Signature Scheme Auditor

signature-scheme-auditor

Domain auditor for classical single-party signature schemes: the verification equations, encodings, and key-validation rules of ECDSA, Schnorr/BIP-340, EdDSA, and RSA signatures.

When to Use

• Auditing generic ECDSA signing/verification across ecosystems and curves (P-256, secp256k1, brainpool, etc.)
• Reviewing Schnorr and BIP-340 x-only key handling, tagged hashes, and verification equations
• Reviewing EdDSA/Ed25519 canonical encoding, cofactor semantics, and batch/single verification consistency
• Reviewing RSA-PSS and PKCS#1 v1.5 verification and padding rules
• Assessing malleability, public-key validation, and hash/prehash semantics

When NOT to Use

• Ethereum-specific EIP-712, ecrecover, or application encoding -> ethereum-crypto-auditor
• BLS signatures, pairings, aggregation -> ecc-pairing-auditor
• Threshold schemes -> dkg-threshold-auditor (FROST/MuSig2) or threshold-ecdsa-auditor (GG/CGGMP)
• Nonce generation/derivation mechanics -> randomness-auditor
• Fault-attack resistance of signing hardware paths -> implementation-safety skills (verify-after-sign handoff)

Core Review Areas

1. Verification equations: exact equation per scheme, all components checked (never trusting a library's partial verification)
2. Range and canonicality: ECDSA r,s in [1, n-1]; Ed25519 s < L and canonical point encodings; BIP-340 x-only lift and even-y convention; PSS salt and v1.5 DigestInfo strictness
3. Malleability: ECDSA (r, s)/(r, n-s); Ed25519 non-canonical s or A; scheme-level implications for consumers that key on signature bytes
4. Public-key validation: on-curve, non-identity, correct subgroup (or cofactored equation), small-order rejection policy
5. Hash and prehash semantics: which hash, truncation rules (FIPS 186-5 leftmost-bits), Ed25519ph/ctx variants, cross-protocol hash-domain reuse
6. Batch vs single consistency: batch equations (cofactored) accepting what single (cofactorless) rejects, and consensus divergence risk
7. Nonce integration: signing paths call correct derivation (RFC 6979 / RFC 8032) — deep RNG review hands off to randomness-auditor
8. Verify-after-sign and fault-check posture at the API boundary

Workflow

Phase 1: Scheme inventory

• Read references/signature-scheme-checklist.md
• Identify every scheme/curve/hash triple in scope and the governing standard for each

Phase 2: Verification-path review

• Execute workflows/signature-verification-review.md

Phase 3: Pattern hunt

• Read references/finding-patterns.md
• Prioritize missing range checks, permissive decoders, batch/single divergence, and v1.5 parsing tolerance

Phase 4: Handoff

• Send surviving findings to crypto-fp-check
• Cross-route nonce lifecycle to randomness-auditor, Ethereum encoding to ethereum-crypto-auditor, BLS/pairing math to ecc-pairing-auditor

Output Contract

Produce a signature-scheme handoff that includes:

• signature_family
• verification_or_signing_path
• equation_or_encoding_invariant
• evidence
• disposition (one of verified, false_positive, unverified, observation, residual_risk)
• next_route

Reference Index

• references/signature-scheme-checklist.md
• references/finding-patterns.md
• references/spec-sources.md
• workflows/signature-verification-review.md