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Security Limited Additional Mechanisms for PKIX and SMIME mldsa milked expanded deprecate This document updates RFC 9881 and RFC 9935 to require the seed representation when encoding Module-Lattice-Based Digital Signature Algorithm (ML-DSA) and Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) private keys for interchange. It deprecates the expanded and combined representations, while permitting their continued support for legacy imports. These changes simplify implementation and improve interoperability by aligning the private-key representation with that used by JSON Object Signing and Encryption (JOSE) and CBOR Object Signing and Encryption (COSE). Internal private-key representations are unaffected. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction and define, respectively, three representations for Module-Lattice-Based Digital Signature Algorithm (ML-DSA) Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) private keys: a 32-octet seed, an expanded private key, and a combination of the seed and expanded private key. The seed deterministically yields both the expanded private key and the public key. Although and recommend the seed representation, they permit all three representations for private-key interchange. Multiple representations complicate interoperability between implementations that support different subsets of these formats. An implementation that supports only seeds cannot import an expanded private key without its seed. The combined representation also introduces a consistency check between the seed and expanded private key, which can require additional implementation support solely for importing that representation. and specify only the seed representation for ML-DSA and ML-KEM private keys in JSON Object Signing and Encryption (JOSE) and CBOR Object Signing and Encryption (COSE). Using the seed representation consistently across these specifications simplifies private-key interchange and avoids the need to support multiple representations and their associated validation logic. This document updates and to deprecate the expanded and combined representations, limiting them to legacy compatibility where applicable.

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 when, and only when, they appear in all capitals, as shown here.

Updates to ML-DSA Private Key Encoding This section updates the private-key encoding requirements and format recommendations in , , and . These updates apply to ML-DSA-44, ML-DSA-65, ML-DSA-87, ML-KEM-512, ML-KEM-768, and ML-KEM-1024. Implementations that decode ML-DSA or ML-KEM private keys for interchange using the structures defined in and MUST support their respective seed representations. Implementations MAY support the both and expandedKey representations for compatibility with legacy systems, but this is optional and not required for conformance with this specification. If decoding the both representation, implementations MUST perform the consistency check described in and . When decoding a private key in either the seed or both representation, and when generating a new ML-DSA or ML-KEM key, implementations MUST retain the seed, unless the private key will never be encoded for interchange. Implementations that encode ML-DSA or ML-KEM private keys for interchange MUST use the seed representation, unless the key is a legacy key whose seed has been discarded. If (and only if) the seed was discarded, implementations MAY use the expandedKey representation. However, this encoding will not be interoperable with all implementations of this specification. Implementations MUST NOT output the both representation.

Compatibility and Migration Considerations Existing private keys can be re-encoded in the seed representation when the original seed is available, including from the both representation. This conversion does not require replacing the key pair or reissuing certificates. Expanded private keys can be computed from seeds, so the seed representation can be imported into a legacy implementation that expects expanded private keys. When doing so, operators SHOULD ensure that seeds are retained for keys that may need to be exported or restored into another implementation. As noted in and , a discarded seed cannot be recovered from the expanded private key. If no copy of the seed is available, migration to an implementation that accepts only seeds requires generating a new key pair and replacing any associated certificates or other public-key bindings. However, such existing keys MAY continue to be used by implementations that support this representation. This document still optionally permits the expandedKey encoding for such cases. It also places no constraints on keys that are never exported for interchange.

Security Considerations The security considerations of and continue to apply. The seed is sufficient to reconstruct the private key, so it requires the same protection as the expanded private key, including when retained in backups or transferred between implementations.

IANA Considerations This document has no IANA actions.

References Normative References Digital signatures are used within X.509 certificates and Certificate Revocation Lists (CRLs), and to sign messages. This document specifies the conventions for using FIPS 204, the Module-Lattice-Based Digital Signature Algorithm (ML-DSA) in Internet X.509 certificates and CRLs. The conventions for the associated signatures, subject public keys, and private key are also described. The Module-Lattice-Based Key-Encapsulation Mechanism (ML-KEM) is a quantum-resistant Key Encapsulation Mechanism. This document specifies the conventions for using the ML-KEM in X.509 Public Key Infrastructure. The conventions for the subject public keys and private keys are also specified. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References This document specifies JSON Object Signing and Encryption (JOSE) and CBOR Object Signing and Encryption (COSE) serializations for the Module-Lattice-Based Digital Signature Standard (ML-DSA), a Post-Quantum Cryptography (PQC) digital signature scheme defined in US NIST FIPS 204. Nokia Nokia University of the Bundeswehr Munich This document describes conventions for using Post-Quantum Key Encapsulation Mechanisms (PQ-KEMs) with CBOR Object Signing and Encryption (COSE).

Acknowledgments TODO acknowledge.