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. For security level 1, SHA-256 ([FIPS180]) is used. For security levels 3 and 5, SHA-512 ([FIPS180]) is used. SHAKE256 ([FIPS202]) is applicable for all security levels.¶

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 signatures. However, signature verification with the small version is faster than with the fast version.¶

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 any new hardness assumptions beyond those inherent to its 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 ongoing security of systems and protocols that use SLH-DSA for digital signatures.¶

However, ML-DSA outperforms SLH-DSA in both signature generation and validation time, as well as in signature size, making it a recommended choice for end-entity certificates. SLH-DSA, in contrast, offers smaller key sizes but larger signature sizes. Given its well-established hardness assumption, SLH-DSA may be preferred for TLS applications where high confidence in security is a priority, such as for long-lived TLS sessions and deployments where computational costs of signature generation and validation are minor compared to data transmission and processing demands of user data. The findings in [PQ-TLS-TTLB] shows that while PQ algorithms increase the TLS 1.3 handshake data size, their effect on connection performance is minimal for large data transfers, especially in low-loss networks. Additionally, SLH-DSA is suitable for use in CA certificates due to its strong cryptographic assurances and smaller key sizes. Its robustness against emerging quantum attacks makes it a dependable choice for trust anchors and long-term security, even though it has larger signature sizes.¶

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 {
  slhdsa_sha2_128s (0x0911),
  slhdsa_sha2_128f (0x0912),
  slhdsa_sha2_192s (0x0913),
  slhdsa_sha2_192f (0x0914),
  slhdsa_sha2_256s (0x0915),
  slhdsa_sha2_256f (0x0916),
  slhdsa_shake_128s (0x0917),
  slhdsa_shake_128f (0x0918),
  slhdsa_shake_192s (0x0919),
  slhdsa_shake_192f (0x091A),
  slhdsa_shake_256s (0x091B),
  slhdsa_shake_256f (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, John Mattsson, D.J. Bernstein, Alicja Kario, and Peter Campbell for the discussion and comments.¶

Authors' Addresses