ML-KEM for HPKE

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Table of Contents

1. Introduction

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. Construction

We construct 'wrapper' KEMs based on ML-KEM to bind the encapsulated shared secret ciphertext into the shared secret value, such that the final KEM has similar binding security properties as the original DHKEM HPKE was designed around.¶

The encapsulation and decapsulation keys are computed, serialized, and deserialized the same as in [FIPS203].¶

We use HKDF-SHA256 and HKDF-SHA512 as the HPKE KDFs and AES-128-GCM and AES-256-GCM as the AEADs for ML-KEM-768 and ML-KEM-1024 respectively.¶

4. Security Considerations

HPKE's IND-CCA2 security relies upon the IND-CCA2 security of the underlying KEM and AEAD schemes. ML-KEM is believed to be IND-CCA secure via multiple analyses.¶

The HPKE key schedule is independent of the encapsulated KEM shared secret ciphertext of the ciphersuite KEM, and dependent on the shared secret produced by the KEM. If HPKE had committed to the encapsulated shared secret ciphertext, we wouldn't have to worry about the binding properties of the ciphersuite KEM's X-BIND-K-CT properties. These computational binding properties for KEMs were formalized in [CDM23].¶

ML-KEM, unlike DHKEM, is also an implicitly-rejecting instantiation of the Fujisaki-Okamoto transform, meaning the ML-KEM output shared secret may be computed differently in case of decryption failure, that reults in different binding properties, such as the lack of X-BIND-CT-PK and X-BIND-CT-K completely.¶

The DHKEM construction in HPKE provides MAL-BIND-K-PK and MAL-BIND-K-CT security (the shared secret 'binds' or uniquely determines the encapsulation key and the encapsualted shared secret ciphertext), where the adversary is able to create the key pairs any way they like in addition to the key generation. ML-KEM as specified provides LEAK-BIND-PK,K-CT security, where the involved key pairs are output by the key generation algorithm of the KEM and then leaked to the adversary. LEAK-BIND-PK,K-CT is a weaker property than the DHKEM properties as it is not resistant in the presence of an actively malicious adversary, and requires both the shared secret _and_the public key together to uniquely bind the ciphertext, so its shared secret alone is insufficient.¶

This results in a wrapper construction around ML-KEM to bind to the encapsulated shared secret ciphertext as the kem_context provided to ExtractAndExpand(). This binds the final shared_secret (K) to the encapsulated shared secret ciphertext (CT), achieving MAL-BIND-K-CT. ML-KEM already is MAL-BIND-K-PK as the hash of the encapsulation key (PK) is an input the computation of the shared secret (K). Together this wrapper KEM matches the binding properties of HPKE's default KEM construction DHKEM in being MAL-BIND-K-CT and MAL-BIND-K-PK.¶

5. IANA Considerations

This document requests/registers two new entries to the "HPKE KEM Identifiers" registry.¶

Value:

0x0070 (please)¶

KEM:

ML-KEM-768¶

Nsecret:

32¶

Nenc:

1088¶

Npk:

1184¶

Nsk:

2400¶

Auth:

no¶

Reference:

This document¶

Value:

0x0080 (please)¶

KEM:

ML-KEM-1024¶

Nsecret:

32¶

Nenc:

1568¶

Npk:

1568¶

Nsk:

3168¶

Auth:

no¶

Reference:

This document¶

Appendix A. Acknowledgments

The authors would like to thank Cas Cremers for their input.¶

Appendix B. Change log

• RFC Editor's Note: Please remove this section prior to publication of a final version of this document.¶

TODO¶

Appendix C. Test Vectors

This section contains test vectors formatted similary to that which are found in [RFC9180], with two changes. First, we only provide vectors for the non-authenticated modes of operation. Secondly, as ML-KEM encapsulation does not involve an ephemeral keypair, we omit the ikmE, skEm, pkEm entries and provide an ier entry instead. The value of ier is the randomness used to encapsulate, so ier[0:32] is the seed that is fed to H in the first step of ML-KEM encapsulation in [FIPS203].¶

Author's Address