Use of SLH-DSA in TLS 1.3

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Table of Contents

1. Introduction

Stateless Hash-Based Digital Signatures (SLH-DSA) [FIPS205] is a quantum-resistant digital signature scheme standardized by the US National Institute of Standards and Technology (NIST) PQC project.¶

This memo specifies how SLH-DSA can be negotiated for authentication in TLS 1.3 via the "signature_algorithms" and "signature_algorithms_cert" extensions.¶

2. SLH-DSA SignatureSchemes Types

SLH-DSA utilizes the concept of stateless hash-based signatures. In contrast to stateful signature algorithms, SLH-DSA eliminates the need for maintaining state information during the signing process. SLH-DSA is designed to sign up to 2^64 messages and it offers three security levels. The parameters for each of the security levels were chosen to provide 128 bits of security, 192 bits of security, and 256 bits of security. This document specifies the use of the SLH-DSA algorithm in TLS at three security levels. It includes the small (S) or fast (F) versions of the algorithm and allows for the use of either SHA-256 [FIPS180] or SHAKE256 [FIPS202] as the hash function. The small version prioritizes smaller signature sizes, making them suitable for resource-constrained environments IoT devices. Conversely, the fast version prioritizes speed over signature size, minimizing the time required to generate and verify signatures.¶

The following combinations are defined in SLH-DSA [FIPS205]:¶

• SLH-DSA-128S-SHA2¶

• SLH-DSA-128F-SHA2¶

• SLH-DSA-192S-SHA2¶

• SLH-DSA-192F-SHA2¶

• SLH-DSA-256S-SHA2¶

• SLH-DSA-256F-SHA2¶

• SLH-DSA-128S-SHAKE¶

• SLH-DSA-128F-SHAKE¶

• SLH-DSA-192S-SHAKE¶

• SLH-DSA-192F-SHAKE¶

• SLH-DSA-256S-SHAKE¶

• SLH-DSA-256F-SHAKE¶

SLH-DSA does not introduce a new hardness assumption beyond those inherent to the underlying hash functions. It builds upon established foundations in cryptography, making it a reliable and robust digital signature scheme for a post-quantum world. While attacks on lattice-based schemes like ML-DSA can compromise their security, SLH-DSA will remain unaffected by these attacks due to its distinct mathematical foundations. This ensures the continued security of systems and protocols that utilize SLH-DSA for digital signatures. However, ML-DSA outperforms SLH-DSA in both signature generation and validation time, as well as signature size. SLH-DSA, in contrast, offers smaller key sizes but larger signature sizes. Due to its well-established hardness assumption, SLH-DSA can be preferred for CA certificates, making it ideal for long-term security as a trust anchor. ML-DSA, on the other hand, is well-suited for end-entity certificates as it provides the computational efficiency required for frequent, real-time authentication, such as during TLS handshake.¶

As defined in [RFC8446], 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 SLH-DSA as follows.¶

enum {
  slhdsa128s_sha256 (0x0911),
  slhdsa128f_sha256 (0x0912),
  slhdsa192s_sha256 (0x0913),
  slhdsa192f_sha256 (0x0914),
  slhdsa256s_sha256 (0x0915),
  slhdsa256f_sha256 (0x0916),
  slhdsa128s_shake (0x0917),
  slhdsa128f_shake (0x0918),
  slhdsa192s_shake (0x0919),
  slhdsa192f_shake (0x091A),
  slhdsa256s_shake (0x091B),
  slhdsa256f_shake (0x091C)
} SignatureScheme;

It is important to note that the slhdsa* entries represent the pure versions of these algorithms and should not be confused with prehashed variant HashSLH-DSA, also defined in [FIPS205].¶

In TLS, the data used for generating a digital signature is unique for each TLS session, as it includes the entire handshake. Thus, SLH-DSA can utilize the deterministic version. The context parameter defined in [FIPS205] Algorithm 23 MUST be an empty string.¶

The signature MUST be computed and verified as specified in Section 4.4.3 of [RFC8446].¶

The corresponding end-entity certificate when negotiated MUST use id-slh-dsa-sha2-128s, id-slh-dsa-sha2-128f, id-slh-dsa-sha2-192s, id-slh-dsa-sha2-192f, id-slh-dsa-sha2-256s, id-slh-dsa-sha2-256f, id-slh-dsa-shake-128s, id-slh-dsa-shake-128f, id-slh-dsa-shake-192s, id-slh-dsa-shake-192f, id-slh-dsa-shake-256s and id-slh-dsa-shake-256f respectively as defined in [I-D.ietf-lamps-x509-slhdsa]}.¶

The schemes defined in this document MUST NOT be used in TLS 1.2 [RFC5246]. 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.¶

3. Security Considerations

The security considerations discussed in Section 11 of [I-D.ietf-lamps-x509-slhdsa] needs to be taken into account.¶

4. IANA Considerations

This document requests new entries to the TLS SignatureScheme registry, according to the procedures in Section 6 of [TLSIANA].¶

5. References

Acknowledgments

Thanks to Bas Westerbaan for the discussion and comments.¶

Authors' Addresses