draft-kampanakis-ml-kem-ikev2-09.txt

IPSECME                                                    P. Kampanakis
Internet-Draft                                                 G. Ravago
Intended status: Standards Track                     Amazon Web Services
Expires: 4 May 2025                                      31 October 2024


    Post-quantum Hybrid Key Exchange with ML-KEM in the Internet Key
                  Exchange Protocol Version 2 (IKEv2)
                    draft-kampanakis-ml-kem-ikev2-09

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

   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
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   This Internet-Draft will expire on 4 May 2025.

Copyright Notice

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   extracted from this document must include Revised BSD License text as
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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  .   4
     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, the Mixing Preshared Keys in IKEv2
   specification [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.  This specification can be used in conjunction with PPK
   as defined in [RFC8784].

   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].





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   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 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.  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.  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-04] 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 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.






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   *  '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.

   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

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 the Multiple Key Exchanges in
   IKEv2 specification [RFC9370].  We summarize them here for
   completeness.




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   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 (sk, pk) 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 continue to the
   IKE_AUTH exchange phase as defined in Section 3.3.2 of the
   Intermediate Exchange in IKEv2 specification [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 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
   [I-D.ietf-pquip-pqt-hybrid-terminology-04] 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

   The KE payload is shown below and the fields inside it has meaning as
   defined in Section 3.4 of the IKEv2 standard [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                       ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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    |   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 keys or ciphertexts
   SHOULD follow the input validation described in the Module-Lattice-
   Based KEM standard [FIPS203].

   Responders SHOULD perform the checks 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 SHOULD 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.  In this case, the
   initiator MAY send a Notify payload of type INVALID_SYNTAX to the
   responder as a separate INFORMATIONAL exchange, usually with no other
   payloads.  This is an exception for the general rule of not starting
   new exchanges based on errors in responses.

   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 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.  As with (EC)DH keys today,
   generating an ephemeral key exchange keypair for ECDH and ML-KEM is
   still REQUIRED per connection by this specification (IND-CPA
   security).




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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 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
   proven to be IND-CCA2 secure although the keys could be reused if the
   nonces are never reused.

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








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   [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>.

   [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-04]
              D, F., P, M., and B. Hale, "Terminology for Post-Quantum
              Traditional Hybrid Schemes", Work in Progress, Internet-
              Draft, draft-ietf-pquip-pqt-hybrid-terminology-04, 10
              September 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-pquip-pqt-hybrid-terminology-04>.





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   [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>.

   [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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