Post-quantum Hybrid Key Exchange with ML-KEM in the Internet Key Exchange Protocol Version 2 (IKEv2)
draft-kampanakis-ml-kem-ikev2-01

IPSECME                                                    P. Kampanakis
Internet-Draft                                                 G. Ravago
Intended status: Standards Track                     Amazon Web Services
Expires: 23 May 2024                                    20 November 2023

    Post-quantum Hybrid Key Exchange with ML-KEM in the Internet Key
                  Exchange Protocol Version 2 (IKEv2)
                    draft-kampanakis-ml-kem-ikev2-01

Abstract

   [EDNOTE: The intention of this draft is to get IANA KE codepoints for
   ML-KEM.  It could be a standards track draft given that ML-KEM will
   see a lot of adoption, an AD sponsored draft, or even an individual
   stable draft which gets codepoints from Expert Review.  The approach
   is to be decided by the IPSECME WG. ]

   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 additionall key exchange
   mechanism in IKEv2 along with traditional (Elliptic Curve) Diffie-
   Hellman.  This hybrid approach allows for negotiating IKE and Child
   SA keys which are safe against cryptanalytically-relevant quantum
   computers and theoretical weaknesses in ML-KEM as it is relatively
   new.

Status of This Memo

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Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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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 messages . . . . . . . . . . .   5
     2.2.  Key Exchange Payload  . . . . . . . . . . . . . . . . . .   6
     2.3.  Recipient Tests . . . . . . . . . . . . . . . . . . . . .   7
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   A Cryptanalytically-relevant Quantum Computer (CRQC), if it became a
   reality, could threaten public key encryption algorithms used today
   for key exhange.  Someone storing encrypted communications which use
   (Elliptic Curve) Diffie-Hellman ((EC)DH) to negotiate keys could
   decrypt these communications in the future after a CRQC was
   available.  This could include Internet Key Exchange Protocol Version
   2 (IKEv2)/IPsec tunnels which negotiate IKE and Child SA keys by
   using ECDH key exchange in their IKE_SA_INIT messages.

   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, [RFC9242] defined how to do
   additional large message exchanges by using a new IKE_INTERMEDIATE
   message.  As post-quantum keys are usualy larger than common network
   Maximum Transport Units (MTU), IKE_INTERMEDIATE messages can be
   fragmented which could allow for the peers to do post-quantum key
   exchanges without IP fragmentation.  [RFC9370] defined how to do up
   to seven additional key exchanges by using IKE_INTERMEDIATE messages

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   and derive new SKEYSEED and KEYMAT key materials.  This allows for
   new post-quantum key exchanges to be used in the derived IKE and
   Child SA keys and provide quantum resistance.

   NIST has been working on a public project [NIST-PQ] for standardizing
   quantum-safe algorithms which include key ensapsulation 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-03].  Kyber was then standardized as
   Module-Lattice-based Key-Encapsulation Mechanism (ML-KEM) in
   [FIPS203-ipd].  ML-KEM was standardized in 2024 [FIPS203]. [ EDNOTE:
   Reference normatively the ratified version
   [I-D.draft-cfrg-schwabe-kyber-03] if it is ever ratified.  Otherwise
   keep a normative reference of [FIPS203].  And remove the reference to
   [FIPS203-ipd]. ]

   This document describes how ML-KEM can be used as the quantum-safe
   KEM in IKEv2 by using one additional IKE_INTERMEDIATE key exchange
   after the classical (EC)DH exchange in IKE_SA_INIT.  This approach is
   commonly called post-quantum hybrid key exchange and combines the
   security of well-established (EC)DH with relatively new quantum-safe
   algorithms which could theoretically have uknown issues.  The result
   is a new Child SA key or an IKE or Child SA rekey with keying
   material which is safe against a CRQC.  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 key 'pk' and a secret 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.

   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

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   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 recently standardized lattice-based key encapsulation
   mechanism [FIPS203]. [ EDNOTE: Reference normatively the ratified
   version [I-D.draft-cfrg-schwabe-kyber-03] if it is ever ratified.
   Otherwise keep a normative reference of [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-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.  Since the ML-KEM-768 and ML-
   KEM-1024 public key and ciphertext sizes can exceed the typical
   network MTU, these key exchanges will usually 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 messages

   ML-KEM key exchanges can be negotiated in IKE_INTERMEDIATE messages
   as defined in [RFC9370].  Section 2.2.2 of [RFC9370] specifies that
   KEi(0), KEr(0) are regular (EC)DH key exchange messages in the first
   IKE_SA_INIT exchange which end up generating a first 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 initiators with ML-KEM KeyGen() and the 256-bit
   ML-KEM shared secret ss encapsulated by the responder to a ciphertext
   ct by using Encaps(pk) respectively.  The public key and the
   ciphertext are encoded as raw bytes in little-endian encoding. [
   EDNOTE: Confirm this makes sense. ] Then the initiator decapsulates
   the 256-bit ML-KEM shared secret ss from the ciphertext ct by using
   its private key sk in Decaps(sk, ct).  Both peers have now reached a
   common ss 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 be used in a post-quantum hybrid exchange to create or
   rekey a Child SA or rekey the IKE SA.  The IKE or Child SA can be
   rekeyed by stirring the new shared secret in SKEYSEED and KEYMAT as
   specified in Section 2.2.4 of [RFC9370].

   IKE_INTERMEDIATE messages carrying ML-KEM public keys and
   ciphertexts, can be fragmented as per [RFC7383] since the ML-KEM-768
   and ML-KEM-1024 public keys and ciphertexts can exceed typical
   network MTUs.  Although, this document focuses on using ML-KEM as the
   second key exchange in a post-quantum hybrid key exchange scenario,
   ML-KEM-768 Key Exchange Method identifier TBD35 MAY be used in
   IKE_SA_INIT as a quantum-safe-only key exchange because the payloads
   can fit in typical network MTUs.  [EDNOTE: Confirm it fits the MTU
   with captures.]  ML-KEM-1024 Key Exchange Method identifier TBD36
   SHOULD only be used in IKE_INTERMEDIATE exchanges.  It SHOULD NOT be
   used in IKE_SA_INIT because they could often be introducing IP
   fragmentation which is not possible in IKE_SA_INIT exchanges.

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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).

   The IKE_INTERMEDIATE payload which is protected with SK_e[i/r] and
   SK_a[i/r] keys from the IKE_SA_INIT ML-KEM key exchange 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                       ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   *  Payload Length: The ML-KEM-768 public key is 1184 bytes, so the
      Payload Length field included in the payload of the
      IKE_INTERMEDIATE message from the initiator is 1192.  The ML-
      KEM-768 ciphertext is 1088 bytes, so the Payload Length of
      IKE_INTERMEDIATE message from the responder is 1096.  The ML-
      KEM-1024 public key is 1568 bytes, so the Payload Length field
      included in the payload of the IKE_INTERMEDIATE from the initiator
      is 1576.  The ML-KEM-1024 ciphertext is 1568 bytes, so the Payload
      Length of IKE_INTERMEDIATE from the responder is 1576.

   *  The Key Exchange Method Num identifier is TBD35 for ML-KEM-768 or
      TBD36 for ML-KEM-1024.

   *  The Key Exchange Data is the 1184 or 1568 octets of the ML-KEM-768
      or ML-KEM-1024 public key respectively for the IKE_INTERMEDIATE
      message from the initiator.  The response from the responder is
      1088 or 1568 octets as the size of the ML-KEM-768 or ML-KEM-1024
      ciphertexts respectively.

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2.3.  Recipient Tests

   Receiving and handling of malformed ML-KEM public key or ciphertext
   MUST follow the input validation described in [FIPS203]. [ EDNOTE:
   Reference normatively the ratified version
   [I-D.draft-cfrg-schwabe-kyber-03] if it is ever ratified.  Otherwise
   keep a normative reference of [FIPS203]. ] In particular, entities
   receiving the ML-KEM public key to encapsulate to MUST perform the
   type and modulus checks in Sections 6.1 of [FIPS203] and reject the
   ML-KEM public key, if malformed.  Entities receiving an ML-KEM
   ciphertext for decapsulation MUST perform the ciphertext and
   decapsulation key type checks in Section 6.2 of [FIPS203] and reject
   the ciphertext or key, if malformed. [ EDNOTE: Reference normatively
   the ratified version [I-D.draft-cfrg-schwabe-kyber-03] if it is ever
   ratified.  Otherwise keep a normative reference of [FIPS203]. ] These
   checks could be performed separately before performing the
   encapsulation or decapsulation steps or be part of them.

   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. [ EDNOTE: Confirm
   implicit rejection is still used after [FIPS203] is ratified or
   change this paragraph. ]

3.  Security Considerations

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

4.  IANA Considerations

   IANA is requested to assign two values for the names "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:

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    +=========+============+========+===================+============+
    | Number  | Name       | Status | Recipient Tests   | Reference  |
    +=========+============+========+===================+============+
    | TBD35   | mlkem-768  |        | [TBD, this draft, | [TBD, this |
    |         |            |        | Section 2.3],     | draft]     |
    +---------+------------+--------+-------------------+------------+
    | TBD36   | mlkem-1024 |        | [TBD, this draft, | [TBD, this |
    |         |            |        | Section 2.3],     | draft]     |
    +---------+------------+--------+-------------------+------------+
    | 37-1023 | Unassigned |        |                   |            |
    +---------+------------+--------+-------------------+------------+

      Table 1: Updates to the IANA "Transform Type 4 - Key Exchange
                       Method Transform IDs" table

5.  References

5.1.  Normative References

   [FIPS203]  "*** BROKEN REFERENCE ***".

   [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>.

   [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

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   [FIPS203-ipd]
              National Institute of Standards and Technology (NIST),
              "Module-Lattice-based Key-Encapsulation Mechanism
              Standard", NIST Federal Information Processing Standards,
              24 August 2023, <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.203.ipd.pdf>.

   [I-D.draft-cfrg-schwabe-kyber-03]
              Schwabe, P. and B. Westerbaan, "Kyber Post-Quantum KEM",
              Work in Progress, Internet-Draft, draft-cfrg-schwabe-
              kyber-03, 25 September 2023,
              <https://datatracker.ietf.org/doc/html/draft-cfrg-schwabe-
              kyber-03>.

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

   [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>.

   [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 for his valuable
   contributions to the document.

Authors' Addresses

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

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

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