| Internet-Draft | ML-DSA in CMS | November 2024 |
| Salter, et al. | Expires 26 May 2025 | [Page] |
The Module-Lattice-Based Digital Signature Algorithm (ML-DSA), as defined in FIPS 204, is a post-quantum digital signature scheme that aims to be secure against an adversary in possession of a Cryptographically Relevant Quantum Computer (CRQC). This document specifies the conventions for using the ML-DSA signature algorithm with the Cryptographic Message Syntax (CMS). In addition, the algorithm identifier and public key syntax are provided.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://lamps-wg.github.io/cms-ml-dsa/draft-ietf-lamps-cms-ml-dsa.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-lamps-cms-ml-dsa/.¶
Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME Working Group mailing list (mailto:spasm@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/spasm/. Subscribe at https://www.ietf.org/mailman/listinfo/spasm/.¶
Source for this draft and an issue tracker can be found at https://github.com/lamps-wg/cms-ml-dsa.¶
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶
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This Internet-Draft will expire on 26 May 2025.¶
Copyright (c) 2024 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.¶
The Module-Lattice-Based Digital Signature Algorithm (ML-DSA) is a digital signature algorithm standardised by NIST as part of their post-quantum cryptography standardization process. It is intended to be secure against both "traditional" cryptographic attacks, as well as attacks utilising a quantum computer. It offers smaller signatures and significantly faster runtimes than SLH-DSA [FIPS205], an alternative post-quantum signature algorithm also standardised by NIST. This document specifies the use of the ML-DSA in CMS at three security levels: ML-DSA-44, ML-DSA-65, and ML-DSA-87. See Appendix B of [I-D.ietf-lamps-dilithium-certificates] for more information on the security levels and key sizes of ML-DSA.¶
Prior to standardisation, ML-DSA was known as Dilithium. ML-DSA and Dilithium are not compatible.¶
For each of the ML-DSA parameter sets, an algorithm identifier OID has been specified.¶
[FIPS204] also specifies a pre-hashed variant of ML-DSA, called HashML-DSA. HashML-DSA is not used in CMS.¶
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.¶
Many ASN.1 data structure types use the AlgorithmIdentifier type to identify cryptographic algorithms. In CMS, AlgorithmIdentifiers are used to identify ML-DSA signatures in the signed-data content type. They may also appear in X.509 certificates used to verify those signatures. The same AlgorithmIdentifiers are used to identify ML-DSA public keys and signature algorithms. [I-D.ietf-lamps-dilithium-certificates] describes the use of ML-DSA in X.509 certificates. The AlgorithmIdentifier type is defined as follows:¶
AlgorithmIdentifier{ALGORITHM-TYPE, ALGORITHM-TYPE:AlgorithmSet} ::=
SEQUENCE {
algorithm ALGORITHM-TYPE.&id({AlgorithmSet}),
parameters ALGORITHM-TYPE.
&Params({AlgorithmSet}{@algorithm}) OPTIONAL
}
¶
The fields in the AlgorithmIdentifier type have the following meanings:¶
The algorithm field contains an OID that identifies the cryptographic algorithm in use. The OIDs for ML-DSA are described below.¶
The parameters field contains parameter information for the algorithm identified by the OID in the algorithm field. Each ML-DSA parameter set is identified by its own algorithm OID, so there is no relevant information to include in this field. As such, parameters MUST be omitted when encoding an ML-DSA AlgorithmIdentifier.¶
The object identifiers for ML-DSA are defined in the NIST Computer Security Objects Register [CSOR], and are reproduced here for convenience.¶
sigAlgs OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16)
us(840) organization(1) gov(101) csor(3) nistAlgorithms(4) 3 }
id-ml-dsa-44 OBJECT IDENTIFIER ::= { sigAlgs 17 }
id-ml-dsa-65 OBJECT IDENTIFIER ::= { sigAlgs 18 }
id-ml-dsa-87 OBJECT IDENTIFIER ::= { sigAlgs 19 }
¶
[RFC5280] defines the SubjectPublicKeyInfo ASN.1 type. In X.509 certificates [RFC5280] and Certificate Management over CMS [RFC5272], the SubjectPublicKeyInfo type is used to encode public keys. It has the following syntax:¶
SubjectPublicKeyInfo {PUBLIC-KEY: IOSet} ::= SEQUENCE {
algorithm AlgorithmIdentifier {PUBLIC-KEY, {IOSet}},
subjectPublicKey BIT STRING
}
¶
The PUBLIC-KEY ASN.1 types for ML-DSA are defined here:¶
pk-ml-dsa-44 PUBLIC-KEY ::= {
IDENTIFIER id-ml-dsa-44
-- KEY no ASN.1 wrapping --
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign }
-- PRIVATE-KEY no ASN.1 wrapping -- }
pk-ml-dsa-65 PUBLIC-KEY ::= {
IDENTIFIER id-ml-dsa-65
-- KEY no ASN.1 wrapping --
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign }
-- PRIVATE-KEY no ASN.1 wrapping -- }
pk-ml-dsa-87 PUBLIC-KEY ::= {
IDENTIFIER id-ml-dsa-87
-- KEY no ASN.1 wrapping --
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign }
-- PRIVATE-KEY no ASN.1 wrapping -- }
ML-DSA-PublicKey ::= OCTET STRING (SIZE (1312 | 1952 | 2592))
ML-DSA-PrivateKey ::= OCTET STRING (SIZE (32))
¶
Algorithm 22 in Section 7.2 of [FIPS204] defines the raw byte string encoding of an ML-DSA public key. When used in a SubjectPublicKeyInfo type, the subjectPublicKey BIT STRING contains the raw byte string encoding of the public key.¶
When an ML-DSA public key appears outside of a SubjectPublicKeyInfo type in an environment that uses ASN.1 encoding, it can be encoded as an OCTET STRING by using the ML-DSA-PublicKey type.¶
[RFC5958] describes the Asymmetric Key Package CMS content type, and the OneAsymmetricKey type for encoding asymmetric keypairs. When an ML-DSA private key or keypair is encoded as a OneAsymmetricKey, it follows the description in Section 6 of [I-D.ietf-lamps-dilithium-certificates].¶
When the ML-DSA private key appears outside of an Asymmetric Key Package in an environment that uses ASN.1 encoding, it can be encoded as an OCTET STRING by using the ML-DSA-PrivateKey type.¶
[RFC5652] specifies that digital signatures for CMS are produced using a digest of the message to be signed, and the signer's private key. At the time of publication of that RFC, all signature algorithms supported in CMS required a message digest to be calculated externally to that algorithm, which would then be supplied to the algorithm implementation when calculating and verifying signatures. Since then, EdDSA [RFC8032] and SLH-DSA [FIPS205] have also been standardised, and these algorithms support both a "pure" and "pre-hash" mode. In the pre-hash mode, a message digest (the "pre-hash") is calculated separately and supplied to the signature algorithm as described above. In the pure mode, the message to be signed or verified is instead supplied directly to the signature algorithm. ML-DSA also supports a pre-hash and pure mode, though this document follows the convention set by EdDSA in CMS [RFC8419] and SLH-DSA in CMS [I-D.ietf-lamps-cms-sphincs-plus] and only specifies use of the pure mode of ML-DSA in CMS.¶
[RFC5652] describes the two methods that are used to calculate and verify signatures in CMS. One method is used when signed attributes are present in the signedAttrs field of the relevant SignerInfo, and another is used when signed attributes are absent. Each method produces a different "message digest" to be supplied to the signature algorithm in question, but because the pure mode of ML-DSA is used, the "message digest" is in fact the entire message. Use of signed attributes is preferred, but the conventions for signed-data without signed attributes is also described below for completeness.¶
When signed attributes are absent, ML-DSA (pure mode) signatures are computed over the content of the signed-data. As described in Section 5.4 of [RFC5652], the "content" of a signed-data is the value of the encapContentInfo eContent OCTET STRING. The tag and length octets are not included.¶
When signed attributes are included, ML-DSA (pure mode) signatures are computed over the complete DER encoding of the SignedAttrs value contained in the SignerInfo's signedAttrs field. As described in Section 5.4 of [RFC5652], this encoding includes the tag and length octets, but an EXPLICIT SET OF tag is used rather than the IMPLICIT [0] tag that appears in the final message. The signedAttrs field MUST at minimum include a content-type attribute and a message-digest attribute. The message-digest attribute contains a hash of the content of the signed-data, where the content is as described for the absent signed attributes case above. Recalculation of the hash value by the recipient is an important step in signature verification.¶
Section 4 of [I-D.ietf-lamps-cms-sphincs-plus] describes how, when the content of a signed-data is large, performance may be improved by including signed attributes. This is as true for ML-DSA as it is for SLH-DSA, although ML-DSA signature generation and verification is significantly faster than SLH-DSA.¶
ML-DSA has a context string input that can be used to ensure that different signatures are generated for different application contexts. When using ML-DSA as described in this document, the context string is not used.¶
When using ML-DSA, the fields of a SignerInfo are used as follows:¶
Per Section 5.3 of [RFC5652], the digestAlgorithm field identifies the message digest algorithm used by the signer, and any associated parameters. To ensure collision resistance, the identified message digest algorithm SHOULD produce a hash value of a size that is at least twice the collision strength of the internal commitment hash used by ML-DSA. SHA-512 [FIPS180] MUST be supported for use with the variants of SLH-DSA in this document; however, other hash functions MAY also be supported. When SHA-512 is used, the id-sha512 [RFC5754] digest algorithm identifier is used and the parameters field MUST be omitted. When signing using ML-DSA without including signed attributes, the algorithm specified in the digestAlgorithm field has no meaning, as ML-DSA computes signatures over entire messages rather than externally computed digests. Nonetheless, it SHOULD specify a digest algorithm that otherwise would have been used if signed attributes were present, such as SHA-512. When processing a SignerInfo signed using ML-DSA, if no signed attributes are present, implementations MUST ignore the content of the digestAlgorithm field.¶
When signing a signed-data using ML-DSA, the signatureAlgorithm field MUST contain one of the ML-DSA signature algorithm OIDs, and the parameters field MUST be absent. The algorithm OID MUST be one of the following OIDs described in Section 2:¶
| Signature algorithm | Algorithm Identifier OID |
|---|---|
| ML-DSA-44 | id-ml-dsa-44 |
| ML-DSA-65 | id-ml-dsa-65 |
| ML-DSA-87 | id-ml-dsa-87 |
The signature field contains the signature value resulting from the use of the ML-DSA signature algorithm identified by the signatureAlgorithm field. The ML-DSA (pure mode) signature generation operation is specified in Section 5.2 of [FIPS204], and the signature verification operation is specified in Section 5.3 of [FIPS204]. Note that Section 5.6 of [RFC5652] places further requirements on the successful verification of a signature.¶
The security considerations [RFC5652] and [I-D.ietf-lamps-dilithium-certificates] apply to this specification as well.¶
Security of the ML-DSA private key is critical. Compromise of the private key will enable an adversary to forge arbitrary signatures.¶
ML-DSA depends on high quality random numbers that are suitable for use in cryptography. The use of inadequate pseudo-random number generators (PRNGs) to generate such values can significantly undermine the security properties offered by a cryptographic algorithm. For instance, an attacker may find it much easier to reproduce the PRNG environment that produced any private keys, searching the resulting small set of possibilities, rather than brute force searching the whole key space. The generation of random numbers of a sufficient level of quality for use in cryptography is difficult, and [RFC4086] offers important guidance in this area.¶
By default ML-DSA signature generation uses randomness from two sources: fresh random data generated during signature generation, and precomputed random data included in the signer's private key. This is referred to as the "hedged" variant of ML-DSA. Inclusion of both sources of random can help mitigate against faulty random number generators, side-channel attacks and fault attacks. [FIPS204] also permits creating deterministic signatures using just the precomputed random data in the signer's private key. The same verification algorithm is used to verify both hedged and deterministic signatures, so this choice does not affect interoperability. The signer SHOULD NOT use the deterministic variant of ML-DSA on platforms where side-channel attacks or fault attacks are a concern. Side channel attacks and fault attacks against ML-DSA are an active area of research [WNGD2023] [KPLG2024]. Future protection against these styles of attack may involve interoperable changes to the implementation of ML-DSA's internal functions. Implementers SHOULD consider implementing such protection measures if it would be beneficial for their particular use cases.¶
To avoid algorithm substitution attacks, the CMSAlgorithmProtection attribute defined in [RFC6211] SHOULD be included in signed attributes.¶
If ML-DSA signing is implemented in a hardware device such as hardware security module (HSM) or portable cryptographic token, implementers might want to avoid sending the full content to the device for performance reasons. By including signed attributes, which necessarily include the message-digest attribute and the content-type attribute as described in Section 5.3 of [RFC5652], the much smaller set of signed attributes are sent to the device for signing.¶
This approach addresses the use case for HashML-DSA, and is one reason why HashML-DSA is not specified for use with CMS in this document. Additionally, the pure variant of ML-DSA does support a form of pre-hash via the mu "message representative" value described in Section 6.2 of [FIPS204]. This value may "optionally be computed in a different cryptographic module" and supplied to the hardware device, rather than requiring the entire message to be transmitted.¶
For the ASN.1 module found in Appendix A, IANA is requested to assign an object identifier for the module identifier (TBD) with a description of "id-mod-ml-dsa-2024". This should be allocated in the "SMI Security for S/MIME Module Identifier" registry (1.2.840.113549.1.9.16.0).¶
This document was heavily influenced by [RFC8419], [I-D.ietf-lamps-cms-sphincs-plus], and [I-D.ietf-lamps-dilithium-certificates]. Thanks go to the authors of those documents.¶
<CODE BEGINS>
ML-DSA-Module-2024
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9)
id-smime(16) id-mod(0) id-mod-ml-dsa-2024(TBD) }
DEFINITIONS IMPLICIT TAGS ::= BEGIN
EXPORTS ALL;
IMPORTS PUBLIC-KEY, SIGNATURE-ALGORITHM, SMIME-CAPS
FROM AlgorithmInformation-2009 -- in [RFC5911]
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-algorithmInformation-02(58) } ;
--
-- Object Identifiers
--
sigAlgs OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16)
us(840) organization(1) gov(101) csor(3) nistAlgorithms(4) 3 }
id-ml-dsa-44 OBJECT IDENTIFIER ::= { sigAlgs 17 }
id-ml-dsa-65 OBJECT IDENTIFIER ::= { sigAlgs 18 }
id-ml-dsa-87 OBJECT IDENTIFIER ::= { sigAlgs 19 }
--
-- Signature Algorithm Identifiers
--
sa-ml-dsa-44 SIGNATURE-ALGORITHM ::= {
IDENTIFIER id-ml-dsa-44
PARAMS ARE absent
PUBLIC-KEYS { pk-ml-dsa-44 }
SMIME-CAPS { IDENTIFIED BY id-ml-dsa-44 } }
sa-ml-dsa-65 SIGNATURE-ALGORITHM ::= {
IDENTIFIER id-ml-dsa-65
PARAMS ARE absent
PUBLIC-KEYS { pk-ml-dsa-65 }
SMIME-CAPS { IDENTIFIED BY id-ml-dsa-65 } }
sa-ml-dsa-87 SIGNATURE-ALGORITHM ::= {
IDENTIFIER id-ml-dsa-87
PARAMS ARE absent
PUBLIC-KEYS { pk-ml-dsa-87 }
SMIME-CAPS { IDENTIFIED BY id-ml-dsa-87 } }
--
-- Public Keys
--
pk-ml-dsa-44 PUBLIC-KEY ::= {
IDENTIFIER id-ml-dsa-44
-- KEY no ASN.1 wrapping --
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign }
-- PRIVATE-KEY no ASN.1 wrapping -- }
pk-ml-dsa-65 PUBLIC-KEY ::= {
IDENTIFIER id-ml-dsa-65
-- KEY no ASN.1 wrapping --
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign }
-- PRIVATE-KEY no ASN.1 wrapping -- }
pk-ml-dsa-87 PUBLIC-KEY ::= {
IDENTIFIER id-ml-dsa-87
-- KEY no ASN.1 wrapping --
CERT-KEY-USAGE
{ digitalSignature, nonRepudiation, keyCertSign, cRLSign }
-- PRIVATE-KEY no ASN.1 wrapping -- }
ML-DSA-PublicKey ::= OCTET STRING (SIZE (1312 | 1952 | 2592))
ML-DSA-PrivateKey ::= OCTET STRING (SIZE (32))
--
-- Expand the signature algorithm set used by CMS [RFC5911]
--
SignatureAlgorithmSet SIGNATURE-ALGORITHM ::= {
sa-ml-dsa-44 |
sa-ml-dsa-65 |
sa-ml-dsa-87,
... }
SMimeCaps SMIME-CAPS ::= {
sa-ml-dsa-44.&smimeCaps |
sa-ml-dsa-65.&smimeCaps |
sa-ml-dsa-87.&smimeCaps,
... }
END
<CODE ENDS>
¶