]> 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 Compositing the post-quantum ML-DSA signature with traditional signature algorithms provides protection against potential breaks or critical bugs in ML-DSA or the ML-DSA implementation. This document specifies how such a composite signature can be formed using ML-DSA with RSA-PKCS#1 v1.5, RSA-PSS, ECDSA, Ed25519, and Ed448 to provide authentication in TLS 1.3.

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 . One practical way to implement a hybrid signature scheme is through a composite signature algorithm. In this approach, the composite signature consists of two signature components, each produced by a different signature algorithm. A composite key is treated as a single key that performs a single cryptographic operation such as key generation, signing and verification by using its internal sequence of component keys as if they form a single key. Certain jurisdictions are already recommending or mandating that PQC lattice schemes be used exclusively within a PQ/T hybrid framework. The use of composite schemes provides a straightforward implementation of hybrid solutions compatible with (and advocated by) some governments and cybersecurity agencies . ML-DSA is a post-quantum signature scheme 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. Hybrid signatures provide additional safety by ensuring protection even if vulnerabilities are discovered in one of the constituent algorithms. For deployments that cannot easily tweak configuration or effectively enable/disable algorithms, a composite signature combining PQC signature algorithm with a traditional signature algorithm offers the most viable solution. The rationale for this approach is based on the limitations of fallback strategies. For example, if a traditional signature system is compromised, reverting to a PQC signature algorithm would prevent attackers from forging new signatures that are no longer accepted. However, such a fallback process leaves systems exposed until the transition to the PQC signature algorithm is complete, which can be slow in many environments. In contrast, using hybrid signatures from the start mitigates this issue, offering robust protection and encouraging faster adoption of PQC. Further, zero-day vulnerabilities, where an exploit is discovered and used before the vulnerability is publicly disclosed, highlights this risk. The time required to disclose such attacks and for organizations to reactively switch to alternative algorithms can leave systems critically exposed. By the time a secure fallback is implemented, attackers may have already caused irreparable damage. Adopting hybrid signatures preemptively helps mitigate this window of vulnerability, ensuring resilience even in the face of unforeseen threats.

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 schemes for authentication via the "signature_algorithms" and "signature_algorithms_cert" extensions. This document adds new SignatureSchemes types for the composite ML-DSA as follows. The SignatureScheme names defined in this document follow the TLS IANA naming convention. In composite ML-DSA schemes, the trailing portion of the name corresponds to the traditional signature algorithm variant, including its associated hash function (for example, RSASSA-PSS with SHA-256). The pre-hash function in the composite ML-DSA algorithm names defined in is not used in the TLS name. The explicit RSA key size (for example, RSA2048, RSA3072, or RSA4096) is included to uniquely identify the composite construction and to align with the composite algorithm definitions in . The hash function indicated in the name applies only to the traditional signature component; ML-DSA internally defines its own hashing as specified in . Each entry specifies a unique combination of an ML-DSA parameter set (ML-DSA-44, ML-DSA-65, or ML-DSA-87, as defined in ) and a traditional signature algorithm. The mldsa* identifiers refer to the pure ML-DSA variants and MUST NOT be confused with prehashed variants (for example, HashML-DSA-44). ML-DSA supports two signing modes: deterministic and hedged. In the deterministic mode, the signature is derived solely from the message and the private key, without requiring fresh randomness at signing time. While this eliminates dependence on an external random number generator (RNG), it may increase susceptibility to side-channel attacks, such as fault injection. The hedged mode mitigates this risk by incorporating both fresh randomness generated at signing time and precomputed randomness embedded in the private key, thereby offering stronger protection against such attacks. In the context of TLS, authentication signatures are computed over a signing input derived from the handshake transcript, making each signature input distinct for every session. This property allows the use of either signing mode. The hedged signing mode can be leveraged to provide protection against the side-channel attack. The choice between deterministic and hedged modes does not affect interoperability, as the verification process is the same for both. In both modes, the context parameter defined in Algorithm 2 and Algorithm 3 of MUST be set to the empty string. The signature in the CertificateVerify message MUST be computed as specified in . When a composite ML-DSA signature scheme defined in this document is negotiated, the TLS 1.3 CertificateVerify signing input constructed as specified in MUST be provided as the message input M to the Composite-ML-DSA.Sign function defined in . The composite signature construction then applies its domain separation, labeling, and pre-hash function as specified by the composite algorithm identifier. Any pre-hash function applied as part of the composite signature construction is determined by the composite algorithm identifier defined in and is independent of the TLS SignatureScheme name. The traditional signature algorithm and its associated parameters including the specific hash function are fully determined by the negotiated composite algorithm identifier (OID). The Trad.Sign operation, as defined in , MUST be performed using the hash function implicitly bound to that OID. Upon receipt of the CertificateVerify message, the peer MUST verify the signature by applying the corresponding Composite-ML-DSA.Verify function to the received signature and the locally constructed TLS 1.3 CertificateVerify signing input, in accordance with . 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.

Signature Algorithm Restrictions TLS 1.3 removed support for RSASSA-PKCS1-v1_5 in CertificateVerify messages, opting for RSASSA-PSS instead. Similarly, this document restricts the use of the composite signature algorithms mldsa44_rsa2048_pkcs1_sha256, mldsa65_rsa3072_pkcs1_sha512, and mldsa65_rsa4096_pkcs1_sha512 algorithms to the "signature_algorithms_cert" extension. These composite signature algorithms MUST NOT be used with the "signature_algorithms" extension. These values refer solely to signatures which appear in certificates (see ) and are not defined for use in signed TLS handshake messages. A peer that receives a CertificateVerify message indicating the use of the RSASSA-PKCS1-v1_5 algorithm as one of the component signature algorithms MUST terminate the connection with a fatal illegal_parameter alert.

Selection Criteria for Composite Signature Algorithms The composite signatures specified in the document are a 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 need to be taken into account. Ed25519 and Ed448 ensure SUF security, which may remain secure even if ML-DSA is broken, at least until CRQCs emerge. Applications that prioritize SUF security may benefit from using them in composite with ML-DSA to mitigate risks if ML-DSA is eventually broken. TLS clients that support both post-quantum and traditional-only signature algorithms are vulnerable to downgrade attacks. In such scenarios, an attacker with access to a CRQC could forge a traditional server certificate and impersonate the server. If the client continues to accept traditional-only certificates for backward compatibility, it remains exposed to this risk. While broader deployment of composite or post-quantum certificates will reduce this exposure, clients remain vulnerable unless stricter authentication continuity policies are enforced. A coordinated “flag day” in which all traditional-only certificates are simultaneously phased out is unlikely due to real-world deployment constraints. The continuity mechanism defined in addresses this deployment challenge by allowing clients to cache and enforce a server’s support for post-quantum or composite authentication, thereby preventing fallback to traditional-only authentication in subsequent connections.

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

Value	Description	Recommended	Reference
TBD1	mldsa44_ecdsa_secp256r1_sha256	N	This document.
TBD2	mldsa65_ecdsa_secp256r1_sha512	N	This document.
TBD3	mldsa65_ecdsa_secp384r1_sha512	N	This document.
TBD4	mldsa87_ecdsa_secp384r1_sha512	N	This document.
TBD5	mldsa44_ed25519	N	This document.
TBD6	mldsa65_ed25519	N	This document.
TBD7	mldsa87_ed448	N	This document.
TBD8	mldsa44_rsa2048_pkcs1_sha256	N	This document.
TBD9	mldsa65_rsa3072_pkcs1_sha512	N	This document.
TBD10	mldsa65_rsa4096_pkcs1_sha512	N	This document.
TBD11	mldsa44_rsa2048_pss_pss_sha256	N	This document.
TBD12	mldsa65_rsa3072_pss_pss_sha512	N	This document.
TBD13	mldsa87_rsa3072_pss_pss_sha512	N	This document.
TBD14	mldsa65_rsa4096_pss_pss_sha512	N	This document.
TBD15	mldsa87_rsa4096_pss_pss_sha512	N	This document.

Restricting Composite Signature Algorithms to the signature_algorithms_cert Extension IANA is requested to add a footnote indicating that the mldsa44_rsa2048_pkcs1_sha256, mldsa65_rsa3072_pkcs1_sha512, and mldsa65_rsa4096_pkcs1_sha512 algorithms are defined exclusively for use with the signature_algorithms_cert extension and are not intended for use with the signature_algorithms extension.

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 and adds a "Comment" column to all active registries that do not already have a "Comment" column. Finally, it updates the registration request instructions. This document updates RFC 8447. Entrust Entrust OpenCA Labs Bundesdruckerei GmbH Cisco Systems This document defines combinations of US NIST ML-DSA in hybrid with traditional algorithms RSASSA-PKCS1-v1.5, RSASSA-PSS, ECDSA, Ed25519, and Ed448. These combinations are tailored to meet regulatory guidelines. Composite ML-DSA is applicable in applications that uses X.509 or PKIX data structures that accept ML-DSA, but where the operator wants extra protection against breaks or catastrophic bugs in ML-DSA, and where EUF-CMA-level security is acceptable. 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] This document provides recommendations for the implementation of public-key cryptography based on the RSA algorithm, covering cryptographic primitives, encryption schemes, signature schemes with appendix, and ASN.1 syntax for representing keys and for identifying the schemes. This document represents a republication of PKCS #1 v2.2 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series. By publishing this RFC, change control is transferred to the IETF. This document also obsoletes RFC 3447. 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) Intuit Nokia As the Internet transitions toward post-quantum cryptography (PQC), many TLS servers will continue supporting traditional certificates to maintain compatibility with legacy clients. However, this coexistence introduces a significant vulnerability: an undetected rollback attack, where a malicious actor strips the PQC or Composite certificate and forces the use of a traditional certificate once quantum-capable adversaries exist. To defend against this, this document defines a TLS extension that allows a client to cache a server's declared commitment to present PQC or composite certificates for a specified duration. On subsequent connections, clients enforce that cached commitment and reject traditional-only certificates that conflict with it. This mechanism, inspired by HTTP Strict Transport Security (HSTS) but operating at the TLS layer provides PQC downgrade protection without requiring changes to certificate authority (CA) infrastructure.

Acknowledgments Thanks to Bas Westerbaan, Alicja Kario, Ilari Liusvaara, Dan Wing, Yaron Sheffer, Daniel Van Geest, Samuel Lee, and Sean Turner for the discussion and comments.