ML-KEM Post-Quantum Key Agreement for TLS 1.3
draft-ietf-tls-mlkem-03

Transport Layer Security                                     D. Connolly
Internet-Draft                                                 SandboxAQ
Intended status: Informational                              22 July 2025
Expires: 23 January 2026

             ML-KEM Post-Quantum Key Agreement for TLS 1.3
                        draft-ietf-tls-mlkem-03

Abstract

   This memo defines ML-KEM-512, ML-KEM-768, and ML-KEM-1024 as a
   standalone NamedGroups for use in TLS 1.3 to achieve post-quantum key
   agreement.

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
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   This Internet-Draft will expire on 23 January 2026.

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Copyright Notice

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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.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     6.1.  IND-CCA . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.2.  Binding properties  . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

1.1.  Motivation

   FIPS 203 (ML-KEM) is a new FIPS standard for post-quantum key
   agreement via lattice-based key establishment mechanism (KEM).
   Having a purely post-quantum (not hybrid) key agreement option for
   TLS 1.3 is necessary for migrating beyond hybrids and for users that
   need to be fully post-quantum.

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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 agreement 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 API:

   *  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

   We define the KEMs as NamedGroups, sent in the supported_groups
   extension.

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4.1.  Negotiation

   Each method is its own solely post-quantum key agreement method,
   which are assigned their own identifiers, registered by IANA in the
   TLS Supported Groups registry:

       enum {

            ...,

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

            ...,

       } NamedGroup;

4.2.  Transmitting encapsulation keys and ciphertexts

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

   In TLS 1.3 a KEM encapsulation key or KEM ciphertext is represented
   as a KeyShareEntry:

       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 client's shares are listed in descending order of client
   preference; the server selects one algorithm and sends its
   corresponding share.

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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, 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 agreement

5.  Discussion

6.  Security Considerations

6.1.  IND-CCA

   The main security property for KEMs is indistinguishability under
   adaptive chosen ciphertext attack (IND-CCA2), which means that shared
   secret values should be indistinguishable from random strings even
   given the ability to have other 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.

   TLS 1.3 does not require that ephemeral public keys be used only in a
   single key exchange session; some implementations may reuse them, at
   the cost of limited forward secrecy.  As a result, any KEM used in

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   the manner described in this document MUST explicitly be designed to
   be secure in the event that the public key is reused.  Finite-field
   and elliptic-curve Diffie-Hellman key exchange methods used in TLS
   1.3 satisfy this criteria.  For generic KEMs, this means satisfying
   IND-CCA2 security or having a transform like the Fujisaki-Okamoto
   transform [FO] [HHK] applied.  ML-KEM satisfies this requirement
   [FIPS203].

   Along with other TLS 1.3 key agreement methods, it is recommended
   that implementations avoid reuse of ML-KEM keys.  Implementations
   MUST NOT reuse randomness in the generation of ML-KEM ciphertexts.

6.2.  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 agreement.

7.  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

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   Comment:  FIPS 203 version of ML-KEM-768

   Value:  0x0202

   Description:  MLKEM1024

   DTLS-OK:  Y

   Recommended:  N

   Reference:  This document

   Comment:  FIPS 203 version of ML-KEM-1024

8.  References

8.1.  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>.

8.2.  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-14, 21 July 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              hybrid-design-14>.

   [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/>.

   [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>.

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Acknowledgments

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

Author's Address

   Deirdre Connolly
   SandboxAQ
   Email: durumcrustulum@gmail.com

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