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Onchain verifier auditor

Skill Yue-Zhou1/zkcrypto-audit/plugins/zk-and-vm-auditors/skills/onchain-verifier-auditor

Audit Solidity, Vyper, or Huff proof-verifier contracts for pairing precompile misuse, missing scalar-field checks on public inputs, calldata decoding errors, and verification-key provenance or upgrade risks. Use when reviewing on-chain SNARK/STARK verifier contracts or EIP-196/197/2537 precompile call sites.From its SKILL.md

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

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SKILL.md

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onchain-verifier-auditor

Domain auditor for on-chain proof-verifier contracts: the EVM-side half of a proof system, where a Solidity/Vyper/Huff contract decodes proof calldata, checks public inputs, and delegates pairing math to precompiles.

When to Use

  • Auditing generated or hand-written verifier contracts (snarkjs, gnark, Halo2, custom) in Solidity, Vyper, or Huff
  • Reviewing calls to the EVM pairing/add/mul precompiles (EIP-196/197 at 0x06/0x07/0x08, EIP-2537 BLS12-381 precompiles)
  • Checking public-input range validation against the scalar field modulus
  • Reviewing verification-key storage, immutability, and upgrade authorization
  • Reviewing ABI decoding and ordering of proof points and public inputs

When NOT to Use

  • Circuit constraint soundness or witness completeness -> zk-circuit-auditor
  • Generic Rust/Go verifier implementations (arkworks, gnark backend) -> zk-circuit-auditor plus ecc-pairing-auditor and language-safety skills
  • Nullifier uniqueness, spent-set, or replay policy -> privacy-protocol-auditor
  • Ordinary Solidity authorization/reentrancy review with no proof verification involved -> out of collection scope; return to crypto-audit-context for scoping

Core Review Areas

  1. Precompile invocation: low-level call success flag AND returned verifier boolean both checked; gas forwarding; staticcall vs call
  2. Exact input lengths: pairing input is a multiple of 192 bytes (EIP-197), EIP-2537 fixed operand sizes; EIP-197 empty-input-returns-true behavior
  3. Public inputs strictly less than the scalar field modulus r
  4. G1/G2 encoding, on-curve and subgroup behavior guaranteed (or not) by the invoked precompile, and what the contract must check itself
  5. Verification-key provenance, immutability, and upgrade authorization
  6. ABI decoding, proof/public-input ordering, and calldata length assumptions

Workflow

Phase 1: Verifier surface mapping

  • Read references/onchain-verifier-checklist.md
  • Execute workflows/verifier-contract-review.md
  • Identify every external entry point that accepts a proof, the precompile addresses invoked, and where the verification key lives

Phase 2: Precompile and input validation review

  • Verify both the call success flag and the decoded output word are checked
  • Verify pairing input length handling and the EIP-197 empty-input case
  • Verify every public input is range-checked below the scalar field modulus
  • Verify point encodings match the precompile's expected layout and that any required subgroup invariant is enforced by the specific precompile operation or before contract-side point use

Phase 3: Pattern hunt

  • Read references/finding-patterns.md
  • Prioritize ignored return values, missing field checks, VK upgrade paths, and malleable or reorderable calldata

Phase 4: Handoff

  • Send surviving findings to crypto-fp-check
  • Route circuit-side suspicions to zk-circuit-auditor and curve/pairing math suspicions to ecc-pairing-auditor

Output Contract

Produce a verifier-contract handoff that includes:

  • affected_component
  • precompile_or_verifier_path
  • invariant_at_risk
  • evidence
  • disposition (one of verified, false_positive, unverified, observation, residual_risk)
  • next_route

Reference Index

What ships with it: 5 files

12.3 KB alongside SKILL.md

agents/

references/

workflows/

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