Post-quantum Hybrid Key Exchange with ML-KEM in the Internet Key Exchange Protocol Version 2 (IKEv2)
draft-kampanakis-ml-kem-ikev2-08

IPSECME                                                    P. Kampanakis
Internet-Draft                                                 G. Ravago
Intended status: Standards Track                     Amazon Web Services
Expires: 22 March 2025                                 18 September 2024

    Post-quantum Hybrid Key Exchange with ML-KEM in the Internet Key
                  Exchange Protocol Version 2 (IKEv2)
                    draft-kampanakis-ml-kem-ikev2-08

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 as an additional key exchange in
   IKEv2 along with traditional key exchanges.  This Post-Quantum
   Traditional Hybrid Key Encapsulation Mechanism approach allows for
   negotiating IKE and Child SA keys which are safe against
   cryptanalytically-relevant quantum computers and theoretical
   weaknesses in ML-KEM.

Status of This Memo

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   This Internet-Draft will expire on 22 March 2025.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  KEMs  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  ML-KEM  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Conventions and Definitions . . . . . . . . . . . . . . .   4
   2.  ML-KEM in IKEv2 . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  ML-KEM in IKE_INTERMEDIATE or CREATE_CHILD_SA messages  .   5
     2.2.  Key Exchange Payload  . . . . . . . . . . . . . . . . . .   6
     2.3.  Recipient Tests . . . . . . . . . . . . . . . . . . . . .   7
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   A Cryptanalytically-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).

   To address this concern, [RFC8784] introduced Post-quantum Preshared
   Keys 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.

   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 [I-D.draft-cfrg-schwabe-kyber-04].  Kyber was then
   standardized as Module-Lattice-based Key-Encapsulation Mechanism (ML-
   KEM) in 2024 [FIPS203].

   As post-quantum publc keys and ciphertexts may make UDP packet sizes
   larger than common network Maximum Transport Units (MTU), [RFC9242]
   defined how to do additional large message exchanges by using new

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   IKE_INTERMEDIATE or IKE_FOLLOWUP_KE messages which can be fragmented
   at the IKEv2 layer before causing IP fragmentation [RFC7383].
   Because [RFC9242] messages can only be used after IKE_SA_INIT, if a
   PQ KEM does not fit inside IKE_SA_INIT without causing IP
   fragmentation, then it should be used after IKE_SA_INIT as an
   additional key establishment algorithm.  [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.  This specification was created to enable new post-quantum
   key exchanges to be used in the derived IKE and Child SA keys and
   provide quantum resistance.

   This document describes how ML-KEM can be used as a quantum-resistant
   KEM in IKEv2 in an IKE_SA_INIT key 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
   [I-D.ietf-pquip-pqt-hybrid-terminology-02] 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 commom network packet payload
   sizes, they will need to be sent in a IKE_FOLLOWUP_KE or
   CREATE_CHILD_SA key exchange which can be fragmented.  This
   specification is a profile of [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'.

   *  'Encaps(pk) -> (ct, ss)': A probabilistic encapsulation algorithm,
      which takes as input a public key 'pk' and outputs a ciphertext
      'ct' and shared secret 'ss'.

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

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   The main security property for KEMs standardized by NIST is
   indistinguishability under adaptive chosen ciphertext attacks (IND-
   CCA2), 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.

1.2.  ML-KEM

   ML-KEM is a standardized lattice-based key encapsulation mechanism
   [FIPS203].

   ML-KEM is using 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.

   This specification introduces ML-KEM-512, ML-KEM-768 and ML-KEM-1024
   to IKEv2 key exchanges as conservative security level parameters
   which will not have material 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 typical network MTU, these
   key exchanges could require two or three network IP packets from both
   the initiator and the responder.

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

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2.1.  ML-KEM in IKE_INTERMEDIATE or CREATE_CHILD_SA messages

   ML-KEM key exchanges can be negotiated in IKE_INTERMEDIATE or
   IKE_FOLLOWUP_KE messages as defined in [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 [RFC9242].  KEi(1) and KEr(1) are the subsequent key
   exchange messages which carry the ML-KEM public key of a keypair (sk,
   pk) generated by the initiator with ML-KEM KeyGen() and the 256-bit
   ML-KEM shared secret SK(1) encapsulated by the responder to a
   ciphertext ct by using Encaps(pk) respectively.  The initiator then
   decapsulates the 256-bit ML-KEM shared secret SK(1) from the
   ciphertext ct by using its private key sk in Decaps(sk, ct).  Both
   peers have now reached a common SK(1) at the end of this KE(1) key
   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 [RFC9370].  Afterwards the peers continue to the IKE_AUTH exchange
   phase as defined in Section 3.3.2 of [RFC9242].

   ML-KEM can also be used to create or rekey a Child SA or rekey the
   IKE SA by using a IKE_FOLLOWUP_KE message after a CREATE_CHILD_SA
   message.  After the ML-KEM additional key exchange KE(1) has taken
   place using and 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].

   ML-KEM-768 and ML-KEM-1024 public keys and ciphertexts may make UDP
   packet sizes larger typical network MTUs (1500 bytes).  Thus,
   IKE_INTERMEDIATE or IKE_FOLLOWUP_KE messages carrying ML-KEM public
   keys and ciphertexts may be IKEv2 fragmented as per [RFC7383].

   Although, this document focuses on using ML-KEM as the second key
   exchange in a PQ/T Hybrid KEM
   [I-D.ietf-pquip-pqt-hybrid-terminology-02] scenario, ML-KEM-512 and
   ML-KEM-768 Key Exchange Method identifiers TBD35 and TBD36
   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 of 1500 bytes.  ML-KEM-1024 Key Exchange Method
   identifier TBD37 SHOULD NOT be used in IKE_SA_INIT messages which
   could exceed typical network MTUs and cannot be IKEv2 fragmented.

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2.2.  Key Exchange Payload

   HDR, the IKE header, of the IKE_INTERMEDIATE messages carrying the
   ML-KEM key exchange has a Next Payload value of 34 (Key Exchange),
   Exchange Type of 43 (IKE_INTERMEDIATE) and Message ID of 1 assuming
   this is the first additional key exchange (ADDKE1).  For
   IKE_FOLLOWUP_KE messages carrying the ML-KEM key exchange, the
   Exchange Type would be 44 (IKE_FOLLOWUP_KE).

   The IKE_INTERMEDIATE or IKE_FOLLOWUP_KE payload is shown below as
   defined in Section 3.4 of [RFC7296]:

                        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                       ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 specific
   in this document.  The public key and the ciphertext in the Key
   Exchange Data are encoded as raw byte arrays as defined in [FIPS203].

     +=============+================+============+===================+
     |     KEM     | Payload Length |    Key     |    Data Size in   |
     |             |  (initiator /  |  Exchange  | Octets (initiator |
     |             |   responder)   | Method Num |    / responder)   |
     +=============+================+============+===================+
     |  ML-KEM-512 |   808 / 776    |   TBD35    |     800 / 768     |
     +-------------+----------------+------------+-------------------+
     |  ML-KEM-768 |  1192 / 1096   |   TBD36    |    1184 / 1088    |
     +-------------+----------------+------------+-------------------+
     | ML-KEM-1024 |  1576 / 1576   |   TBD37    |    1568 / 1568    |
     +-------------+----------------+------------+-------------------+

                    Table 1: Key Exchange Payload Fields

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2.3.  Recipient Tests

   Receiving and handling of malformed ML-KEM public key or ciphertext
   SHOULD follow the input validation described in [FIPS203].  In
   particular, entities receiving the ML-KEM public key to encapsulate
   to SHOULD perform the checks in Section 7.1 of [FIPS203] and reject
   the ML-KEM public key, if malformed.  Entities receiving an ML-KEM
   ciphertext for decapsulation SHOULD perform the ciphertext checks in
   Section 7.2 of [FIPS203] and reject the ciphertext, if malformed.
   These checks could be performed separately before performing the
   encapsulation or decapsulation steps or be part of them.  The
   response message to an IKE_SA_INIT, or IKE_INTERMEDIATE, or
   IKE_FOLLOWUP_KE with malformed ML-KEM public keys or ciphertext MUST
   be a Notify payload of type INVALID_SYNTAX.

   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.

3.  Security Considerations

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

   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 should not be generated
   from the same seed.

   SKEYSEED and KEYMAT in this specification are generated from PQ/T
   Hybrid key exchanges 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
   IND-CCA2 secure although the keys could be reused if the nonces are
   never reused.  Alternatively, the PRF combiner that derives the PQ/T
   Hybrid SKEYSEED and KEYMAT could be updated to include the
   ciphertexts which could be proven IND-CCA2 secure following similar
   methodology to [PQ-PROOF2].  In any ocassion, reusing the same ML-KEM
   keypair does not offer perfect forward secrecy.  As is the case with
   (EC)DH, the initiator ought to generate a new ML-KEM keypair with
   every ML-KEM key exchange.

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4.  IANA Considerations

   IANA is requested to assign three values for the names "mlkem-512",
   "mlkem-768", and "mlkem-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  |
    +=========+=============+========+===================+============+
    | TBD35   | ml-kem-512  |        | [TBD, this draft, | [TBD, this |
    |         |             |        | Section 2.3],     | draft]     |
    +---------+-------------+--------+-------------------+------------+
    | TBD36   | ml-kem-768  |        | [TBD, this draft, | [TBD, this |
    |         |             |        | Section 2.3],     | draft]     |
    +---------+-------------+--------+-------------------+------------+
    | TBD37   | ml-kem-1024 |        | [TBD, this draft, | [TBD, this |
    |         |             |        | Section 2.3],     | draft]     |
    +---------+-------------+--------+-------------------+------------+
    | 38-1023 | Unassigned  |        |                   |            |
    +---------+-------------+--------+-------------------+------------+

       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>.

   [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/rfc/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/rfc/rfc7296>.

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   [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/rfc/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/rfc/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/rfc/rfc9370>.

5.2.  Informative References

   [I-D.draft-cfrg-schwabe-kyber-04]
              Schwabe, P. and B. Westerbaan, "Kyber Post-Quantum KEM",
              Work in Progress, Internet-Draft, draft-cfrg-schwabe-
              kyber-04, 2 January 2024,
              <https://datatracker.ietf.org/doc/html/draft-cfrg-schwabe-
              kyber-04>.

   [I-D.ietf-pquip-pqt-hybrid-terminology-02]
              D, F., "Terminology for Post-Quantum Traditional Hybrid
              Schemes", Work in Progress, Internet-Draft, draft-ietf-
              pquip-pqt-hybrid-terminology-02, 2 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-pquip-
              pqt-hybrid-terminology-02>.

   [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>.

   [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/rfc/rfc7383>.

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

Acknowledgments

   The authors would like to thank Valery Smyslov, Graham Bartlett,
   Scott Fluhrer, Ben S, and Leonie Bruckert for their valuable
   feedback.

Authors' Addresses

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

   Gerardo Ravago
   Amazon Web Services
   Email: gcr@amazon.com

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