]> Nokia

Bangalore Karnataka India kondtir@gmail.com

DigiCert

Pittsburgh USA tim.hollebeek@digicert.com

Entrust Limited

2500 Solandt Road – Suite 100 Ottawa, Ontario K2K 3G5 Canada john.gray@entrust.com

Cisco Systems

sfluhrer@cisco.com

Security TLS ML-DSA FIPS204 Composite This document specifies how the post-quantum signature scheme ML-DSA , in combination with traditional algorithms RSA-PKCS#1v1.5,RSA-PSS, ECDSA, Ed25519, and Ed448 can be used for authentication in TLS 1.3. The composite ML-DSA approach is beneficial in deployments where operators seek additional protection against potential breaks or catastrophic bugs in ML-DSA.

Introduction The advent of quantum computing poses a significant threat to current cryptographic systems. Traditional cryptographic algorithms such as RSA, Diffie-Hellman, DSA, and their elliptic curve variants are vulnerable to quantum attacks. During the transition to post-quantum cryptography (PQC), there is considerable uncertainty regarding the robustness of both existing and new cryptographic algorithms. While we can no longer fully trust traditional cryptography, we also cannot immediately place complete trust in post-quantum replacements until they have undergone extensive scrutiny and real-world testing to uncover and rectify potential implementation flaws. Unlike previous migrations between cryptographic algorithms, the decision of when to migrate and which algorithms to adopt is far from straightforward. Even after the migration period, it may be advantageous for an entity's cryptographic identity to incorporate multiple public-key algorithms to enhance security. Cautious implementers may opt to combine cryptographic algorithms in such a way that an attacker would need to break all of them simultaneously to compromise the protected data. These mechanisms are referred to as Post-Quantum/Traditional (PQ/T) Hybrids . Certain jurisdictions are already recommending or mandating that PQC lattice schemes be used exclusively within a PQ/T hybrid framework. The use of Composite scheme provides a straightforward implementation of hybrid solutions compatible with (and advocated by) some governments and cybersecurity agencies . ML-DSA is a post-quantum signature schemes standardised by NIST. It is a module-lattice based scheme. This memo specifies how a composite ML-DSA can be negotiated for authentication in TLS 1.3 via the "signature_algorithms" and "signature_algorithms_cert" extensions. The use of composite ML-DSA is valuable in deployments where disabling algorithms is complex or slow. Hybrid signatures provide additional safety by ensuring protection even if vulnerabilities are discovered in one of the constituent algorithms.

Conventions and Terminology 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. These words may also appear in this document in lower case as plain English words, absent their normative meanings. This document is consistent with the terminology defined in . It defines composites as:

• Composite Cryptographic Element: A cryptographic element that incorporates multiple component cryptographic elements of the same type in a multi-algorithm scheme.

ML-DSA SignatureSchemes Types As defined in , the SignatureScheme namespace is used for the negotiation of signature scheme for authentication via the "signature_algorithms" and "signature_algorithms_cert" extensions. This document adds new SignatureSchemes types for the composite ML-DSA as follows. Each entry specifies a unique combination of an ML-DSA parameter, an elliptic curve or RSA variant, and a hashing function. The first algorithm corresponds to ML-DSA-44, ML-DSA-65, and ML-DSA-87, as defined in . It is important to note that the mldsa* entries represent the pure versions of these algorithms and should not be confused with prehashed variants, such as HashML-DSA-44, also defined in . In TLS, the data used for generating a digital signature is unique for each TLS session, as it includes the entire handshake. Thus, ML-DSA can utilize the deterministic version. The context parameter defined in Algorithm 2/Algorithm 3 MUST be an empty string. The signature MUST be computed and verified as specified in . The corresponding end-entity certificate when negotiated MUST use the First AlgorithmID and Second AlgorithmID respectively as defined in . The schemes defined in this document MUST NOT be used in TLS 1.2 . A peer that receives ServerKeyExchange or CertificateVerify message in a TLS 1.2 connection with schemes defined in this document MUST abort the connection with an illegal_parameter alert.

Selection Criteria for Composite Signature Algorithms The composite signatures specified in the document are restricted set of cryptographic pairs, chosen from the intersection of two sources:

• The composite algorithm combinations as recommended in , which specify both PQC and traditional signature algorithms.
• The mandatory-to-support or recommended traditional signature algorithms listed in TLS 1.3.

By limiting algorithm combinations to those defined in both and TLS 1.3, this specification ensures that each pair:

• Meets established security standards for composite signatures in a post-quantum context, as described in .
• Is compatible with traditional digital signatures recommended in TLS 1.3, ensuring interoperability and ease of adoption within the TLS ecosystem.

This conservative approach reduces the risk of selecting unsafe or incompatible configurations, promoting security by requiring only trusted and well-vetted pairs. Future updates to this specification may introduce additional algorithm pairs as standards evolve, subject to similar vetting and inclusion criteria.

Security Considerations The security considerations discussed in Section 11 of needs to be taken into account.

IANA Considerations This document requests new entries to the TLS SignatureScheme registry, according to the procedures in .

Value	Description	Recommended	Reference
0x0907	mldsa44_ecdsa_secp256r1_sha256	Y	This document.
0x0908	mldsa65_ecdsa_secp384r1_sha384	Y	This document.
0x0909	mldsa87_ecdsa_secp384r1_sha384	Y	This document.
0x090A	mldsa44_ed25519	Y	This document.
0x090B	mldsa65_ed25519	Y	This document.
0x090C	mldsa44_rsa_pkcs1_sha256	Y	This document.
0x090D	mldsa65_rsa_pkcs1_sha384	Y	This document.
0x090E	mldsa44_rsa_pss_pss_sha256	Y	This document.
0x090F	mldsa65_rsa_pss_pss_sha384	Y	This document.
0x0910	mldsa87_ed448	Y	This document.

References Normative References This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Venafi sn3rd This document updates the changes to TLS and DTLS IANA registries made in RFC 8447. It adds a new value "D" for discouraged to the recommended column of the selected TLS registries. This document updates the following RFCs: 3749, 5077, 4680, 5246, 5705, 5878, 6520, 7301, and 8447. Entrust Limited Entrust Limited OpenCA Labs Bundesdruckerei GmbH Cisco Systems This document defines combinations of ML-DSA [FIPS.204] in hybrid with traditional algorithms RSA-PKCS#1v1.5, RSA-PSS, ECDSA, Ed25519, and Ed448. These combinations are tailored to meet security best practices and regulatory requirements. Composite ML-DSA is applicable in any application that uses X.509, PKIX, and CMS data structures and protocols that accept ML-DSA, but where the operator wants extra protection against breaks or catastrophic bugs in ML-DSA. 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 Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] UK National Cyber Security Centre UK National Cyber Security Centre Naval Postgraduate School One aspect of the transition to post-quantum algorithms in cryptographic protocols is the development of hybrid schemes that incorporate both post-quantum and traditional asymmetric algorithms. This document defines terminology for such schemes. It is intended to be used as a reference and, hopefully, to ensure consistency and clarity across different protocols, standards, and organisations. Federal Office for Information Security (BSI)

Acknowledgments Thanks to Bas Westerbaan for the discussion and comments.