Post-quantum Hybrid Key Exchange with ML-KEM in the Internet Key Exchange Protocol Version 2 (IKEv2)
draft-ietf-ipsecme-ikev2-mlkem-03

IPSECME                                                    P. Kampanakis
Internet-Draft                                       Amazon Web Services
Intended status: Standards Track                       29 September 2025
Expires: 2 April 2026

    Post-quantum Hybrid Key Exchange with ML-KEM in the Internet Key
                  Exchange Protocol Version 2 (IKEv2)
                   draft-ietf-ipsecme-ikev2-mlkem-03

Abstract

   NIST recently standardized ML-KEM, a new key encapsulation mechanism,
   which can be used for quantum-resistant key establishment.  This
   draft specifies how to use ML-KEM by itself or as an additional key
   exchange in IKEv2 along with a traditional key exchange.  These
   options allow for negotiating IKE and Child SA keys which are safe
   against cryptographically relevant quantum computers and theoretical
   weaknesses in ML-KEM or implementation issues.

Status of This Memo

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Copyright Notice

   Copyright (c) 2025 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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  KEMs  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  ML-KEM  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Conventions and Definitions . . . . . . . . . . . . . . .   5
   2.  ML-KEM in IKEv2 . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  ML-KEM in IKE_INTERMEDIATE, CREATE_CHILD_SA, or
           IKE_FOLLOWUP_KE messages  . . . . . . . . . . . . . . . .   5
     2.2.  Key Exchange Payload  . . . . . . . . . . . . . . . . . .   6
     2.3.  Recipient Tests . . . . . . . . . . . . . . . . . . . . .   7
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   A Cryptographically Relevant Quantum Computer (CRQC), if it became a
   reality, could threaten today's public key establishment algorithms.
   Someone storing encrypted communications that use (Elliptic Curve)
   Diffie-Hellman ((EC)DH) to establish keys could decrypt these
   communications in the future after a CRQC became available to them.
   Such communications include Internet Key Exchange Protocol Version 2
   (IKEv2).

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   To address this concern, the Mixing Preshared Keys in IKEv2
   specification [RFC8784] introduced Post-quantum Preshared Keys (PPK)
   as a temporary option for stirring a pre-shared key of adequate
   entropy in the derived Child SA encryption keys in order to provide
   quantum-resistance.  This specification can be used in conjunction
   with PPK as defined in [RFC8784].  Alternatively,
   [I-D.ietf-ipsecme-ikev2-qr-alt] can be used for mixing pre-shared
   keys in IKEv2, as it provides better security properties than
   [RFC8784] and, since the PPK negotiation can be combined with
   additional ML-KEM key exchanges and the extra round trip penalty can
   be avoided.

   Since then, NIST has been working on a public project [NIST-PQ] for
   standardizing quantum-resistant algorithms which include key
   encapsulation and signatures.  At the end of Round 3, they picked
   Kyber as the first Key Encapsulation Mechanism (KEM) for
   standardization. . Kyber was then standardized as Module-Lattice-
   based Key-Encapsulation Mechanism (ML-KEM) in 2024 [FIPS203].

   As post-quantum public keys and ciphertexts may make UDP packet sizes
   larger than common network Maximum Transport Units (MTU), the
   Intermediate Exchange in IKEv2 document [RFC9242] defined how to do
   additional large message exchanges by using new IKE_INTERMEDIATE
   messages.  IKE_INTERMEDIATE messages can only be used after
   IKE_SA_INIT.  The Multiple Key Exchanges in IKEv2 specification
   [RFC9370] defined how to do up to seven additional key exchanges by
   using IKE_INTERMEDIATE or IKE_FOLLOWUP_KE messages and by deriving
   new SKEYSEED and KEYMAT key materials.  These messages can be
   fragmented at the IKEv2 layer before causing IP fragmentation
   [RFC7383].  If a post-quantum KEM does not fit inside IKE_SA_INIT
   without causing IP fragmentation, then it can be used after
   IKE_SA_INIT in an IKE_INTERMEDIATE, CREATE_CHILD_SA, or
   IKE_FOLLOWUP_KE message as an additional key establishment algorithm.

   This document describes how ML-KEM can be used as a quantum-resistant
   KEM in IKEv2 in an IKE_SA_INIT or CREATE_CHILD_SA exchange, or in one
   additional IKE_INTERMEDIATE or IKE_FOLLOWUP_KE key exchange after an
   initial IKE_SA_INIT or CREATE_CHILD_SA respectively.  This approach
   of combining a quantum-resistant with a traditional algorithm, is
   commonly called Post-Quantum Traditional (PQ/T) Hybrid [RFC9794] key
   exchange and combines the security of a well-established algorithm
   with relatively new quantum-resistant algorithms.  The result is a
   new Child SA key or an IKE or Child SA rekey with keying material
   which is safe against a CRQC.  Another use of a PQ/T Hybrid key
   exchange in IKEv2 is for someone that wants to exchange keys using
   the high security parameter of ML-KEM.  As these may not fit in
   common network packet payload sizes, they will need to be sent in a
   IKE_FOLLOWUP_KE or CREATE_CHILD_SA key exchange which can be

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   fragmented.  This specification is a profile of the Multiple Key
   Exchanges in IKEv2 specification [RFC9370] and registers new
   algorithm identifiers for ML-KEM key exchanges in IKEv2.

1.1.  KEMs

   In the context of the NIST Post-Quantum Cryptography Standardization
   Project [NIST-PQ], key exchange algorithms are formulated as KEMs,
   which consist of three steps:

   *  'KeyGen() -> (pk, sk)': A probabilistic key generation algorithm,
      which generates a public / encapsulation key 'pk' and a private /
      decapsulation key 'sk'.  The resulting pk is sent to the responder
      in the KEi payload.

   *  'Encaps(pk) -> (ct, ss)': A probabilistic encapsulation algorithm,
      which takes as input a public key pk (from the KEi) and outputs a
      ciphertext 'ct' and shared secret 'ss'.  The ct is sent back to
      intiator in the KEr payload.

   *  'Decaps(sk, ct) -> ss': A decapsulation algorithm, which takes as
      input a secret key sk and ciphertext ct (from the KEr) and outputs
      a shared secret ss, or in some rare cases a distinguished error
      value.

1.2.  ML-KEM

   ML-KEM is a standardized lattice-based key encapsulation mechanism
   [FIPS203].  It uses Module Learning with Errors as its underlying
   primitive which is a structured lattices variant that offers good
   performance and relatively small and balanced key and ciphertext
   sizes.  ML-KEM was standardized with three parameters, ML-KEM-512,
   ML-KEM-768, and ML-KEM-1024.  These were mapped by NIST to the three
   security levels defined in the NIST PQC Project, Level 1, 3, and 5.
   These levels correspond to the hardness of breaking AES-128, AES-192
   and AES-256 respectively.

   ML-KEM-512, ML-KEM-768 and ML-KEM-1024 key exchanges will not have
   noticeable performance impact on IKEv2/IPsec tunnels which usually
   stay up for long periods of time and transfer sizable amounts of
   data.  Since the ML-KEM-768 and ML-KEM-1024 public key and ciphertext
   sizes can exceed the network MTU, these key exchanges could require
   two or three network IP packets from both the initiator and the
   responder.

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1.3.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.  ML-KEM in IKEv2

2.1.  ML-KEM in IKE_INTERMEDIATE, CREATE_CHILD_SA, or IKE_FOLLOWUP_KE
      messages

   ML-KEM key exchanges can be negotiated in IKE_INTERMEDIATE or
   IKE_FOLLOWUP_KE messages as defined in the Multiple Key Exchanges in
   IKEv2 specification [RFC9370].  We summarize them here for
   completeness.

   Section 2.2.2 of [RFC9370] specifies that KEi(0), KEr(0) are regular
   key exchange messages in the first IKE_SA_INIT exchange which end up
   generating a set of keying material, SK_d, SK_a[i/r], and SK_e[i/r].
   The peers then perform an IKE_INTERMEDIATE exchange, carrying new Key
   Exchange payloads.  These are protected with the SK_e[i/r] and
   SK_a[i/r] keys which were derived from the IKE_SA_INIT as per
   Section 3.3.1 of the Intermediate Exchange in IKEv2 document
   [RFC9242].  The initiator generates an ML-KEM keypair (pk, sk) using
   KeyGen(), and sends the public key (pk) to the responder inside a
   KEi(1) payload.  The responder will encapsulate a shared secret ss
   using Encaps(pk) and the resulting ciphertext (ct) is sent to
   initiator using the KEr(1).  After the initiator receives KEr(1), it
   will decapsulate it using Decaps(sk, ct).  Both Encaps and Decaps
   return the shared secret (ss) and both peers have a common shared
   secret SK(1) at the end of this KE(1) exchange.  The ML-KEM shared
   secret is stirred into new keying material SK_d, SK_a[i/r], and
   SK_e[i/r] as defined in Section 2.2.2 of the Multiple Key Exchanges
   in IKEv2 document [RFC9370].  Afterwards the peers can perform more
   exchanges if necessary and then continue to the IKE_AUTH exchange
   phase as defined in Section 3.3.2 of the Intermediate Exchange in
   IKEv2 specification [RFC9242].

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   ML-KEM can also be used to create or rekey a Child SA or rekey the
   IKE SA in a PQ/T Hybrid approach by using a IKE_FOLLOWUP_KE exchange
   which follows a traditional CREATE_CHILD_SA.  After the additional
   ML-KEM key exchange KE(1) has taken place in the IKE_FOLLOWUP_KE
   exchange, the IKE or Child SA are rekeyed by stirring the new ML-KEM
   shared secret SK(1) in SKEYSEED and KEYMAT as specified in
   Section 2.2.4 of [RFC9370].  Alternatively, ML-KEM can still be used
   on its own in a CREATE_CHILD_SA that rekeys the IKE or IPsec SAs
   without any other key exchanges as per [RFC7296].

   ML-KEM-768 and ML-KEM-1024 public keys and ciphertexts may make UDP
   packet sizes larger than typical network MTUs.  Thus,
   IKE_INTERMEDIATE or IKE_FOLLOWUP_KE messages carrying ML-KEM public
   keys and ciphertexts may be IKEv2 fragmented as per the IKEv2 Message
   Fragmentation specification [RFC7383].

   Although, this document focuses on using ML-KEM as the second key
   exchange in a PQ/T Hybrid KEM [RFC9794] scenario, ML-KEM-512 and ML-
   KEM-768 Key Exchange Method identifiers 35 and 36 respectively MAY be
   used in IKE_SA_INIT as a quantum-resistant-only key exchange.  The
   encapsulation key and ciphertext sizes for these ML-KEM parameters
   may not push the UDP packet to size larger than typical network MTUs.
   On the other hand, IKE_SA_INIT messages using ML-KEM-1024 Key
   Exchange Method identifier 37 could exceed typical network MTUs and
   could not be IKEv2 fragmented.  Thus, implementations transporting
   IKE over UDP and not performing Path MTU (PMTU) discovery SHOULD NOT
   use ML-KEM-1024 in the IKE_SA_INIT exchange on networks where the
   PMTU is unknown or restricted.  However, when reliable transport is
   used for IKE (e.g. [RFC9329],
   [I-D.smyslov-ipsecme-ikev2-reliable-transport]) or a sufficient PMTU
   is guaranteed, implementations MAY use ML-KEM-1024 in an IKE_SA_INIT
   exchange.

2.2.  Key Exchange Payload

   The KE payload is shown below and the fields inside it has meaning as
   defined in Section 3.4 of the IKEv2 standard [RFC7296]:

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                        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|  RESERVED   |         Payload Length        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Key Exchange Method Num     |           RESERVED            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~                       Key Exchange Data                       ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Key Exchange Data from the initiator to the responder contains
   the public key (pk) from the KeyGen() operation encoded as a raw byte
   array (i.e., output of ByteEncode) as defined in Section 7.1 of
   Module-Lattice-Based KEM standard [FIPS203].

   The Key Exchange Data from the responder to the initiator contains
   the ciphertext (ct) from the Encaps operation encoded as a raw byte
   array.

   Table 1 shows the Payload Length, Key Exchange Method Num identifier
   and the Key Exchange Data Size in octets for Key Exchange Payloads
   from the initiator and the responder for the ML-KEM variants
   specified in this document.

     +=============+================+============+===================+
     |     KEM     | Payload Length |    Key     |    Data Size in   |
     |             |  (initiator /  |  Exchange  | Octets (initiator |
     |             |   responder)   | Method Num |    / responder)   |
     +=============+================+============+===================+
     |  ML-KEM-512 |   808 / 776    |     35     |     800 / 768     |
     +-------------+----------------+------------+-------------------+
     |  ML-KEM-768 |  1192 / 1096   |     36     |    1184 / 1088    |
     +-------------+----------------+------------+-------------------+
     | ML-KEM-1024 |  1576 / 1576   |     37     |    1568 / 1568    |
     +-------------+----------------+------------+-------------------+

                    Table 1: Key Exchange Payload Fields

2.3.  Recipient Tests

   Receiving and handling of malformed ML-KEM public keys or ciphertexts
   must follow the input validation described in the Module-Lattice-
   Based KEM standard [FIPS203].

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   Responders MUST perform the checks on the initiator public key
   specified in section 7.2 of the Module-Lattice-Based KEM standard
   [FIPS203] before the Encaps(pk) operation.  If the checks fail, the
   responder SHOULD send a Notify payload of type INVALID_SYNTAX as a
   response to the request from initiator.

   Initiators MUST perform the Ciphertext type check specified in
   section 7.3 of the Module-Lattice-Based KEM standard [FIPS203] before
   the Decaps(sk, ct) operation.  If the check fails, the initiator MUST
   reject the ciphertext and MUST fail the exchange, log the error, and
   stop creating the SA (i.e. not initiate IKE_AUTH or next
   IKE_INTERMEDIATE).  If the error occurs in the CREATE_CHILD_SA or
   IKE_FOLLOWUP_KE exchanges, the initiator MUST delete the existing IKE
   SA and send a Delete payload in a new INFORMATIONAL exchange for the
   responder to also remove it.

   Note that during decapsulation, ML-KEM uses implicit rejection which
   leads the decapsulating entity to implicitly reject the decapsulated
   shared secret by setting it to a hash of the ciphertext together with
   a random value stored in the ML-KEM secret when the re-encrypted
   shared secret does not match the original one.

   Section 4 of [SP800227] includes guidelines for using KEMs securely
   in applications.

3.  Security Considerations

   All security considerations from [RFC9242] and [RFC9370] apply to the
   ML-KEM exchanges described in this specification.

   The main security property for KEMs standardized by NIST is
   indistinguishability under adaptive chosen ciphertext attacks (IND-
   CCA2) [FIPS203], which means that shared secret values should be
   indistinguishable from random strings even given the ability to have
   arbitrary ciphertexts decapsulated.  IND-CCA2 corresponds to security
   against an active attacker, and the public key / secret key pair can
   be treated as a long-term key or reused.  A weaker security notion is
   indistinguishability under chosen plaintext attacks (IND-CPA), which
   means that the shared secret values should be indistinguishable from
   random strings given a copy of the public key.  IND-CPA roughly
   corresponds to security against a passive attacker, and sometimes
   corresponds to one-time key exchange.  Generating an ephemeral
   keypair and ciphertext for each ML-KEM key exchange is REQUIRED by
   this specification.  Note that this is also common practice for
   (EC)DH keys today.  Responders also MUST NOT reuse randomness in the
   generation of KEM ciphertexts.

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   The ML-KEM public key generated by the initiator and the ciphertext
   generated by the responder use randomness (usually a seed) which MUST
   be independent of any other random seed used in the IKEv2
   negotiation.  For example, at the initiator, the ML-KEM and (EC)DH
   keypairs used in a PQ/T Hybrid key exchange MUST NOT be generated
   from the same seed.

   When using PQ/T Hybrid key exchanges, SKEYSEED and KEYMAT in this
   specification are generated by using shared secrets, nonces, and SPIs
   with a pseudorandom function as defined in [RFC9370].  As discussed
   in [PQ-PROOF2], such PQ/T Hybrid key derivations are IND-CPA, but not
   proven to be IND-CCA2 secure.

   IKEv2 is susceptible to downgrade attacks where an active man-in-the-
   middle could force the peers to negotiate the weakest key exchange
   method supported by both.  In particular, if both peers support some
   sequence of key exchanges that involve only traditional algorithms,
   an active, on-path attacker with a CRQC may be able to convince the
   peers to use it even if they both support ML-KEM as well.  Note that
   to achieve such a downgrade, the adversary needs to break traditional
   (EC)DH IKE_SA_INIT ephemeral exchanges while the negotiation is still
   taking place and completely control the flow to delay or drop
   legitimate IKEv2 messages.  IKEv2 downgrades is a known issue
   [DOWN-RES] caused by the way IKEv2 authenticates messages only in one
   direction of the exchange; [PQIKEV2-FA] concluded that
   IKE_INTERMEDIATE [RFC9370] does not introduce additional attacks with
   respect to IKEv2's original security model.

   The simplest way to prevent such active attacks is to disable support
   for traditional-only sequences of key exchanges whenever possible.
   If the responder knows out-of-band that initiators support ML-KEM,
   then it SHOULD reject any proposal that doesn't include ML-KEM in the
   IKE_SA_INIT or IKE_INTERMEDIATE.  Likewise, if the initiator knows
   out-of-band that a responder supports ML-KEM, it SHOULD only include
   proposals for ML-KEM or abort the negotiation if the responder
   selects a proposal that doesn't include ML-KEM.  A long-term solution
   for the downgrade issue in IKEv2 is proposed in
   [I-D.smyslov-ipsecme-ikev2-downgrade-prevention].

   As an alternative, in cases where only a subset of peer identities is
   known to have been upgraded to support ML-KEM the peers can enforce a
   policy to not encrypt any data until an ML_KEM exchange has taken
   place.  [RFC9370] supports Childless IKE SAs which can be followed by
   a new Child SA after doing more key exchanges.  To ensure that data
   is encrypted over a quantum-resistant IPsec Child SA, the peers could
   enforce a policy which first establishes a Childless IKE SA [RFC6023]
   (or a Child SA which does not encrypt any data) with a traditional
   key exchange and without an IKE_INTERMEDIATE exchange.  Subsequently

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   the peers can rekey the initial IKE SA and derive a new Child SA (or
   rekey the existing Child SA that did not encrypt any data) with ML-
   KEM in a CREATE_CHILD_SA exchange or with ML-KEM as an additional key
   exchange in a IKE_FOLLOWUP_KE exchange which follows a traditional
   CREATE_CHILD_SA exchange.  Section 2.2.5.1 of [RFC9370] discusses the
   details of the latter PQ/T Hybrid approach.  This approach has the
   disadvantage that an adversary with a CRQC that could decrypt the
   IKE_SA_INIT exchange has access to all the information exchanged over
   the initial IKE SA or Child SA before the rekey.  This information
   includes the identities of the peers, configuration parameters, and
   all negotiated SA information (including traffic selectors), but not
   the information and data encrypted after the CREATE_CHILD_SA (and
   IKE_FOLLOWUP_KE with ML-KEM)

4.  IANA Considerations

   IANA is requested to assign three values for the names "ml-kem-512",
   "ml-kem-768", and "ml-kem-1024" in the IKEv2 "Transform Type 4 - Key
   Exchange Method Transform IDs" and has listed this document as the
   reference.  The Recipient Tests field should also point to this
   document:

      +========+=============+========+=================+===========+
      | Number | Name        | Status | Recipient Tests | Reference |
      +========+=============+========+=================+===========+
      | 35     | ml-kem-512  |        | [TBD, this RFC, | [TBD,     |
      |        |             |        | Section 2.3],   | this RFC] |
      +--------+-------------+--------+-----------------+-----------+
      | 36     | ml-kem-768  |        | [TBD, this RFC, | [TBD,     |
      |        |             |        | Section 2.3],   | this RFC] |
      +--------+-------------+--------+-----------------+-----------+
      | 37     | ml-kem-1024 |        | [TBD, this RFC, | [TBD,     |
      |        |             |        | Section 2.3],   | this RFC] |
      +--------+-------------+--------+-----------------+-----------+

            Table 2: Updates to the IANA "Transform Type 4 - Key
                    Exchange Method Transform IDs" table

5.  References

5.1.  Normative References

   [FIPS203]  National Institute of Standards and Technology (NIST),
              "Module-Lattice-Based Key-Encapsulation Mechanism
              Standard", NIST Federal Information Processing Standards,
              13 August 2024, <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.203.pdf>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <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>.

   [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>.

   [RFC9242]  Smyslov, V., "Intermediate Exchange in the Internet Key
              Exchange Protocol Version 2 (IKEv2)", RFC 9242,
              DOI 10.17487/RFC9242, May 2022,
              <https://www.rfc-editor.org/info/rfc9242>.

   [RFC9370]  Tjhai, CJ., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
              Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
              Key Exchanges in the Internet Key Exchange Protocol
              Version 2 (IKEv2)", RFC 9370, DOI 10.17487/RFC9370, May
              2023, <https://www.rfc-editor.org/info/rfc9370>.

5.2.  Informative References

   [DOWN-RES] Bhargavan, K., Brzuska, C., Fournet, C., Green, M.,
              Kohlweiss, M., and S. Zanella-Béguelin, "Downgrade
              Resilience in Key-Exchange Protocols", 2016,
              <https://ieeexplore.ieee.org/document/7546520>.

   [I-D.ietf-ipsecme-ikev2-qr-alt]
              Smyslov, V., "Mixing Preshared Keys in the
              IKE_INTERMEDIATE and in the CREATE_CHILD_SA Exchanges of
              IKEv2 for Post-quantum Security", Work in Progress,
              Internet-Draft, draft-ietf-ipsecme-ikev2-qr-alt-10, 23 May
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              ipsecme-ikev2-qr-alt-10>.

   [I-D.smyslov-ipsecme-ikev2-downgrade-prevention]
              Smyslov, V. and C. Patton, "Prevention Downgrade Attacks
              on the Internet Key Exchange Protocol Version 2 (IKEv2)",
              Work in Progress, Internet-Draft, draft-smyslov-ipsecme-
              ikev2-downgrade-prevention-02, 28 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-smyslov-
              ipsecme-ikev2-downgrade-prevention-02>.

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   [I-D.smyslov-ipsecme-ikev2-reliable-transport]
              Smyslov, V. and T. Reddy.K, "Separate Transports for IKE
              and ESP", Work in Progress, Internet-Draft, draft-smyslov-
              ipsecme-ikev2-reliable-transport-04, 15 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-smyslov-
              ipsecme-ikev2-reliable-transport-04>.

   [IKEv2-A]  Petcher, A. and E. Assuncao, "Analyzing IKEv2: Security
              Proofs, Known Attacks, and Other Insights", 2025,
              <https://ethz.ch/content/dam/ethz/special-interest/infk/
              inst-infsec/appliedcrypto/education/theses/semester-
              project_eduarda-assuncao.pdf>.

   [NIST-PQ]  National Institute of Standards and Technology (NIST),
              "Post-Quantum Cryptography",
              https://csrc.nist.gov/projects/post-quantum-cryptography .

   [PQ-PROOF2]
              Petcher, A. and M. Campagna, "Security of Hybrid Key
              Establishment using Concatenation", 2023,
              <https://eprint.iacr.org/2023/972>.

   [PQIKEV2-FA]
              Gazdag, S., Grundner-Culemann, S., Guggemos, T., Heider,
              T., and D. Loebenberger, "A formal analysis of IKEv2’s
              post-quantum extension", 2021, <https://www.mnm-
              team.org/pub/Publikationen/gggh21b/PDF-Version/
              gggh21b.pdf>.

   [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>.

   [RFC7383]  Smyslov, V., "Internet Key Exchange Protocol Version 2
              (IKEv2) Message Fragmentation", RFC 7383,
              DOI 10.17487/RFC7383, November 2014,
              <https://www.rfc-editor.org/info/rfc7383>.

   [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,
              <https://www.rfc-editor.org/info/rfc8784>.

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   [RFC9329]  Pauly, T. and V. Smyslov, "TCP Encapsulation of Internet
              Key Exchange Protocol (IKE) and IPsec Packets", RFC 9329,
              DOI 10.17487/RFC9329, November 2022,
              <https://www.rfc-editor.org/info/rfc9329>.

   [RFC9794]  Driscoll, F., Parsons, M., and B. Hale, "Terminology for
              Post-Quantum Traditional Hybrid Schemes", RFC 9794,
              DOI 10.17487/RFC9794, June 2025,
              <https://www.rfc-editor.org/info/rfc9794>.

   [SP800227] National Institute of Standards and Technology (NIST),
              "Recommendations for Key-Encapsulation Mechanisms",
              NIST Federal Information Processing Standards, 18
              September 2025,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-227.pdf>.

Acknowledgments

   The authors would like to thank Valery Smyslov, Graham Bartlett,
   Scott Fluhrer, Ben S, Leonie Bruckert, Tero Kivinen, Rebecca Guthrie,
   Wang Guilin, Michael Richardson, John Mattsson, and Gerardo Ravago
   for their valuable feedback.  Special thanks to Chris Patton for
   bringing up the downgrade issue.

Author's Address

   Panos Kampanakis
   Amazon Web Services
   Email: kpanos@amazon.com

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