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Post-quantum hybrid ECDHE-MLKEM Key Agreement for TLSv1.3
draft-kwiatkowski-tls-ecdhe-mlkem-02

Document Type Active Internet-Draft (individual)
Authors Kris Kwiatkowski , Panos Kampanakis , Bas Westerbaan , Douglas Stebila
Last updated 2024-09-09
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draft-kwiatkowski-tls-ecdhe-mlkem-02
Transport Layer Security                                  K. Kwiatkowski
Internet-Draft                                                  PQShield
Intended status: Informational                             P. Kampanakis
Expires: 14 March 2025                                               AWS
                                                        B. E. Westerbaan
                                                              Cloudflare
                                                              D. Stebila
                                                  University of Waterloo
                                                       10 September 2024

       Post-quantum hybrid ECDHE-MLKEM Key Agreement for TLSv1.3
                  draft-kwiatkowski-tls-ecdhe-mlkem-02

Abstract

   This draft defines two hybrid key agreements for TLS 1.3:
   X25519MLKEM768 and SecP256r1MLKEM768, 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://post-
   quantum-cryptography.github.io/draft-kwiatkowski-tls-ecdhe-mlkem/.
   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-kwiatkowski-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/post-quantum-cryptography/draft-kwiatkowski-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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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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 14 March 2025.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Negotiated Groups . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Construction  . . . . . . . . . . . . . . . . . . . . . .   3
       3.1.1.  Client share  . . . . . . . . . . . . . . . . . . . .   4
       3.1.2.  Server share  . . . . . . . . . . . . . . . . . . . .   4
       3.1.3.  Shared secret . . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  SecP256r1MLKEM768 . . . . . . . . . . . . . . . . . . . .   5
     5.2.  X25519MLKEM768  . . . . . . . . . . . . . . . . . . . . .   5
     5.3.  Obsoleted Supported Groups  . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Change log . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

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1.1.  Motivation

   ML-KEM is a key encapsulation method (KEM) defined in the [FIPS203].
   It is designed to withstand cryptanalytic attacks from quantum
   computers.

   This document introduces two new supported groups for hybrid post-
   quantum key agreements in TLS 1.3: X25519MLKEM768 and
   SecP256r1MLKEM768.  Both combine ML-KEM-768 with ECDH in the manner
   of [hybrid].

   The first one uses X25519 [rfc7748] and is an update to
   X25519Kyber768Draft00 [xyber], the most widely deployed PQ/T hybrid
   combiner for TLS v1.3 deployed in 2024.

   The second one uses secp256r1 (NIST P-256) [ECDSA] [DSS].  The goal
   of this group is to support a use case that requires both shared
   secrets to be generated by FIPS-approved mechanisms.

   Both constructions aim to provide a FIPS-approved key-establishment
   scheme (as per [SP56C]).

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

   Both groups enable the derivation of TLS session keys using FIPS-
   approved schemes.  NIST's special publication 800-56Cr2 [SP56C]
   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 ubiqutous for
   secp256r1 in the coming years.

   For this reason we put the ML-KEM-768 shared secret first in
   X25519MLKEM768, and the secp256r1 shared secret first in
   SecP256r1MLKEM768.

3.1.  Construction

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3.1.1.  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).

   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).

3.1.2.  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).

   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).

   For both groups, 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.

3.1.3.  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).

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4.  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.

5.  IANA Considerations

   This document requests/registers two 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 [FIPS203].

5.1.  SecP256r1MLKEM768

   Value:  4587 (0x11EB)

   Description:  SecP256r1MLKEM768

   DTLS-OK:  Y

   Recommended:  N

   Reference:  This document

   Comment:  Combining secp256r1 ECDH with ML-KEM-768

5.2.  X25519MLKEM768

   Value:  4588 (0x11EC)

   Description:  X25519MLKEM768

   DTLS-OK:  Y

   Recommended:  N

   Reference:  This document

   Comment:  Combining X25519 ECDH with ML-KEM-768

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5.3.  Obsoleted Supported Groups

   This document obsoletes 25497 and 25498 in the TLS Supported Groups
   registry.

6.  References

6.1.  Normative References

   [FIPS203]  "Module-Lattice-Based Key-Encapsulation Mechanism
              Standard", National Institute of Standards and Technology,
              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>.

   [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>.

   [SP56C]    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>.

6.2.  Informative References

   [DSS]      Chen, L., Moody, D., Regenscheid, A., Robinson, A., and K.
              Randall, "Recommendations for Discrete Logarithm-based
              Cryptography:: Elliptic Curve Domain Parameters", National
              Institute of Standards and Technology,
              DOI 10.6028/nist.sp.800-186, February 2023,
              <https://doi.org/10.6028/nist.sp.800-186>.

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   [ECDSA]    American National Standards Institute, "Public Key
              Cryptography for the Financial Services Industry: The
              Elliptic Curve Digital Signature Algorithm (ECDSA)",
              ANSI ANS X9.62-2005, November 2005.

   [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-10, 5 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              hybrid-design-10>.

   [tlsiana]  Salowey, J. A. and S. Turner, "IANA Registry Updates for
              TLS and DTLS", Work in Progress, Internet-Draft, draft-
              ietf-tls-rfc8447bis-09, 30 April 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              rfc8447bis-09>.

   [xyber]    Westerbaan, B. and D. Stebila, "X25519Kyber768Draft00
              hybrid post-quantum key agreement", Work in Progress,
              Internet-Draft, draft-tls-westerbaan-xyber768d00-03, 24
              September 2023, <https://datatracker.ietf.org/doc/html/
              draft-tls-westerbaan-xyber768d00-03>.

Appendix A.  Change log

   *  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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