Pqc Signature Auditor

pqc-signature-auditor

Domain auditor for standardized post-quantum signatures, in two families with different failure modes: stateless schemes (ML-DSA, SLH-DSA, FN-DSA) where the risks are sampling and bound enforcement, and stateful hash-based schemes (XMSS/LMS/HSS) where the dominant risk is one-time-key state reuse.

Do not fold ML-KEM/Kyber findings into this skill — KEM review belongs to pqc-kem-auditor.

When to Use

• Auditing ML-DSA (FIPS 204) signing/verification: rejection sampling, hint computation, deterministic vs hedged modes
• Auditing SLH-DSA (FIPS 205): FORS/WOTS+ structure, addressing, and randomizer handling
• Auditing FN-DSA/Falcon implementations (with its standardization status pinned in source notes — FIPS 206 not yet final)
• Auditing XMSS/LMS/HSS under NIST SP 800-208: OTS index monotonicity, crash recovery, backup/restore, cloning, and hardware-binding requirements

When NOT to Use

• ML-KEM/Kyber or any KEM decapsulation review -> pqc-kem-auditor
• Classical signatures (ECDSA, Schnorr, EdDSA, RSA) -> signature-scheme-auditor
• Generic lattice parameter/noise design for custom schemes -> lattice-auditor
• Timing measurement methodology for samplers -> side-channel-auditor

Core Review Areas

1. Parameter-set and version provenance: FIPS 204/205 final vs round-3 Dilithium/SPHINCS+ differences; Falcon flagged as pre-standard
2. ML-DSA signing: rejection-sampling loop exactness (norm checks on z, r0, hint count), iteration bounds, and no leakage of rejected candidates
3. Signing modes: deterministic vs hedged (rnd) handling per FIPS 204; SLH-DSA opt_rand per FIPS 205; fault posture of deterministic modes
4. Verification bounds: every norm/weight/count bound checked (z bound, hint weight, FORS/WOTS checksum rules) — skipping any bound admits forgery
5. Stateful OTS index management (XMSS/LMS/HSS): monotonic persistent counter committed BEFORE signature release, crash recovery, backups, replication, and SP 800-208 hardware-binding requirements
6. Encoding/serialization: canonical encodings, length checks, context strings (FIPS 204/205 ctx parameter) bound correctly

Workflow

Phase 1: Family and version mapping

• Read references/pqc-signature-checklist.md
• Pin the scheme, parameter set, and spec version; queue spec-delta-checker for conformance claims

Phase 2: Family-specific review

• Stateless (ML-DSA, SLH-DSA, FN-DSA): execute workflows/pqc-signature-review.md
• Stateful (XMSS/LMS/HSS): execute workflows/stateful-hash-signature-review.md

Phase 3: Pattern hunt

• Read references/finding-patterns.md
• Prioritize skipped verification bounds, rejection-loop shortcuts, and OTS index rollback paths

Phase 4: Handoff

• Send surviving findings to crypto-fp-check
• Cross-route sampler leakage to side-channel-auditor, RNG lifecycle to randomness-auditor, KEM questions to pqc-kem-auditor

Output Contract

Produce a PQ-signature handoff that includes:

• signature_family_and_parameter_set
• sign_or_verify_path
• state_or_sampling_invariant
• evidence
• disposition (one of verified, false_positive, unverified, observation, residual_risk)
• next_route

Reference Index

• references/pqc-signature-checklist.md
• references/finding-patterns.md
• references/spec-sources.md
• workflows/pqc-signature-review.md
• workflows/stateful-hash-signature-review.md