is added.

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   o  Clarification about key derivation in case of multiple key
      exchanges in CREATE_CHILD_SA is added.

   o  Resolving rekey collisions in case of multiple key exchanges is


   o  Using multiple key exchanges CREATE_CHILD_SA is defined.


   o  Use new transform types to negotiate additional key exchanges,
      rather than using the KE payloads of IKE SA.


   o  Use IKE_INTERMEDIATE to perform multiple key exchanges in

   o  Handle fragmentation by keeping the first key exchange (a standard
      IKE_SA_INIT with a few extra notifies) small, and encrypting the
      rest of the key exchanges.

   o  Simplify the negotiation of the 'extra' key exchanges.


   o  We added a feature to allow more than one post-quantum key
      exchange algorithms to be negotiated and used to exchange a post-
      quantum shared secret.

   o  Instead of relying on TCP encapsulation to deal with IP level
      fragmentation, we introduced a new key exchange payload that can
      be sent as multiple fragments within IKE_SA_INIT message.

1.4.  Document Organization

   The remainder of this document is organized as follows.  Section 2
   summarizes design criteria.  Section 3 describes how multiple key
   exchanges are performed between two IKE peers and how keying
   materials are derived for both SAs and Child SAs.  A summary of
   alternative approaches that have been considered, but later
   discarded, are described in Appendix A.  Section 4 discusses IANA
   considerations for the namespaces introduced in this document, and
   lastly Section 5 discusses security considerations.

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   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.  Design Criteria

   The design of the proposed extension is driven by the following

   1)   Need for post-quantum cryptography in IPsec.  Quantum computers
        might become feasible in the near future.  If current Internet
        communications are monitored and recorded today (D), the
        communications could be decrypted as soon as a quantum- computer
        is available (e.g., year Q) if key negotiation only relies on
        non post-quantum primitives.  This is a high threat for any
        information that must remain confidential for a long period of
        time T > Q-D.  The need is obvious if we assume that Q is 2040,
        D is 2020, and T is 30 years.  Such a value of T is typical in
        classified or healthcare data.

   2)   Hybrid.  Currently, there does not exist a post-quantum key
        exchange that is trusted at the level that ECDH is trusted
        against conventional (non-quantum) adversaries.  A hybrid post-
        quantum algorithm to be introduced next to well-established
        primitives, since the overall security is at least as strong as
        each individual primitive.

   3)   Focus on quantum-resistant confidentiality.  A passive attacker
        can eavesdrop on IPsec communication today and decrypt it once a
        quantum computer is available in the future.  This is a very
        serious attack for which we do not have a solution.  An attacker
        can only perform active attacks such as impersonation of the
        communicating peers once a quantum computer is available,
        sometime in the future.  Thus, our design focuses on quantum-
        resistant confidentiality due to the urgency of this problem.
        This document does not address quantum-resistant authentication
        since it is less urgent at this stage.

   4)   Limit amount of exchanged data.  The protocol design should be
        such that the amount of exchanged data, such as public-keys, is
        kept as small as possible even if initiator and responder need
        to agree on a hybrid group or multiple public-keys need to be

   5)   Future proof.  Any cryptographic algorithm could be potentially
        broken in the future by currently unknown or impractical

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        attacks: quantum computers are merely the most concrete example
        of this.  The design does not categorize algorithms as "post-
        quantum" or "non post-quantum" nor does it create assumptions
        about the properties of the algorithms, meaning that if
        algorithms with different properties become necessary in the
        future, this extension can be used unchanged to facilitate
        migration to those algorithms.

   6)   Limited amount of changes.  A key goal is to limit the number of
        changes required when enabling a post-quantum handshake.  This
        ensures easier and quicker adoption in existing implementations.

   7)   Localized changes.  Another key requirement is that changes to
        the protocol are limited in scope, in particular, limiting
        changes in the exchanged messages and in the state machine, so
        that they can be easily implemented.

   8)   Deterministic operation.  This requirement means that the hybrid
        post-quantum exchange, and thus, the computed keys, will be
        based on algorithms that both client and server wish to support.

   9)   Fragmentation support.  Some PQC algorithms could be relatively
        bulky and they might require fragmentation.  Thus, a design goal
        is the adaptation and adoption of an existing fragmentation
        method or the design of a new method that allows for the
        fragmentation of the key shares.

   10)  Backwards compatibility and interoperability.  This is a
        fundamental requirement to ensure that hybrid post-quantum IKEv2
        and non-post-quantum IKEv2 implementations are interoperable.

   11)  Federal Information Processing Standards (FIPS) compliance.
        IPsec is widely used in Federal Information Systems and FIPS
        certification is an important requirement.  However, algorithms
        that are believed to be post-quantum are not FIPS compliant yet.
        Still, the goal is that the overall hybrid post-quantum IKEv2
        design can be FIPS compliant.

   12)  Ability to use this method with multiple classical (EC)DH key
        exchanges.  In some situations peers have no single mutually
        trusted key exchange algorithm (e.g., due to local policy
        restrictions).  The ability to combine two (or more) key
        exchange methods in such a way that the resulting shared key
        depends on all of them allows peers to communicate in this

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3.  Multiple Key Exchanges

3.1.  Overall Design

   This design assigns new Transform Type 4 identifiers to the various
   post-quantum key exchanges (which will be defined later).  We
   specifically do not make a distinction between classical (DH and
   ECDH) and post-quantum key exchanges, nor post-quantum algorithms
   which are true key exchanges versus post-quantum algorithms that act
   as key transport mechanisms; all are treated equivalently by the
   protocol.  To be more specific, this document renames Transform Type
   4 from "Diffie-Hellman Group (D-H)" to "Key Exchange Method (KE)" and
   renames a field in the Key Exchange Payload from "Diffie-Hellman
   Group Num" to "Key Exchange Method".  The corresponding IANA registry
   is also renamed from "Diffie-Hellman Group Transform IDs" to "Key
   Exchange Method Transform IDs".

   In order to support IKE fragmentation for additional key exchanges
   that may have long public keys, the proposed framework utilizes the
   IKE_INTERMEDIATE exchange defined in

   In order to minimize communication overhead, only the key shares that
   are agreed to be used are actually exchanged.  In order to achieve
   this several new Transform Types are defined, each sharing possible
   Transform IDs with Transform Type 4.  The IKE_SA_INIT message
   includes one or more newly defined SA transforms that lists the extra
   key exchange policy required by the initiator; the responder selects
   a single transform of each type, and returns them in the response
   IKE_SA_INIT message.  Then, provided that additional key exchanges
   are negotiated, the initiator and the responder perform one or more
   IKE_INTERMEDIATE exchanges; each such exchange includes a KE payload
   for one of the negotiated key exchanges.

   Here is an overview of the initial exchanges:

   Initiator                             Responder
   <-- IKE_SA_INIT (additional key exchanges negotiation) -->

   <-- {IKE_INTERMEDIATE (additional key exchange)} -->


   <-- {IKE_INTERMEDIATE (additional key exchange)} -->

   <-- {IKE_AUTH} -->

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   The additional key exchanges may use algorithms that are currently
   considered to be resistant to quantum computer attacks.  These
   algorithms are collectively referred to as post-quantum algorithms in
   this document.  However, it is also possible to use classical (EC)DH
   primitives for non post-quantum requirements.

   Most post-quantum key agreement algorithms are relatively new, and
   thus are not fully trusted.  There are also many proposed algorithms,
   with different trade-offs and relying on different hard problems.
   The concern is that some of these hard problems may turn out to be
   easier to solve than anticipated and thus the key agreement algorithm
   may not be as secure as expected.  A hybrid solution allows us to
   deal with this uncertainty by combining a classical key exchange with
   a post-quantum one, as well as leaving open the possibility of
   multiple post-quantum key exchanges.

   The method that we use to perform additional key exchanges also
   addresses the fragmentation issue.  The initial IKE_INIT messages do
   not have any inherent fragmentation support within IKE; however that
   can include a relatively short KE payload (e.g. one for group 14, 19
   or 31).  The rest of the KE payloads are encrypted within
   IKE_INTERMEDIATE messages; because they are encrypted, the standard
   IKE fragmentation solution [RFC7383] is available.

   The fact that all Additional Key Exchange Transform Types share the
   same registry with Transform Type 4 allows additional key exchanges
   to be of any type - either post-quantum ones or classical (EC)DH
   ones.  This approach allows any combination of defined key exchange
   methods to take place.  This also allows performing a single post-
   quantum key exchange in the IKE_SA_INIT without additional key
   exchanges, provided that IP fragmentation is not an issue and that
   hybrid key exchange is not needed.

3.2.  Overall Protocol

   In the simplest case, the initiator is happy with a single key
   exchange (and has no interest in supporting multiple), and it is not
   concerned with possible fragmentation of the IKE_SA_INIT messages
   (either because the key exchange it selects is small enough not to
   fragment, or the initiator is confident that fragmentation will be
   handled either by IP fragmentation, or transport via TCP).

   In this case, the initiator performs the IKE_SA_INIT as standard,
   inserting a preferred key exchange (which is possibly a post-quantum
   algorithm) as the listed Transform Type 4, and including the
   initiator KE payload.  If the responder accepts the policy, it
   responds with an IKE_SA_INIT response, and IKE continues as usual.

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   If the initiator desires to negotiate multiple key exchanges, or it
   needs IKE to handle any possible fragmentation, then the initiator
   uses the protocol listed below.

3.2.1.  IKE_SA_INIT Round: Negotiation

   Multiple key exchanges are negotiated using the standard IKEv2
   mechanism, via SA payload.  For this purpose several new transform
   types, namely Additional Key Exchange 1, Additional Key Exchange 2,
   Additional Key Exchange 3, etc., are defined.  They are collectively
   called Additional Key Exchanges and have slightly different semantics
   than existing IKEv2 transform types.  They are interpreted as
   additional key exchanges that peers agreed to perform in a series of
   IKE_INTERMEDIATE exchanges.  The possible transform IDs for these
   transform types are the same as IDs for the Transform Type 4, so they
   all share a single IANA registry for transform IDs.

   Key exchange methods negotiated via Transform Type 4 MUST always take
   place in the IKE_SA_INIT exchange.  Additional key exchanges
   negotiated via newly defined transforms MUST take place in a series
   of IKE_INTERMEDIATE exchanges, in an order of the values of their
   transform types, so that key exchange negotiated using Transform Type
   n always precedes that of Transform Type n + 1.  Each
   IKE_INTERMEDIATE exchange MUST bear exactly one key exchange method.
   Note that with this semantics, Additional Key Exchanges transforms
   are not associated with any particular type of key exchange and do
   not have any specific per transform type transform IDs IANA registry.
   Instead they all share a single registry for transform IDs - "Key
   Exchange Method Transform IDs", as well as Transform Type 4.  All new
   key exchange algorithms (both classical or post-quantum) should be
   added to this registry.  This approach gives peers flexibility in
   defining the ways they want to combine different key exchange

   When forming a proposal the initiator adds transforms for the
   IKE_SA_INIT exchange using Transform Type 4.  In most cases they will
   contain classical key exchange methods (DH or ECDH), however it is
   not a requirement.  Additional key exchange methods are proposed
   using Additional Key Exchanges transform types.  All these transform
   types are optional, the initiator is free to select any of them for
   proposing additional key exchange methods.  Consequently, if none of
   Additional Key Exchange transforms are included in the proposal, then
   this proposal indicates performing standard IKEv2, as defined in
   [RFC7296].  If the initiator includes any transform of type n (where
   n is among Additional Key Exchanges) in the proposal, the responder
   MUST select one of the algorithms proposed using this type.  A
   transform ID NONE may be added to those transform types which contain
   key exchange methods that the initiator believes are optional.

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   If the initiator includes any Additional Key Exchanges transform
   types into SA payload, it MUST also negotiate using IKE_INTERMEDIATE
   exchange as described in [I-D.ietf-ipsecme-ikev2-intermediate], by
   including INTERMEDIATE_EXCHANGE_SUPPORTED notification in the
   IKE_SA_INIT request message.  If the responder agrees to use
   additional key exchanges, it MUST also return this notification, thus
   confirming that IKE_INTERMEDIATE exchange is supported and will be
   used for transferring additional key exchange data.  The presence of
   Additional Key Exchanges transform types in SA payload without
   negotiation of using IKE_INTERMEDIATE exchange MUST be treated as
   protocol error by both initiator and responder.

   The responder performs negotiation using standard IKEv2 procedure
   described in Section 3.3 of [RFC7296].  However, for the Additional
   Key Exchange types the responder's choice MUST NOT contain equal
   transform IDs (apart from NONE), and the ID selected for Transform
   Type 4 MUST NOT appear in any of Additional Key Exchange transforms.
   In other words, all selected key exchange methods must be different.

3.2.2.  IKE_INTERMEDIATE Round: Additional Key Exchanges

   For each extra key exchange agreed to in the IKE_SA_INIT exchange,
   the initiator and the responder perform one IKE_INTERMEDIATE
   exchange, as described in [I-D.ietf-ipsecme-ikev2-intermediate].

   These exchanges are as follows:

   Initiator                             Responder
   HDR, SK {KEi(n)}     -->
                               <--       HDR, SK {KEr(n)}

   The initiator sends key exchange data in the KEi(n) payload.  This
   packet is protected with the current SK_ei/SK_ai keys.

   On receiving this, the responder sends back key exchange payload
   KEr(n); again, this packet is protected with the current SK_er/SK_ar

   The former "Diffie-Hellman Group Num" (now called "Key Exchange
   Method") field in the KEi(n) and KEr(n) payloads MUST match the n-th
   negotiated additional key exchange.  Note that the negotiated
   transform types (the encryption type, integrity type, prf type) are
   not modified.

   Once this exchange is done, then both sides compute an updated keying

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               SKEYSEED(n) = prf(SK_d(n-1), KE(n) | Ni | Nr)

   where KE(n) is the resulting shared secret of this key exchange, Ni
   and Nr are nonces from the IKE_SA_INIT exchange and SK_d(n-1) is the
   last generated SK_d, (derived from the previous IKE_INTERMEDIATE
   exchange, or the IKE_SA_INIT if there have not already been any
   IKE_INTERMEDIATE exchanges).  Then, SK_d, SK_ai, SK_ar, SK_ei, SK_er,
   SK_pi, SK_pr are updated as:

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

   Both the initiator and the responder use these updated key values in
   the next exchange.

3.2.3.  IKE_AUTH Exchange

   After all IKE_INTERMEDIATE exchanges have completed, the initiator
   and the responder perform an IKE_AUTH exchange.  This exchange is the
   standard IKE exchange, except that the initiator and responder signed
   octets are modified as described in

3.2.4.  CREATE_CHILD_SA Exchange

   The CREATE_CHILD_SA exchange is used in IKEv2 for the purpose of
   creating additional Child SAs, rekeying them and rekeying IKE SA
   itself.  When creating or rekeying Child SAs, the peers may
   optionally perform a Diffie-Hellman key exchange to add a fresh
   entropy into the session keys.  In case of IKE SA rekey, the key
   exchange is mandatory.

   If the IKE SA was created using multiple key exchange methods, the
   peers may want to continue using multiple key exchanges in the
   CREATE_CHILD_SA exchange too.  If the initiator includes any
   Additional Key Exchanges transform in the SA payload (along with
   Transform Type 4) and the responder agrees to perform additional key
   exchanges, then the additional key exchanges are performed in a
   series of new IKE_FOLLOWUP_KE exchanges that follows the
   CREATE_CHILD_SA exchange.  The IKE_FOLLOWUP_KE exchange is introduced
   as a dedicated exchange type to transfer data of additional key
   exchanges following the key exchange performed in the
   CREATE_CHILD_SA.  Its Exchange Type is <TBA by IANA>.

   These key exchanges are performed in an order of the values of their
   transform types, so that key exchange negotiated using Transform Type
   n always precedes key exchange negotiated using Transform Type n + 1.
   Each IKE_FOLLOWUP_KE exchange MUST bear exactly one key exchange

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   method.  Key exchange negotiated via Transform Type 4 always takes
   place in the CREATE_CHILD_SA exchange, as per IKEv2 specification.

   Since after IKE SA is created the window size may be greater than one
   and multiple concurrent exchanges may be in progress, it is essential
   to link the IKE_FOLLOWUP_KE exchanges together and with the
   corresponding CREATE_CHILD_SA exchange.  A new status type
   notification ADDITIONAL_KEY_EXCHANGE is used for this purpose.  Its
   Notify Message Type is <TBA by IANA>, Protocol ID and SPI Size are
   both set to 0.  The data associated with this notification is a blob
   meaningful only to the responder, so that the responder can correctly
   link successive exchanges.  For the initiator the content of this
   notification is an opaque blob.

   The responder MUST include this notification in a CREATE_CHILD_SA or
   IKE_FOLLOWUP_KE response message in case the next exchange is
   expected, filling it with some data that would allow linking this
   exchange to the next one.  The initiator MUST copy the received
   notification with its content intact into the request message of the
   next exchange.

   Below is an example of three additional key exchanges.

   Initiator                             Responder
                             <--  HDR(CREATE_CHILD_SA), SK {SA, Nr, KEr,

                                  <--  HDR(IKE_FOLLOWUP_KE), SK {KEr(1),

                                  <--  HDR(IKE_FOLLOWUP_KE), SK {KEr(2),

                                  <--  HDR(IKE_FOLLOWUP_KE), SK {KEr(3)}

   The former "Diffie-Hellman Group Num" (now called "Key Exchange
   Method") field in the KEi(n) and KEr(n) payloads MUST match the n-th
   negotiated additional key exchange.

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   It is possible that due to some unexpected events (e.g. reboot) the
   initiator could forget that it is in the process of performing
   additional key exchanges and never starts next IKE_FOLLOWUP_KE
   exchanges.  The responder MUST handle this situation gracefully and
   delete the associated state if it does not receive the next expected
   IKE_FOLLOWUP_KE request after some reasonable period of time.

   If responder receives IKE_FOLLOWUP_KE request containing
   ADDITIONAL_KEY_EXCHANGE notification and the content of this notify
   does not correspond to any active key exchange state the responder
   has, it MUST send back a new error type notification STATE_NOT_FOUND.
   This is a non-fatal error notification, its Notify Message Type is
   <TBA by IANA>, Protocol ID and SPI Size are both set to 0 and the
   data is empty.  If the initiator receives this notification in
   response to IKE_FOLLOWUP_KE exchange performing additional key
   exchange, it MUST cancel this exchange and MUST treat the whole
   series of exchanges started from the CREATE_CHILD_SA exchange as
   failed.  In most cases, the receipt of this notification is caused by
   premature deletion of the corresponding state on the responder (the
   time period between IKE_FOLLOWUP_KE exchanges appeared too long from
   responder's point of view, e.g. due to a temporary network failure).
   After receiving this notification the initiator MAY start a new
   CREATE_CHILD_SA exchange (eventually followed by the IKE_FOLLOWUP_KE
   exchanges) to retry the failed attempt.  If the initiator continues
   to receive STATE_NOT_FOUND notifications after several retries, it
   MUST treat this situation as a fatal error and delete IKE SA by
   sending a DELETE payload.

   When rekeying IKE SA or Child SA, it is possible that the peers start
   doing this at the same time, which is called simultaneous rekeying.
   Sections 2.8.1 and 2.8.2 of [RFC7296] describes how IKEv2 handles
   this situation.  In a nutshell IKEv2 follows the rule that if in case
   of simultaneous rekeying two identical new IKE SAs (or two pairs of
   Child SAs) are created, then one of them should be deleted.  Which
   one is to be deleted is determined by comparing the values of four
   nonces, that were used in the colliding CREATE_CHILD_SA exchanges -
   the IKE SA (or pair of Child SAs) that was created by the exchange in
   which the smallest nonce was used should be deleted by the initiator
   of this exchange.

   With multiple key exchanges the SAs are not yet created when the
   CRETE_CHILD_SA is completed, they would be created only after the
   series of IKE_FOLLOWUP_KE exchanges is finished.  For this reason if
   additional key exchanges were negotiated in the CREATE_CHILD_SA
   initiated by the losing side, there is nothing to delete and this
   side just stops the rekeying process - this side MUST not initiate
   IKE_FOLLOWUP_KE exchange with next key exchange.

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   In most cases, rekey collisions are resolved in the CREATE_CHILD_SA
   exchange.  However, a situation may occur when due to packet loss,
   one of the peers receives CREATE_CHILD_SA message requesting rekeying
   SA that is already being rekeyed by this peer (i.e. the
   CREATE_CHILD_SA exchange initiated by this peer has been already
   completed and the series of IKE_FOLLOWUP_KE exchanges is in
   progress).  In this case, a TEMPORARY_FAILURE notification MUST be
   sent in response to such a request.

   If multiple key exchanges were negotiated in the CREATE_CHILD_SA
   exchange, then the resulting keys are computed as follows.  In case
   of IKE SA rekey:

          SKEYSEED = prf(SK_d, KE  | Ni | Nr | KE(1) | ... KE(n))

   In case of Child SA creation or rekey:

         KEYMAT = prf+ (SK_d, KE  | Ni | Nr | KE(1) |  ... KE(n))

   In both cases SK_d is from existing IKE SA; KE, Ni, Nr are the shared
   key and nonces from the CREATE_CHILD_SA respectively; KE(1)...KE(n)
   are the shared keys from additional key exchanges.

4.  IANA Considerations

   This document adds new exchange type into the "IKEv2 Exchange Types"


   This document renames Transform Type 4 defined in "Transform Type
   Values" registry from "Diffie-Hellman Group (D-H)" to "Key Exchange
   Method (KE)".

   This document renames IKEv2 registry "Transform Type 4 - Diffie-
   Hellman Group Transform IDs" to "Transform Type 4 - Key Exchange
   Method Transform IDs".

   This document adds the following Transform Types to the "Transform
   Type Values" registry:

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   Type     Description                   Used In
   <TBA>    Additional Key Exchange 1     (optional in IKE, AH, ESP)
   <TBA>    Additional Key Exchange 2     (optional in IKE, AH, ESP)
   <TBA>    Additional Key Exchange 3     (optional in IKE, AH, ESP)
   <TBA>    Additional Key Exchange 4     (optional in IKE, AH, ESP)
   <TBA>    Additional Key Exchange 5     (optional in IKE, AH, ESP)
   <TBA>    Additional Key Exchange 6     (optional in IKE, AH, ESP)
   <TBA>    Additional Key Exchange 7     (optional in IKE, AH, ESP)

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


   and a new Notify Message Type in the "Notify Message Types - Error
   Types" registry:


5.  Security Considerations

   The key length of the Encryption Algorithm (Transform Type 1), the
   Pseudorandom Function (Transform Type 2) and the Integrity Algorithm
   (Transform Type 3), all have to be of sufficient length to prevent
   attacks using Grover's algorithm [GROVER].  In order to use the
   extension proposed in this document, the key lengths of these
   transforms SHALL be at least 256 bits long in order to provide
   sufficient resistance to quantum attacks.  Accordingly the post-
   quantum security level achieved is at least 128 bits.

   SKEYSEED is calculated from shared KE(x) using an algorithm defined
   in Transform Type 2.  While a quantum attacker may learn the value of
   KE(x), if this value is obtained by means of a classical key
   exchange, other KE(x) values generated by means of a quantum-
   resistant algorithm ensure that the final SKEYSEED is not
   compromised.  This assumes that the algorithm defined in the
   Transform Type 2 is post-quantum.

   The main focus of this document is to prevent a passive attacker
   performing a "harvest and decrypt" attack.  In other words, an
   attacker that records messages exchanges today and proceeds to
   decrypt them once he owns a quantum computer.  This attack is
   prevented due to the hybrid nature of the key exchange.  Other
   attacks involving an active attacker using a quantum-computer are not
   completely solved by this document.  This is for two reasons.

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   The first reason is because the authentication step remains
   classical.  In particular, the authenticity of the SAs established
   under IKEv2 is protected using a pre-shared key, RSA, DSA, or ECDSA
   algorithms.  Whilst the pre-shared key option, provided the key is
   long enough, is post-quantum, the other algorithms are not.
   Moreover, in implementations where scalability is a requirement, the
   pre-shared key method may not be suitable.  Quantum-safe authenticity
   may be provided by using a quantum-safe digital signature and several
   quantum-safe digital signature methods are being explored by IETF.
   For example, if the implementation is able to reliably track state,
   the hash based method, XMSS has the status of an RFC, see [RFC8391].
   Currently, quantum-safe authentication methods are not specified in
   this document, but are planned to be incorporated in due course.

   It should be noted that the purpose of post-quantum algorithms is to
   provide resistance to attacks mounted in the future.  The current
   threat is that encrypted sessions are subject to eavesdropping and
   archived with decryption by quantum computers taking place at some
   point in the future.  Until quantum computers become available there
   is no point in attacking the authenticity of a connection because
   there are no possibilities for exploitation.  These only occur at the
   time of the connection, for example by mounting a man-in-the-middle
   (MitM) attack.  Consequently there is not such a pressing need for
   quantum-safe authenticity.

   This draft does not attempt to address key exchanges with KE payloads
   longer than 64k; the current IKE payload format does not allow that
   as a possibility.  If such huge KE payloads are required, a work
   around (such as making the KE payload a URL and a hash of the real
   payload) would be needed.  At the current time, it appears likely
   that there will be plenty of key exchanges available that would not
   require such a workaround.

6.  Acknowledgements

   The authors would like to thanks Frederic Detienne and Olivier
   Pelerin for their comments and suggestions, including the idea to
   negotiate the post-quantum algorithms using the existing KE payload.
   The authors are also grateful to Tobias Heider and Tobias Guggemos
   for valuable comments.

7.  References

7.1.  Normative References

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              Smyslov, V., "Intermediate Exchange in the IKEv2
              Protocol", draft-ietf-ipsecme-ikev2-intermediate-04 (work
              in progress), June 2020.

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

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative 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.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              DOI 10.17487/RFC4302, December 2005,

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,

   [RFC8229]  Pauly, T., Touati, S., and R. Mantha, "TCP Encapsulation
              of IKE and IPsec Packets", RFC 8229, DOI 10.17487/RFC8229,
              August 2017, <https://www.rfc-editor.org/info/rfc8229>.

   [RFC8391]  Huelsing, A., Butin, D., Gazdag, S., Rijneveld, J., and A.
              Mohaisen, "XMSS: eXtended Merkle Signature Scheme",
              RFC 8391, DOI 10.17487/RFC8391, May 2018,

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   [RFC8784]  Fluhrer, S., Kampanakis, P., McGrew, D., and V. Smyslov,
              "Mixing Preshared Keys in the Internet Key Exchange
              Protocol Version 2 (IKEv2) for Post-quantum Security",
              RFC 8784, DOI 10.17487/RFC8784, June 2020,

Appendix A.  Alternative Design

   This section gives an overview on a number of alternative approaches
   that we have considered, but later discarded.  These approaches are:

   o  Sending the classical and post-quantum key exchanges as a single

      We considered combining the various key exchanges into a single
      large KE payload; this effort is documented in a previous version
      of this draft (draft-tjhai-ipsecme-hybrid-qske-ikev2-01).  This
      does allow us to cleanly apply hybrid key exchanges during the
      child SA; however it does add considerable complexity, and
      requires an independent fragmentation solution.

   o  Sending post-quantum proposals and policies in KE payload only

      With the objective of not introducing unnecessary notify payloads,
      we considered communicating the hybrid post-quantum proposal in
      the KE payload during the first pass of the protocol exchange.
      Unfortunately, this design is susceptible to the following
      downgrade attack.  Consider the scenario where there is an MitM
      attacker sitting between an initiator and a responder.  The
      initiator proposes, through SAi payload, to use a hybrid post-
      quantum group and as a backup a Diffie-Hellman group, and through
      KEi payload, the initiator proposes a list of hybrid post-quantum
      proposals and policies.  The MitM attacker intercepts this traffic
      and replies with N(INVALID_KE_PAYLOAD) suggesting to downgrade to
      the backup Diffie-Hellman group instead.  The initiator then
      resends the same SAi payload and the KEi payload containing the
      public value of the backup Diffie-Hellman group.  Note that the
      attacker may forward the second IKE_SA_INIT message only to the
      responder, and therefore at this point in time, the responder will
      not have the information that the initiator prefers the hybrid
      group.  Of course, it is possible for the responder to have a
      policy to reject an IKE_SA_INIT message that (a) offers a hybrid
      group but not offering the corresponding public value in the KEi
      payload; and (b) the responder has not specifically acknowledged
      that it does not supported the requested hybrid group.  However,
      the checking of this policy introduces unnecessary protocol
      complexity.  Therefore, in order to fully prevent any downgrade
      attacks, using KE payload alone is not sufficient and that the

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      initiator MUST always indicate its preferred post-quantum
      proposals and policies in a notify payload in the subsequent
      IKE_SA_INIT messages following a N(INVALID_KE_PAYLOAD) response.

   o  New payload types to negotiate hybrid proposal and to carry post-
      quantum public values

      Semantically, it makes sense to use a new payload type, which
      mimics the SA payload, to carry a hybrid proposal.  Likewise,
      another new payload type that mimics the KE payload, could be used
      to transport hybrid public value.  Although, in theory a new
      payload type could be made backwards compatible by not setting its
      critical flag as per Section 2.5 of RFC7296, we believe that it
      may not be that simple in practice.  Since the original release of
      IKEv2 in RFC4306, no new payload type has ever been proposed and
      therefore, this creates a potential risk of having a backward
      compatibility issue from non-conforming RFC IKEv2 implementations.
      Since we could not see any other compelling advantages apart from
      a semantic one, we use the existing transform type and notify
      payloads instead.  In fact, as described above, we use the KE
      payload in the first IKE_SA_INIT request round and the notify
      payload to carry the post-quantum proposals and policies.  We use
      one or more of the existing KE payloads to carry the hybrid public

   o  Hybrid public value payload

      One way to transport the negotiated hybrid public payload, which
      contains one classical Diffie-Hellman public value and one or more
      post-quantum public values, is to bundle these into a single KE
      payload.  Alternatively, these could also be transported in a
      single new hybrid public value payload, but following the same
      reasoning as above, this may not be a good idea from a backward
      compatibility perspective.  Using a single KE payload would
      require an encoding or formatting to be defined so that both peers
      are able to compose and extract the individual public values.
      However, we believe that it is cleaner to send the hybrid public
      values in multiple KE payloads--one for each group or algorithm.
      Furthermore, at this point in the protocol exchange, both peers
      should have indicated support of handling multiple KE payloads.

   o  Fragmentation

      Handling of large IKE_SA_INIT messages has been one of the most
      challenging tasks.  A number of approaches have been considered
      and the two prominent ones that we have discarded are outlined as

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      The first approach was to treat the entire IKE_SA_INIT message as
      a stream of bytes, which we then split it into a number of
      fragments, each of which is wrapped onto a payload that would fit
      into the size of the network MTU.  The payload that wraps each
      fragment is a new payload type and it was envisaged that this new
      payload type will not cause a backward compatibility issue because
      at this stage of the protocol, both peers should have indicated
      support of fragmentation in the first pass of the IKE_SA_INIT
      exchange.  The negotiation of fragmentation is performed using a
      notify payload, which also defines supporting parameters such as
      the size of fragment in octets and the fragment identifier.  The
      new payload that wraps each fragment of the messages in this
      exchange is assigned the same fragment identifier.  Furthermore,
      it also has other parameters such as a fragment index and total
      number of fragments.  We decided to discard this approach due to
      its blanket approach to fragmentation.  In cases where only a few
      payloads need to be fragmented, we felt that this approach is
      overly complicated.

      Another idea that was discarded was fragmenting an individual
      payload without introducing a new payload type.  The idea was to
      use the 9-th bit (the bit after the critical flag in the RESERVED
      field) in the generic payload header as a flag to mark that this
      payload is fragmented.  As an example, if a KE payload is to be
      fragmented, it may look as follows.

                      1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     | Next Payload  |C|F| RESERVED  |         Payload Length        |
     |  Diffie-Hellman Group Number  |     Fragment Identifier       |
     |         Fragment Index        |        Total Fragments        |
     |                  Total KE Payload Data Length                 |
     |                                                               |
     ~                       Fragmented KE Payload                   ~
     |                                                               |

      When the flag F is set, this means the current KE payload is a
      fragment of a larger KE payload.  The Payload Length field denotes
      the size of this payload fragment in octets--including the size of
      the generic payload header.  The two-octet RESERVED field
      following Diffie-Hellman Group Number was to be used as a fragment
      identifier to help assembly and disassembly of fragments.  The

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      Fragment Index and Total Fragments fields are self-explanatory.
      The Total KE Payload Data Length indicates the size of the
      assembled KE payload data in octets.  Finally, the actual fragment
      is carried in Fragment KE Payload field.

      We discarded this approach because we believe that the working
      group may not be happy using the RESERVED field to change the
      format of a packet and that implementers may not like the
      complexity added from checking the fragmentation flag in each
      received payload.  More importantly, fragmenting the messages in
      this way may leave the system to be more prone to denial of
      service (DoS) attacks.  By using IKE_INTERMEDIATE to transport the
      large post-quantum key exchange payloads, there is no longer any
      issue with fragmentation.

   o  Group sub-identifier

      As discussed before, each group identifier is used to distinguish
      a post-quantum algorithm.  Further classification could be made on
      a particular post-quantum algorithm by assigning additional value
      alongside the group identifier.  This sub- identifier value may be
      used to assign different security parameter sets to a given post-
      quantum algorithm.  However, this level of details does not fit
      the principles of the document where it should deal with generic
      hybrid key exchange protocol, not a specific ciphersuite.
      Furthermore, there are enough Diffie- Hellman group identifiers
      should this be required in the future.

Authors' Addresses

   C. Tjhai

   Email: cjt@post-quantum.com

   M. Tomlinson

   Email: mt@post-quantum.com

   G. Bartlett
   Quantum Secret

   Email: graham.ietf@gmail.com

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   S. Fluhrer
   Cisco Systems

   Email: sfluhrer@cisco.com

   D. Van Geest
   ISARA Corporation

   Email: daniel.vangeest@isara.com

   O. Garcia-Morchon

   Email: oscar.garcia-morchon@philips.com

   Valery Smyslov

   Email: svan@elvis.ru

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