ML-KEM Post-Quantum Key Agreement for TLS 1.3

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Table of Contents

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 4.2.7 of [RFC8446bis] and key_share Section 4.2.8 of [RFC8446bis] 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.¶

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]¶

    enum {

         ...,

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

         ...,

    } NamedGroup;

5. Security Considerations

This document defines standalone ML-KEM key establishment for TLS 1.3. Hybrid key establishment mechanisms, which support combining a post-quantum algorithm with a traditional algorithm such as ECDH, are supported generically via [HYBRID] with some concrete definitions in [ECDHE-MLKEM]. Hybrid mechanisms provide security as long as at least one of the component algorithms remains unbroken, such as combining quantum-resistant and traditional cryptographic assumptions. Standalone ML-KEM relies on lattice-based and hash function cryptographic assumptions for its security. Proponents of hybrid PQ/T key establishment generally consider it a conservative approach to deployment of newer post-quantum schemes alongside older traditional schemes, retaining at least the security currently offered by traditional algorithms.¶

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 encapsulation key / secret decapsulation key pair can be treated as a long-term key or reused in generic usage. ML-KEM satisfies IND-CCA security in the random oracle model [KYBERV] via a variant of the Fujisaki-Okamoto (FO) transform [FO][HHK]. Use of KEMs for key agreement in TLS 1.3 has been analyzed and discussed 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 and secure in case of reuse [GHS25] [RFC8446bis].¶

[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.¶

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].¶

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¶

Value:

0x0202¶

Description:

MLKEM1024¶

DTLS-OK:

Y¶

Recommended:

N¶

Reference:

This document¶

Comment:

FIPS 203 version of ML-KEM-1024¶

7. References

Acknowledgments

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

Author's Address