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Vrf auditor

Skill Yue-Zhou1/zkcrypto-audit/plugins/protocol-auditors/skills/vrf-auditor

Audit Verifiable Random Function implementations (RFC 9381 ECVRF and RSA-FDH-VRF) for key validation, ciphersuite/suite-string domain separation, encode-to-curve and cofactor handling, proof-to-hash ordering, and uniqueness/pseudorandomness assumptions, plus application-level output grinding. Use when reviewing VRF provers, verifiers, or consumers of VRF outputs (leader election, lotteries, randomness beacons).From its SKILL.md

Install
npx -y skills add Yue-Zhou1/zkcrypto-audit --skill vrf-auditor

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SKILL.md

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vrf-auditor

Domain auditor for VRFs: proof generation, verification, and the trust rules consumers must follow before treating a VRF output as random.

When to Use

  • Auditing RFC 9381 ECVRF (any ciphersuite) or RSA-FDH-VRF provers and verifiers
  • Reviewing encode_to_curve/hash_to_curve usage, cofactor handling, and ciphersuite/suite-string domain separation
  • Reviewing proof_to_hash ordering and whether outputs are consumed before verification
  • Assessing uniqueness (full uniqueness vs trusted-key uniqueness), pseudorandomness assumptions, and application-level grinding resistance

When NOT to Use

  • VDF sequential-delay proofs (Wesolowski/Pietrzak) -> vdf-auditor
  • Ordinary signature scheme review without VRF output semantics -> signature-scheme-auditor
  • Elliptic-curve point validation/subgroup mechanics in isolation -> ecc-pairing-auditor
  • Nonce/CSPRNG generation mechanics -> randomness-auditor
  • Line-by-line RFC conformance diffing -> spec-delta-checker

Core Review Areas

  1. Key validation: ECVRF public keys decoded and validated per the ciphersuite's validate_key rules (small-order/identity rejection for full uniqueness/collision resistance claims); RSA key sanity
  2. Domain separation: suite string, DST, and challenge-generation separation octets exactly per RFC 9381; no cross-suite or cross-protocol hash reuse
  3. encode_to_curve: correct method for the ciphersuite (TAI vs hash_to_curve per RFC 9380), including the salt and loop rules
  4. Cofactor handling: Gamma multiplied by the cofactor in proof_to_hash; verification equation component checks
  5. Nonce generation in proving: RFC 9381 §5.4.2 deterministic derivation (RFC 6979-style / RFC 8032-style) — reuse leaks the secret key
  6. proof_to_hash ordering: beta computed only from a proof that verified; consumers must never use outputs before verify returns VALID
  7. Uniqueness and pseudorandomness caveats: which property holds for adversarial keys under the chosen ciphersuite, and whether the application needs full uniqueness
  8. Application grinding: can a participant re-key, re-input, or withhold outputs to bias the consuming protocol (leader election, lotteries)

Workflow

Phase 1: Construction and ciphersuite mapping

  • Read references/vrf-checklist.md
  • Pin the exact construction (ECVRF ciphersuite ID or RSA-FDH-VRF parameters) and the RFC 9381 sections that govern it

Phase 2: Prove/verify path review

  • Execute workflows/vrf-review.md

Phase 3: Pattern hunt

  • Read references/finding-patterns.md
  • Prioritize outputs used before verification, missing key validation, wrong cofactor handling, and grinding-by-withholding

Phase 4: Handoff

  • Send surviving findings to crypto-fp-check
  • Cross-route RFC conformance deltas to spec-delta-checker, EC point rules to ecc-pairing-auditor, and nonce generation to randomness-auditor

Output Contract

Produce a VRF handoff that includes:

  • vrf_family
  • prove_or_verify_path
  • uniqueness_or_pseudorandomness_invariant
  • evidence
  • disposition (one of verified, false_positive, unverified, observation, residual_risk)
  • next_route

Reference Index

What ships with it: 5 files

10.9 KB alongside SKILL.md

agents/

references/

workflows/

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