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 [FIPS205] 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 security levels 1, 3, and 5 were chosen to provide AES-128, AES-192, and AES-256 bits of security respectively (see Table 2 in Section 10 of [I-D.ietf-pquip-pqc-engineers]). This document specifies the use of the SLH-DSA algorithm in TLS at each level.¶

This document specifies the use of the SLH-DSA algorithm in TLS at three security levels. Each security level (1, 3, and 5) defines two variants of the algorithm: a small (S) version and a fast (F) version. 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. For hash function selection, the algorithm uses SHA-256 ([FIPS180]) for security level 1 and SHA-512 ([FIPS180]) for security levels 3 and 5. Alternatively, SHAKE256 ([FIPS202]) can be used across all security levels.¶

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 are currently hypothetical at the time of writing this document, such attacks, if realized, could compromise their security. SLH-DSA would 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].¶

SLH-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 unique handshake transcripts, 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 side-channel attacks. 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 22 and Algorithm 24 of [FIPS205] MUST be set to the 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 8 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