Deprecating Expanded ML-DSA and ML-KEM Private Key Encodings

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Table of Contents

1. Introduction

[X509-MLDSA] and [X509-MLKEM] 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 Section 8.1 of [X509-MLDSA] and Section 6 of [X509-MLKEM] 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.¶

[JOSE-COSE-MLDSA] and [JOSE-COSE-MLKEM] 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 [X509-MLDSA] and [X509-MLKEM] to deprecate the expanded and combined representations, limiting them to legacy compatibility where applicable.¶

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. Updates to ML-DSA Private Key Encoding

This section updates the private-key encoding requirements and format recommendations in Section 6 of [X509-MLDSA], Section 6 of [X509-MLKEM], and Section 8.1 of [X509-MLDSA]. 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 [X509-MLDSA] and [X509-MLKEM] 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 Section 8.2 of [X509-MLDSA] and Section 8 of [X509-MLKEM].¶

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.¶

4. 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 Section 8.1 of [X509-MLDSA] and Section 7 of [X509-MLKEM], 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.¶

5. Security Considerations

The security considerations of [X509-MLDSA] and [X509-MLKEM] 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.¶

6. IANA Considerations

This document has no IANA actions.¶

7. References

Acknowledgments

TODO acknowledge.¶

Authors' Addresses