ML-KEM Post-Quantum Key Agreement for TLS 1.3
draft-ietf-tls-mlkem-09

Transport Layer Security                                     D. Connolly
Internet-Draft                                                 SandboxAQ
Intended status: Informational                              20 July 2026
Expires: 21 January 2027

             ML-KEM Post-Quantum Key Agreement for TLS 1.3
                        draft-ietf-tls-mlkem-09

Abstract

   This memo defines ML-KEM-512, ML-KEM-768, and ML-KEM-1024 as
   NamedGroups and registers IANA values in the TLS Supported Groups
   registry for use in TLS 1.3 to achieve post-quantum (PQ) key
   establishment.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-tls-mlkem/.

   Discussion of this document takes place on the Transport Layer
   Security Working Group mailing list (mailto:tls@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/tls/.  Subscribe
   at https://www.ietf.org/mailman/listinfo/tls/.

   Source for this draft and an issue tracker can be found at
   https://github.com/tlswg/draft-ietf-tls-mlkem.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 21 January 2027.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   2
   3.  Key encapsulation mechanisms  . . . . . . . . . . . . . . . .   3
   4.  Construction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Negotiation . . . . . . . . . . . . . . . . . . . . . . .   3
     4.2.  Transmitting encapsulation keys and ciphertexts . . . . .   4
     4.3.  Shared secret calculation . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   ML-KEM [FIPS203] is a FIPS standard for post-quantum [RFC9794] key
   establishment via a lattice-based key encapsulation mechanism (KEM).
   This document defines key establishment options for TLS 1.3 via the
   existing supported_groups Section 4.3.7 of [RFC9846] and key_share
   Section 4.3.8 of [RFC9846] extensions.

2.  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.

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3.  Key encapsulation mechanisms

   This document models key establishment as key encapsulation
   mechanisms (KEMs), which consist of three algorithms:

   *  KeyGen() -> (pk, sk): A probabilistic key generation algorithm,
      which generates a public encapsulation key pk and a secret
      decapsulation key sk.

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

   *  Decaps(sk, ct) -> shared_secret: A decapsulation algorithm, which
      takes as input a secret decapsulation key sk and ciphertext ct and
      outputs a shared secret shared_secret.

   ML-KEM-512, ML-KEM-768 and ML-KEM-1024 conform to this interface:

   *  ML-KEM-512 has encapsulation keys of size 800 bytes, expanded
      decapsulation keys of 1632 bytes, decapsulation key seeds of size
      64 bytes, ciphertext size of 768 bytes, and shared secrets of size
      32 bytes

   *  ML-KEM-768 has encapsulation keys of size 1184 bytes, expanded
      decapsulation keys of 2400 bytes, decapsulation key seeds of size
      64 bytes, ciphertext size of 1088 bytes, and shared secrets of
      size 32 bytes

   *  ML-KEM-1024 has encapsulation keys of size 1568 bytes, expanded
      decapsulation keys of 3168 bytes, decapsulation key seeds of size
      64 bytes, ciphertext size of 1568 bytes, and shared secrets of
      size 32 bytes

4.  Construction

   The KEMs are defined as NamedGroups, sent in the supported_groups
   extension.  Section 4.3.7 of [RFC9846]

4.1.  Negotiation

   Each parameter set of ML-KEM is assigned an identifier, registered by
   IANA in the TLS Supported Groups registry:

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       enum {

            ...,

             /* ML-KEM Key Establishment Methods */
             mlkem512(0x0200),
             mlkem768(0x0201),
             mlkem1024(0x0202)

            ...,

       } NamedGroup;

4.2.  Transmitting encapsulation keys and ciphertexts

   The public encapsulation key and ciphertext values are each directly
   encoded with fixed lengths as in [FIPS203].

   In TLS 1.3 a KEM public encapsulation key pk or ciphertext ct is
   represented as a KeyShareEntry as specified in Section 4.3.8 of
   [RFC9846].  These are transmitted in the extension_data fields of
   KeyShareClientHello and KeyShareServerHello extensions.

   For the client's share, the key_exchange value contains the pk output
   of the corresponding ML-KEM parameter set's KeyGen algorithm.

   For the server's share, the key_exchange value contains the ct output
   of the corresponding ML-KEM parameter set's Encaps algorithm.

   For all parameter sets, the server MUST perform the encapsulation key
   check described in Section 7.2 of [FIPS203] on the client's
   encapsulation key, and abort with an illegal_parameter alert if it
   fails.

   For all parameter sets, the client MUST check if the ciphertext
   length matches the selected parameter set, and abort with an
   illegal_parameter alert if it fails.

   If ML-KEM decapsulation fails for any other reason, the connection
   MUST be aborted with an internal_error alert.

   Implementations MUST NOT reuse randomness in the generation of ML-KEM
   ciphertexts— it follows that ML-KEM ciphertexts also MUST NOT be
   reused.

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   During encapsulation, ML-KEM draws the encapsulation randomness from
   a random bit generator; the peer holding the decapsulation key sk
   recovers this randomness exactly.  Any information that this
   randomness provides about other outputs of the generator is therefore
   available to that peer.

4.3.  Shared secret calculation

   The fixed-length shared secret output from the ML-KEM Encaps and
   Decaps algorithms over the appropriate keypair and ciphertext results
   in the same shared secret shared_secret as its peer, which is
   inserted into the TLS 1.3 key schedule in place of the (EC)DHE shared
   secret, as shown in Section 7.1 of [RFC9846].

5.  Security Considerations

   This document defines standalone ML-KEM key establishment for TLS
   1.3.  Use of KEMs for key agreement in TLS 1.3 has been analyzed in
   multiple settings and security models [DOWLING] [KEMTLS] [HV22]
   [CHSW22] [CZCJWH25] [ZJZ24]; ML-KEM's IND-CCA security exceeds the
   requirements for ephemeral key establishment [GHS25] [RFC9846].
   Multiple formal analyses, including pen-and-paper computational
   proofs and machine-checked symbolic analysis using ProVerif
   [KOBEISSI26], demonstrate that replacing Diffie-Hellman with an IND-
   CCA-secure KEM preserves the security properties of the TLS
   handshake.  Formal analysis has also shown that hybrid key
   establishment (e.g., [HYBRID], [ECDHE-MLKEM]) provides compositional
   security: the exchange remains secure as long as at least one of the
   component algorithms is unbroken [BJ24] [CPWB25].

   TLS 1.3's key schedule commits to the ML-KEM encapsulation key and
   the ciphertext as the key_exchange field of the key_share extension
   is populated with those values, which are included as part of the
   handshake messages.  This provides resilience against re-
   encapsulation attacks against KEMs used for key establishment
   [CDM23].

   [NIST-SP-800-227] includes guidelines and requirements for
   implementations on using KEMs securely.  Implementers are encouraged
   to use implementations resistant to side-channel attacks, especially
   those that can be applied by remote attackers.

   Implementers must evaluate their specific security, performance, and
   operational constraints when deciding whether to deploy standalone
   ML-KEM or a hybrid construction.  The recommended column in the IANA
   TLS Supported Groups registry contains the IETF's current guidance on
   the recommended use of these algorithms for general purposes.

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   The disclosure of the output(s) of an insecure random number
   generator (RNG) when used in TLS and other protocols can be used in
   an attack to compromise the state of the insecure RNG itself as
   described in [DUALEC-TLS].  The encapsulation randomness in ML-KEM is
   an additional place where raw RNG output may be disclosed, therefore
   it is important to follow the RNG guidance in [FIPS203] and
   [RFC9846].  Implementers can choose to implement mechanisms from
   [RFC8937] for additional protection across sessions.

6.  IANA Considerations

   This document requests/registers three new entries to the TLS Named
   Group (or Supported Group) registry, according to the procedures in
   Section 6 of [RFC9847].

   +======+=============+=======+=============+=========+=============+
   |Value | Description |DTLS-OK| Recommended |Reference| Comment     |
   +======+=============+=======+=============+=========+=============+
   |0x0200| MLKEM512    |Y      | N           |This     | FIPS 203    |
   |      |             |       |             |document.| version of  |
   |      |             |       |             |         | ML-KEM-512  |
   +------+-------------+-------+-------------+---------+-------------+
   |0x0201| MLKEM768    |Y      | N           |This     | FIPS 203    |
   |      |             |       |             |document.| version of  |
   |      |             |       |             |         | ML-KEM-768  |
   +------+-------------+-------+-------------+---------+-------------+
   |0x0202| MLKEM1024   |Y      | N           |This     | FIPS 203    |
   |      |             |       |             |document.| version of  |
   |      |             |       |             |         | ML-KEM-1024 |
   +------+-------------+-------+-------------+---------+-------------+

                                 Table 1

   As defined in Section 3 of [RFC9847], the value N indicates:

      That the item has not been evaluated by the IETF and that the IETF
      has made no statement about the suitability of the associated
      mechanism.  This does not necessarily mean that the mechanism is
      flawed, only that no consensus exists.  The IETF might have
      consensus to leave an item marked as "N" on the basis of the item
      having limited applicability or usage constraints.

7.  References

7.1.  Normative References

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   [FIPS203]  "Module-lattice-based key-encapsulation mechanism
              standard", National Institute of Standards and Technology
              (U.S.), DOI 10.6028/nist.fips.203, August 2024,
              <https://doi.org/10.6028/nist.fips.203>.

   [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>.

   [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>.

   [RFC9846]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 9846, DOI 10.17487/RFC9846, July 2026,
              <https://www.rfc-editor.org/rfc/rfc9846>.

7.2.  Informative References

   [BJ24]     Blanchet, B. and C. Jacomme, "Post-Quantum Sound
              CryptoVerif and Verification of Hybrid TLS and SSH Key-
              Exchanges", Proceedings of CSF 2024 , 2024,
              <https://bblanche.gitlabpages.inria.fr/publications/
              BlanchetJacommeCSF24.pdf>.

   [CDM23]    Cremers, C., Dax, A., and N. Medinger, "Keeping Up with
              the KEMs: Stronger Security Notions for KEMs and automated
              analysis of KEM-based protocols", 2023,
              <https://eprint.iacr.org/2023/1933.pdf>.

   [CHSW22]   Celi, S., Hoyland, J., Stebila, D., and T. Wiggers, "A
              Tale of Two Models: Formal Verification of KEMTLS via
              Tamarin", Proceedings of ESORICS 2022 , 2022,
              <https://doi.org/10.1007/978-3-031-17143-7_4>.

   [CPWB25]   Chen, J., Peng, W., Wang, Y., and Y. Bian, "On the
              Security and Efficiency of TLS 1.3 Handshake with Hybrid
              Key Exchange from CPA-Secure KEMs", Entropy 27(12):1242 ,
              2025, <https://doi.org/10.3390/e27121242>.

   [CZCJWH25] "Post-Quantum {TLS} 1.3 Handshake from {CPA}-Secure {KEMs}
              with Tighter Reductions", n.d.,
              <https://eprint.iacr.org/2025/1748.pdf>.

   [DOWLING]  "A Cryptographic Analysis of the TLS 1.3 Handshake
              Protocol", Journal of Cryptology 2021 , 2020,
              <https://doi.org/10.1007/s00145-021-09384-1>.

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   [DUALECTLS]
              "On the Practical Exploitability of Dual EC in TLS
              Implementations", 2014,
              <https://www.usenix.org/system/files/conference/
              usenixsecurity14/sec14-paper-checkoway.pdf>.

   [ECDHE-MLKEM]
              Kwiatkowski, K., Kampanakis, P., Westerbaan, B., and D.
              Stebila, "Post-quantum hybrid ECDHE-MLKEM Key Agreement
              for TLSv1.3", Work in Progress, Internet-Draft, draft-
              ietf-tls-ecdhe-mlkem-05, 26 May 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              ecdhe-mlkem-05>.

   [GHS25]    Glabush, L., Hovelmanns, K., and D. Stebila, "On The
              Multi-target Security of Post-Quantum Key Encapsulation
              Mechanisms", Cryptology ePrint Archive, Report 2025/343 ,
              2025, <https://eprint.iacr.org/2025/343.pdf>.

   [HV22]     Huguenin-Dumittan, L. and S. Vaudenay, "On IND-qCCA
              Security in the ROM and Its Applications - CPA Security Is
              Sufficient for TLS 1.3", Proceedings of Eurocrypt 2022 ,
              n.d., <https://link.springer.com/
              chapter/10.1007/978-3-031-07082-2_22>.

   [HYBRID]   Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
              exchange in TLS 1.3", Work in Progress, Internet-Draft,
              draft-ietf-tls-hybrid-design-16, 7 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              hybrid-design-16>.

   [KEMTLS]   Schwabe, P., Stebila, D., and T. Wiggers, "Post-Quantum
              TLS Without Handshake Signatures", ACM, Proceedings of the
              2020 ACM SIGSAC Conference on Computer and Communications
              Security pp. 1461-1480, DOI 10.1145/3372297.3423350,
              October 2020, <https://doi.org/10.1145/3372297.3423350>.

   [KOBEISSI26]
              "FATT Chance: On the Robustness of Standalone and Hybrid
              ML-KEM Key Exchange in TLS 1.3", n.d.,
              <https://eprint.iacr.org/2026/1147.pdf>.

   [NIST-SP-800-227]
              Alagic, G., Barker, E., Chen, L., Moody, D., Robinson, A.,
              Silberg, H., and N. Waller, "Recommendations for key-
              encapsulation mechanisms", National Institute of Standards
              and Technology (U.S.), DOI 10.6028/nist.sp.800-227,
              September 2025, <https://doi.org/10.6028/nist.sp.800-227>.

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   [RFC8937]  Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
              and C. Wood, "Randomness Improvements for Security
              Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
              <https://www.rfc-editor.org/rfc/rfc8937>.

   [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/rfc/rfc9794>.

   [RFC9847]  Salowey, J. and S. Turner, "IANA Registry Updates for TLS
              and DTLS", RFC 9847, DOI 10.17487/RFC9847, December 2025,
              <https://www.rfc-editor.org/rfc/rfc9847>.

   [ZJZ24]    Zhou, B., Jiang, H., and Y. Zhao, "CPA-Secure KEMs are
              also Sufficient for Post-quantum TLS 1.3", Proceedings of
              Asiacrypt 2024 , n.d.,
              <https://doi.org/10.1007/978-981-96-0891-1_14>.

Acknowledgments

   Thanks to Douglas Stebila for consultation on the draft-ietf-tls-
   hybrid-design design, and to Scott Fluhrer, Eric Rescorla, John Preuß
   Mattsson, Martin Thomson, and Rebecca Guthrie for reviews.

Author's Address

   Deirdre Connolly
   SandboxAQ
   Email: durumcrustulum@gmail.com

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