| Internet-Draft | hpke-mlkem | October 2024 |
| Connolly | Expires 17 April 2025 | [Page] |
This document defines Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) KEM options for Hybrid Public-Key Encryption (HPKE). ML-KEM is believed to be secure even against adversaries who possess a cryptographically-relevant quantum computer.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://dconnolly.github.io/draft-connolly-cfrg-hpke-mlkem/draft-connolly-cfrg-hpke-mlkem.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-connolly-cfrg-hpke-mlkem/.¶
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ML-KEM [FIPS203] is a Key-Encapsulation Mechanism (KEM) which is believed to be secure against both classical and quantum cryptographic attacks. For parties that must move to exclusively post-quantum algorithms, this document defines pure post-quantum algorithms for the Hybrid Public-Key Encryption (HPKE) protocol [RFC9180]. ML-KEM as a post-quantum IND-CCA2-secure KEM fits nicely into HPKE's design. Supporting multiple security levels for ML-KEM allows a spectrum of use cases including settings where the (United States) National Institute of Standards (NIST) security category 5 is required.¶
ML-KEM is a plain KEM that does not support the static-static key exchange that allows HPKE based on Diffie-Hellman (DH) based KEMs and their (optional) authenticated modes.¶
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.¶
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.¶
[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.¶
TODO: Describe the mapping between GenerateKeyPair, DeriveKeyPair,
SerializePublicKey, DeserializePublicKey, Encap, and Decap in HPKE
and the functions defined in [FIPS203].¶
HPKE-ML-KEM is not an authenticated KEM and does not support AuthEncap() or AuthDecap(), see Section 1.2.¶
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.¶
This document requests/registers two new entries to the "HPKE KEM Identifiers" registry.¶
0x0040 (please)¶
ML-KEM-512¶
32¶
768¶
800¶
1632¶
no¶
This document¶
0x0041 (please)¶
ML-KEM-768¶
32¶
1088¶
1184¶
2400¶
no¶
This document¶
0x0042 (please)¶
ML-KEM-1024¶
32¶
1568¶
1568¶
3168¶
no¶
This document¶
The authors would like to thank Cas Cremers for their input.¶
RFC Editor's Note: Please remove this section prior to publication of a final version of this document.¶
TODO¶
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].¶
TODO¶
TODO¶
TODO¶