Post-quantum hybrid ECDHE-MLKEM Key Agreement for TLSv1.3
draft-ietf-tls-ecdhe-mlkem-01

Transport Layer Security                                  K. Kwiatkowski
Internet-Draft                                                  PQShield
Intended status: Informational                             P. Kampanakis
Expires: 2 April 2026                                                AWS
                                                        B. E. Westerbaan
                                                              Cloudflare
                                                              D. Stebila
                                                  University of Waterloo
                                                       29 September 2025

       Post-quantum hybrid ECDHE-MLKEM Key Agreement for TLSv1.3
                     draft-ietf-tls-ecdhe-mlkem-01

Abstract

   This draft defines three hybrid key agreements for TLS 1.3:
   X25519MLKEM768, SecP256r1MLKEM768, and SecP384r1MLKEM1024 which
   combine a post-quantum KEM with an elliptic curve Diffie-Hellman
   (ECDHE).

About This Document

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

   The latest revision of this draft can be found at
   https://tlswg.github.io/draft-ietf-tls-ecdhe-mlkem/draft-ietf-tls-
   ecdhe-mlkem.html.  Status information for this document may be found
   at https://datatracker.ietf.org/doc/draft-ietf-tls-ecdhe-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-ecdhe-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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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Negotiated Groups . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Client share  . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Server share  . . . . . . . . . . . . . . . . . . . . . .   4
     3.3.  Shared secret . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  SecP256r1MLKEM768 . . . . . . . . . . . . . . . . . . . .   6
     6.2.  X25519MLKEM768  . . . . . . . . . . . . . . . . . . . . .   7
     6.3.  SecP384r1MLKEM1024  . . . . . . . . . . . . . . . . . . .   7
     6.4.  Obsoleted Supported Groups  . . . . . . . . . . . . . . .   7
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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1.  Introduction

   The [hybrid] document defines a framework for combining traditional
   key exchanges with next-generation key exchange in TLS 1.3.  The goal
   of this approach is to provide security against both classical and
   quantum adversaries while maintaining compatibility with existing
   infrastructure and protocols.

   This document applies the framework to ML-KEM, a key encapsulation
   mechanism defined in [NIST-FIPS-203], and specifies code points for
   the hybrid groups.

1.1.  Motivation

   This document introduces three new supported groups for hybrid post-
   quantum key agreements in TLS 1.3: the X25519MLKEM768,
   SecP256r1MLKEM768, and SecP384r1MLKEM1024 which combine ML-KEM with
   ECDH in the manner of [hybrid].

   *  The first one uses X25519 [rfc7748], is widely deployed, and often
      serves as the most practical choice for a single PQ/T hybrid
      combiner in TLS 1.3.

   *  The second group uses secp256r1 (NIST P-256).  This group supports
      use cases that require both shared secrets to be generated by
      FIPS-approved mechanisms.

   *  The third group uses secp384r1 (NIST P-384).  This group is
      intended for high-security environments that require FIPS-approved
      mechanisms with an increased security margin.

   Key establishment using NIST curves is outlined in Section 6.1.1.2 of
   [KEYAGREEMENT].

1.2.  Terminology

   The [hybrid] document defines "traditional" algorithms as those that
   are already widely adopted and "next-generation" algorithms as those
   that are not yet widely adopted, such as post-quantum algorithms.  In
   this document, ECDH using Curve25519, P-256, or P-384 is considered
   traditional, while ML-KEM is considered next-generation.

   The [hybrid] document defines a "hybrid" key exchange as one that
   combines a traditional key exchange with a next-generation key
   exchange.  This document uses the term "hybrid" in the same way.

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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.  Negotiated Groups

3.1.  Client share

   When the X25519MLKEM768 group is negotiated, the client's
   key_exchange value is the concatenation of the client's ML-KEM-768
   encapsulation key and the client's X25519 ephemeral share.  The size
   of the client share is 1216 bytes (1184 bytes for the ML-KEM part and
   32 bytes for X25519).

   Note: The group name X25519MLKEM768 does not adhere to the naming
   convention outlined in Section 3.2 of [hybrid].  Specifically, the
   order of shares in the concatenation has been reversed.  This is due
   to historical reasons.

   When the SecP256r1MLKEM768 group is negotiated, the client's
   key_exchange value is the concatenation of the secp256r1 ephemeral
   share and ML-KEM-768 encapsulation key.  The ECDHE share is the
   serialized value of the uncompressed ECDH point representation as
   defined in Section 4.2.8.2 of [RFC8446].  The size of the client
   share is 1249 bytes (65 bytes for the secp256r1 part and 1184 bytes
   for ML-KEM).

   When the SecP384r1MLKEM1024 group is negotiated, the client's
   key_exchange value is the concatenation of the secp384r1 ephemeral
   share and the ML-KEM-1024 encapsulation key.  The ECDH share is the
   serialized value of the uncompressed ECDH point representation as
   defined in Section 4.2.8.2 of [RFC8446].  The size of the client
   share is 1665 bytes (97 bytes for the secp384r1 and the 1568 for the
   ML-KEM).

3.2.  Server share

   When the X25519MLKEM768 group is negotiated, the server's key
   exchange value is the concatenation of an ML-KEM ciphertext returned
   from encapsulation to the client's encapsulation key, and the
   server's ephemeral X25519 share.  The size of the server share is
   1120 bytes (1088 bytes for the ML-KEM part and 32 bytes for X25519).

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   When the SecP256r1MLKEM768 group is negotiated, the server's key
   exchange value is the concatenation of the server's ephemeral
   secp256r1 share encoded in the same way as the client share and an
   ML-KEM ciphertext returned from encapsulation to the client's
   encapsulation key.  The size of the server share is 1153 bytes (1088
   bytes for the ML-KEM part and 65 bytes for secp256r1).

   When the SecP384r1MLKEM1024 group is negotiated, the server's key
   exchange value is the concatenation of the server's ephemeral
   secp384r1 share encoded in the same way as the client share and an
   ML-KEM ciphertext returned from encapsulation to the client's
   encapsulation key.  The size of the server share is 1665 bytes (1568
   bytes for the ML-KEM part and 97 bytes for secp384r1)

   For all groups, the server MUST perform the encapsulation key check
   described in Section 7.2 of [NIST-FIPS-203] on the client's
   encapsulation key, and abort with an illegal_parameter alert if it
   fails.

   For all groups, the client MUST check if the ciphertext length
   matches the selected group, 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.

   For all groups, both client and server MUST process the ECDH part as
   described in Section 4.2.8.2 of [RFC8446], including all validity
   checks, and abort with an illegal_parameter alert if it fails.

3.3.  Shared secret

   For X25519MLKEM768, the shared secret is the concatenation of the ML-
   KEM shared secret and the X25519 shared secret.  The shared secret is
   64 bytes (32 bytes for each part).

   For SecP256r1MLKEM768, the shared secret is the concatenation of the
   ECDHE and ML-KEM shared secret.  The ECDHE shared secret is the
   x-coordinate of the ECDH shared secret elliptic curve point
   represented as an octet string as defined in Section 7.4.2 of
   [RFC8446].  The size of the shared secret is 64 bytes (32 bytes for
   each part).

   For SecP384r1MLKEM1024, the shared secret is the concatenation of the
   ECDHE and ML-KEM shared secret.  The ECDHE shared secret is the
   x-coordinate of the ECDH shared secret elliptic curve point
   represented as an octet string as defined in Section 7.4.2 of
   [RFC8446].  The size of the shared secret is 80 bytes (48 bytes for
   the ECDH part and 32 bytes for the ML-KEM part).

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   For all groups, both client and server MUST calculate the ECDH part
   of the shared secret as described in Section 7.4.2 of [RFC8446],
   including the all-zero shared secret check for X25519, and abort the
   connection with an illegal_parameter alert if it fails.

4.  Discussion

   *FIPS-compliance*. All groups defined in this document permit FIPS-
   approved key derivation as per [NIST-SP-800-56C] and
   [NIST-SP-800-135].  NIST's special publication 800-56Cr2
   [NIST-SP-800-56C] approves the usage of HKDF [HKDF] with two distinct
   shared secrets, with the condition that the first one is computed by
   a FIPS-approved key-establishment scheme.  FIPS also requires a
   certified implementation of the scheme, which will remain more
   ubiquitous for secp256r1 in the coming years.  For this reason we put
   the ML-KEM shared secret first in X25519MLKEM768, and the ECDH shared
   secret first in SecP256r1MLKEM768 and SecP384r1MLKEM1024.  This means
   that for SecP256r1MLKEM768 and SecP384r1MLKEM1024, the ECDH
   implementation must be certified whereas the ML-KEM implementation
   does not require certification.  In contrast, for X25519MLKEM768, the
   ML-KEM implementation must be certified.

5.  Security Considerations

   The same security considerations as those described in [hybrid] apply
   to the approach used by this document.  The security analysis relies
   crucially on the TLS 1.3 message transcript, and one cannot assume a
   similar hybridisation is secure in other protocols.

   Implementers are encouraged to use implementations resistant to side-
   channel attacks, especially those that can be applied by remote
   attackers.

   All groups defined in this document use and generate fixed-length
   public keys, ciphertexts, and shared secrets, which complies with the
   requirements described in Section 6 of [hybrid].

6.  IANA Considerations

   This document requests/registers three new entries to the TLS
   Supported Groups registry, according to the procedures in Section 6
   of [tlsiana].  These identifiers are to be used with the final,
   ratified by NIST, version of ML-KEM which is specified in
   [NIST-FIPS-203].

6.1.  SecP256r1MLKEM768

   Value:  4587 (0x11EB)

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   Description:  SecP256r1MLKEM768
   DTLS-OK:  Y
   Recommended:  N
   Reference:  This document
   Comment:  Combining secp256r1 ECDH with ML-KEM-768

6.2.  X25519MLKEM768

   Value:  4588 (0x11EC)
   Description:  X25519MLKEM768
   DTLS-OK:  Y
   Recommended:  N
   Reference:  This document
   Comment:  Combining X25519 ECDH with ML-KEM-768

6.3.  SecP384r1MLKEM1024

   Value:  4589 (0x11ED)
   Description:  SecP384r1MLKEM1024
   DTLS-OK:  Y
   Recommended:  N
   Reference:  This document
   Comment:  Combining secp384r1 ECDH with ML-KEM-1024

6.4.  Obsoleted Supported Groups

   This document obsoletes X25519Kyber768Draft00 (25497) and
   SecP256r1Kyber768Draft00 (25498) in the TLS Supported Groups
   registry.

7.  References

7.1.  Normative References

   [KEYAGREEMENT]
              Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for pair-wise key-establishment
              schemes using discrete logarithm cryptography", National
              Institute of Standards and Technology,
              DOI 10.6028/nist.sp.800-56ar3, April 2018,
              <https://doi.org/10.6028/nist.sp.800-56ar3>.

   [NIST-FIPS-203]
              "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>.

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   [NIST-SP-800-135]
              Dang, Q., "Recommendation for existing application-
              specific key derivation functions", National Institute of
              Standards and Technology, DOI 10.6028/nist.sp.800-135r1,
              2011, <https://doi.org/10.6028/nist.sp.800-135r1>.

   [NIST-SP-800-56C]
              Barker, E., Chen, L., and R. Davis, "Recommendation for
              Key-Derivation Methods in Key-Establishment Schemes",
              National Institute of Standards and Technology,
              DOI 10.6028/nist.sp.800-56cr2, August 2020,
              <https://doi.org/10.6028/nist.sp.800-56cr2>.

   [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>.

   [rfc7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/rfc/rfc7748>.

   [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

   [HKDF]     Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC Editor,
              DOI 10.17487/rfc5869, May 2010,
              <https://doi.org/10.17487/rfc5869>.

   [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>.

   [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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Appendix A.  Change log

   *  draft-ietf-tls-ecdhe-mlkem-01:

      -  Alignment with the final version of [hybrid]

      -  Added new section called Discussion and moved FIPS-compliance
         and Failures text there.

      -  The Construction section has been removed.

   *  draft-ietf-tls-ecdhe-mlkem-00:

      -  Change a name of the draft, following adoption by TLS WG

      -  Fixes references to the to NIST ECC CDH

   *  draft-kwiatkowski-tls-ecdhe-mlkem-03:

      -  Adds P-384 combined with ML-KEM-1024

      -  Adds text that describes error-handling and outlines how the
         client and server must ensure the integrity of the key exchange
         process.

      -  Adds note on the incompatibility of the codepoint name
         X25519MLKEM768 with [hybrid].

      -  Various cosmetic changes.

   *  draft-kwiatkowski-tls-ecdhe-mlkem-02:

      -  Adds section that mentions supported groups that this document
         obsoletes.

      -  Fix a reference to encapsulation in the FIPS 203.

   *  draft-kwiatkowski-tls-ecdhe-mlkem-01:

      -  Add X25519MLKEM768

   *  draft-kwiatkowski-tls-ecdhe-mlkem-00:

      -  Change Kyber name to ML-KEM

      -  Swap reference to I-D.cfrg-schwabe-kyber with FIPS-203

      -  Change codepoint.  New value is equal to old value + 1.

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   *  draft-kwiatkowski-tls-ecdhe-kyber-01: Fix size of key shares
      generated by the client and the server

   *  draft-kwiatkowski-tls-ecdhe-kyber-00: updates following IANA
      review

Authors' Addresses

   Kris Kwiatkowski
   PQShield
   Email: kris@amongbytes.com

   Panos Kampanakis
   AWS
   Email: kpanos@amazon.com

   Bas Westerbaan
   Cloudflare
   Email: bas@cloudflare.com

   Douglas Stebila
   University of Waterloo
   Email: dstebila@waterloo.ca

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