



Transport Layer Security                                     D. Connolly
Internet-Draft                                                 SandboxAQ
Intended status: Informational                           3 November 2025
Expires: 7 May 2026


             ML-KEM Post-Quantum Key Agreement for TLS 1.3
                        draft-ietf-tls-mlkem-05

Abstract

   This memo defines ML-KEM-512, ML-KEM-768, and ML-KEM-1024 as
   NamedGroups and 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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 7 May 2026.




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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Key encapsulation mechanisms  . . . . . . . . . . . . . . . .   3
   4.  Construction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     4.1.  Negotiation . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Transmitting encapsulation keys and ciphertexts . . . . .   4
     4.3.  Shared secret calculation . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     5.1.  IND-CCA . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Binding properties  . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

1.1.  Motivation

   FIPS 203 (ML-KEM) [FIPS203] is a FIPS standard for post-quantum
   [RFC9794] key establishment via lattice-based key establishment
   mechanism (KEM).  Having a purely post-quantum (not hybrid) key
   establishment option for TLS 1.3 is necessary for migrating beyond
   hybrids and for users that want or need post-quantum security without
   hybrids.







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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.

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 4.2.7 of [RFC8446]





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4.1.  Negotiation

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

       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 Section 4.2.8 of [RFC8446]:

       struct {
           NamedGroup group;
           opaque key_exchange<1..2^16-1>;
       } KeyShareEntry;

   These are transmitted in the extension_data fields of
   KeyShareClientHello and KeyShareServerHello extensions:

       struct {
           KeyShareEntry client_shares<0..2^16-1>;
       } KeyShareClientHello;

       struct {
           KeyShareEntry server_share;
       } KeyShareServerHello;

   The KeyShareClientHello includes a list of KeyShareEntry structs that
   represent the key establishment algorithms the client supports.  For
   each parameter of ML-KEM the client supports, the corresponding
   KeyShareEntry consists of a NamedGroup that indicates the appropriate
   parameter, and a key_exchange value that is the pk output of the
   KeyGen algorithm.



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   For the client's share, the key_exchange value contains the pk output
   of the corresponding KEM NamedGroup's KeyGen algorithm.

   For the server's share, the key_exchange value contains the ct output
   of the corresponding KEM NamedGroup'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.

4.3.  Shared secret calculation

   The 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 honest peer, which is inserted
   into the TLS 1.3 key schedule in place of the (EC)DHE shared secret,
   as shown in Figure 1.


























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                                       0
                                       |
                                       v
                         PSK ->  HKDF-Extract = Early Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
                shared_secret -> HKDF-Extract = Handshake Secret
                ^^^^^^^^^^^^^          |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       |
                                       v
                                 Derive-Secret(., "derived", "")
                                       |
                                       v
                            0 -> HKDF-Extract = Master Secret
                                       |
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)
                                       +-----> Derive-Secret(...)

                Figure 1: Key schedule for key establishment

5.  Security Considerations

5.1.  IND-CCA

   The main security property for KEMs is indistinguishability under
   adaptive chosen ciphertext attack (IND-CCA), which means that shared
   secret values should be indistinguishable from random strings even
   given the ability to have other arbitrary ciphertexts decapsulated.
   IND-CCA corresponds to security against an active attacker, and the
   public key / secret key pair can be treated as a long-term key or
   reused.  ML-KEM satisfies IND-CCA security in the random oracle model
   [KYBERV].

   TLS 1.3 does not prohibit key re-use; some implementations may use
   the same ephemeral public key for more than one key establishment at
   the cost of limited forward secrecy.  Care must be taken to ensure
   that keys are only re-used if the algorithms from which they are



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   derived are designed to be secure under key-reuse.  ML-KEM's IND-CCA
   security satisfies this requirement such that the public key/secret
   key pair can be used long-term or re-used without compromising the
   security of the keys.  However, it is still recommended that
   implementations avoid re-use of any keys (including ML-KEM keys) to
   ensure perfect forward secrecy.

   Implementations MUST NOT reuse randomness in the generation of ML-KEM
   ciphertexts.

5.2.  Binding properties

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

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 [tlsiana].

   Value:  0x0200

   Description:  MLKEM512

   DTLS-OK:  Y

   Recommended:  N

   Reference:  This document

   Comment:  FIPS 203 version of ML-KEM-512

   Value:  0x0201

   Description:  MLKEM768

   DTLS-OK:  Y

   Recommended:  N

   Reference:  This document

   Comment:  FIPS 203 version of ML-KEM-768




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   Value:  0x0202

   Description:  MLKEM1024

   DTLS-OK:  Y

   Recommended:  N

   Reference:  This document

   Comment:  FIPS 203 version of ML-KEM-1024

7.  References

7.1.  Normative References

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

7.2.  Informative References

   [AVIRAM]   Nimrod Aviram, Benjamin Dowling, Ilan Komargodski, Kenny
              Paterson, Eyal Ronen, and Eylon Yogev, "[TLS] Combining
              Secrets in Hybrid Key Exchange in TLS 1.3", 1 September
              2021, <https://mailarchive.ietf.org/arch/msg/tls/
              F4SVeL2xbGPaPB2GW_GkBbD_a5M/>.

   [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>.





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   [DOWLING]  Dowling, B., Fischlin, M., Günther, F., and D. Stebila, "A
              Cryptographic Analysis of the TLS 1.3 Handshake Protocol",
              Springer Science and Business Media LLC, Journal of
              Cryptology vol. 34, no. 4, DOI 10.1007/s00145-021-09384-1,
              July 2021, <https://doi.org/10.1007/s00145-021-09384-1>.

   [FO]       Fujisaki, E. and T. Okamoto, "Secure Integration of
              Asymmetric and Symmetric Encryption Schemes", Springer
              Science and Business Media LLC, Journal of Cryptology vol.
              26, no. 1, pp. 80-101, DOI 10.1007/s00145-011-9114-1,
              December 2011,
              <https://doi.org/10.1007/s00145-011-9114-1>.

   [HHK]      Hofheinz, D., Hövelmanns, K., and E. Kiltz, "A Modular
              Analysis of the Fujisaki-Okamoto Transformation", Springer
              International Publishing, Lecture Notes in Computer
              Science pp. 341-371, DOI 10.1007/978-3-319-70500-2_12,
              ISBN ["9783319704999", "9783319705002"], 2017,
              <https://doi.org/10.1007/978-3-319-70500-2_12>.

   [HPKE]     Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.

   [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>.

   [KYBERV]   "Formally verifying Kyber Episode V: Machine-checked IND-
              CCA security and correctness of ML-KEM in EasyCrypt",
              n.d., <https://eprint.iacr.org/2024/843.pdf>.

   [LUCKY13]  Al Fardan, N. J. and K. G. Paterson, "Lucky Thirteen:
              Breaking the TLS and DTLS record protocols", n.d.,
              <https://ieeexplore.ieee.org/
              iel7/6547086/6547088/06547131.pdf>.

   [RACCOON]  Merget, R., Brinkmann, M., Aviram, N., Somorovsky, J.,
              Mittmann, J., and J. Schwenk, "Raccoon Attack: Finding and
              Exploiting Most-Significant-Bit-Oracles in TLS-DH(E)",
              September 2020, <https://raccoon-attack.com/>.

   [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>.



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   [tlsiana]  Salowey, J. A. and S. Turner, "IANA Registry Updates for
              TLS and DTLS", Work in Progress, Internet-Draft, draft-
              ietf-tls-rfc8447bis-15, 21 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              rfc8447bis-15>.

Acknowledgments

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

Author's Address

   Deirdre Connolly
   SandboxAQ
   Email: durumcrustulum@gmail.com


































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