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.¶

The following terms are used throughout this document to describe the operations, roles, and behaviors of HPKE:¶

• concat(x0, ..., xN): returns the concatenation of byte strings. concat(0x01, 0x0203, 0x040506) = 0x010203040506.¶

• random(n): return a pseudorandom byte string of length n bytes produced by a cryptographically-secure random number generator, specifically MUST be a FIPS-approved random bit generator (RBG) as described in section 3.3 of [FIPS203].¶

GenerateKeyPair, DeriveKeyPair, SerializePublicKey, DeserializePublicKey, Encap, Decap, AuthEncap, AuthDecap, Nsecret, Nenc, Npk, and Nsk are defined in Section 4 of [RFC9180].¶

When used in the Security Consideration section, PK refers to public key and CT refers to ciphertext as modeled in [CDM23].¶

TODO: explain or reference IND-CCA, IND-CCA2, MAL-BIND-K-PK, MAL-BIND-K-CT, and LEAK-BIND-K-PK.¶

3. Usage

[FIPS203] supports two different key formats - this document only supports the 64-byte seed (d, z). This format supports stronger binding properties for ML-KEM than the expanded format. The 64-byte seed format protects against re-encapsulation attacks. This format provides properties closer to the generic DHKEM binding properties defined in Section 4.1 of [RFC9180].¶

The encapsulation and decapsulation keys are computed, serialized, and deserialized as described in [FIPS203] where the decapsulation keys MUST be in the 64-byte (d, z) format. The 'expanded' format where the decapsulation key is expanded into a variable size based on the parameter set but includes the hash of the encapsulation key is not used.¶

def GenerateKeyPair():
    d = random(32) # `random(n)` MUST comply with {{FIPS203}}'s RBG requirements
    z = random(32) # `random(n)` MUST comply with {{FIPS203}}'s RBG requirements
    (ek, _) = ML-KEM.KeyGen_internal(d, z)
    return (concat(d, z), ek)

def Decap(enc, skR):
    (sk, _) = DeriveKeyPair(skR) # expand decapsulation key from 64-byte format
    return ML-KEM.Decaps(sk, enc)

4. Security Considerations

HPKE's IND-CCA2 security relies upon the IND-CCA and IND-CCA2 security of the underlying KEM and AEAD schemes, respectively. 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 and public key of the ciphersuite KEM, and dependent on the shared secret produced by the KEM. If HPKE had committed to the encapsulated shared secret ciphertext and public key, we wouldn't have to worry about the binding properties of the ciphersuite KEM's X-BIND-K-CT and X-BIND-K-PK properties. These computational binding properties for KEMs were formalized in [CDM23]. [CDM23] describes DHKEM as MAL-BIND-K-PK and MAL-BIND-K-CT secure as a result of the inclusion of the serialized DH public keys (the KEM's PK and CT) in the DHKEM Key Derivation Function (KDF). MAL-BIND-K-PK and MAL-BIND-K-CT security ensures that the shared secret K 'binds' (is uniquely determined by) the encapsulation key and/or the ciphertext, even when the adversary is able to create or modify the key pairs or has access to honestly-generated leaked key material.¶

ML-KEM as specified in [FIPS203] with the seed key format provides MAL-BIND-K-CT security and LEAK-BIND-K-PK security [KEMMY24]. LEAK-BIND-K-PK security is resiliant where the involved key pairs are output by the honest key generation algorithm of the KEM and then leaked to the adversary. MAL-BIND-K-CT security strongly binds the shared secret and the ciphertext even when an adversary can manipulate key material like the decapsulation key.¶

ML-KEM using the seed key format (providing MAL-BIND-K-CT and LEAK-BIND-K-PK) nearly matches the binding properties of DHKEM (the default HPKE KEM construction). The ML-KEM ciphertext is strongly bound by the shared secret. The encapsulation key is more weakly bound, and protocols integrating HPKE using ML-KEM even with the seed key format should evaluate whether they need to strongly bind to the PK elsewhere (outside of ML-KEM or HPKE) to be resilient against a MAL adversary, or to achieve other tight binding to the encapsulation key PK to achieve properties like implicit authentication or session independence.¶

5. IANA Considerations

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

Value:

0x0040 (please)¶

KEM:

ML-KEM-512¶

Nsecret:

32¶

Nenc:

768¶

Npk:

800¶

Nsk:

64¶

Auth:

no¶

Reference:

This document¶

Value:

0x0041 (please)¶

KEM:

ML-KEM-768¶

Nsecret:

32¶

Nenc:

1088¶

Npk:

1184¶

Nsk:

64¶

Auth:

no¶

Reference:

This document¶

Value:

0x0042 (please)¶

KEM:

ML-KEM-1024¶

Nsecret:

32¶

Nenc:

1568¶

Npk:

1568¶

Nsk:

64¶

Auth:

no¶

Reference:

This document¶

6. References

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].¶

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