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 4.3.7 of [RFC9846] and key_share Section 4.3.8 of [RFC9846] 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 4.3.7 of [RFC9846]¶

    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. Use of KEMs for key agreement in TLS 1.3 has been analyzed 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 [GHS25] [RFC9846]. Multiple formal analyses, including pen-and-paper computational proofs and machine-checked symbolic analysis using ProVerif [KOBEISSI26], demonstrate that replacing Diffie-Hellman with an IND-CCA-secure KEM preserves the security properties of the TLS handshake. Formal analysis has also shown that hybrid key establishment (e.g., [HYBRID], [ECDHE-MLKEM]) provides compositional security: the exchange remains secure as long as at least one of the component algorithms is unbroken [BJ24] [CPWB25].¶

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].¶

[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.¶

Implementers must evaluate their specific security, performance, and operational constraints when deciding whether to deploy standalone ML-KEM or a hybrid construction. The recommended column in the IANA TLS Supported Groups registry contains the IETF's current guidance on the recommended use of these algorithms for general purposes.¶

The disclosure of the output(s) of an insecure random number generator (RNG) when used in TLS and other protocols can be used in an attack to compromise the state of the insecure RNG itself as described in [DUALEC-TLS]. The encapsulation randomness in ML-KEM is an additional place where raw RNG output may be disclosed, therefore it is important to follow the RNG guidance in [FIPS203] and [RFC9846]. Implementers can choose to implement mechanisms from [RFC8937] for additional protection across sessions.¶

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 [RFC9847].¶

As defined in Section 3 of [RFC9847], the value N indicates:¶

• That the item has not been evaluated by the IETF and that the IETF has made no statement about the suitability of the associated mechanism. This does not necessarily mean that the mechanism is flawed, only that no consensus exists. The IETF might have consensus to leave an item marked as "N" on the basis of the item having limited applicability or usage constraints.¶

7. References

Acknowledgments

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

Author's Address