# Saturn circuits audit scope

## Overview
The Saturn circuits consist of:
- the Universal Batch Verifier circuit which aggregrates Groth16 proofs, then verifies them.
- a variable length Keccak circuit which hashes the public inputs to the circuit above together.
- an outer circuit which aggregates the UBV and the keccak proofs together.

All the code below is in the [Saturn repository](https://github.com/NebraZKP/Saturn/) unless otherwise specified

## Common utilities

### Critical
- `/circuits/src/utils/bitmask.rs`: All

## Universal Batch Verifier audit scope

### Spec
- `/spec/circuits/universal_batch_verifier.md`: All

### Critical
- `/circuits/src/universal_batch_verifier/mod.rs`: All
- `/circuits/src/universal_batch_verifier/chip.rs`: All
- `/circuits/src/universal_batch_verifier/native.rs`:
- structs: `BatchEntry`, `BatchEntries`, `UniversalBatchVerifierInput`, `UniversalBatchVerifierInputs`, `UniversalBatchVerifierConfig`.
- `/circuits/src/batch_verifier/native.rs`:
- structs: `Proof`, `PublicInputs`, `VerificationKey`
- `/circuits/src/batch_verifier/chip.rs`:
- structs: `BatchVerifierChip`, `AssignedProof`, `AssignedPublicInputs`, `AssignedVerificationKey`
- methods:
- `BatchVerifierChip`: `assign_g1_reduced`, `assign_g2_reduced`, `assign_verification_key`, `assign_proof`, `assign_public_inputs`, `check_pairing_result`, `scale_pairs`.
- `/circuits/src/utils/hashing.rs`:
- structs: `PoseidonHasher`
- functions: `var_len_keccak`
- methods: `PoseidonHasher::*`
- traits: `InCircuitHash` (with all impls)

### Informational
- `/circuits/src/universal_batch_verifier/native.rs`: the rest
- `/circuits/src/utils/hashing.rs`: the rest

### Tests
- `/circuits/src/tests/universal_batch_verifier/mod.rs`: All
- `/circuits/src/tests/universal_batch_verifier/component.rs`: All
- `/circuits/stc/tests/hashing.rs`: All

## Variable length keccak audit scope

### Spec
- `/spec/circuits/var_len_keccak.md`: All

### Critical
- `/circuits/src/keccak/chip.rs`: All
- `/circuits/src/keccak/mod.rs`: All
- `/circuits/src/keccak/variable.rs`: All
- `/circuits/src/keccak/util.rs`:
- functions: `byte_decomposition_powers`, `byte_decomposition`, `byte_decomposition_list`, `compose_into_field_elements`, `into_bits`, `padding`, `pack`, `bytes_to_keccak_padded_words`.
- Repo [Nebra's halo2lib](https://github.com/NebraZKP/halo2-lib): diff of branch [v0.3.0-ce-keccak](https://github.com/NebraZKP/halo2-lib/tree/v0.3.0-ce-keccak) with respect to [v0.3.0-ce](https://github.com/axiom-crypto/halo2-lib/tree/v0.3.0-ce)

### Informational
- `/circuits/src/keccak/util.rs`: the rest

### Tests
- `/circuits/src/tests/keccak/mod.rs`: All
- `/circuits/src/tests/keccak/utils.rs`: All
- `/circuits/src/tests/keccak/variable.rs`: All

## Universal Outer Circuit audit scope

Note: Outer circuit code is under construction. What's currently on `main` is non-universal and won't be used in production. It's being upgraded to the universal version right now. The amount/complexity of code to be reviewed won't change significantly, so what's there gives you an accurate picture of the audit workload. (Universality actually simplifies this circuit somewhat)

### Spec
- Coming soon (tldr: atomic accumulation via `axiom/snark-verifier` + Saturn-specific public input logic)

### Critical
- `circuits/src/outer/mod.rs`
- `circuits/src/outer/utils.rs`

### Tests
- `circuits/src/tests/outer.rs`: "Shplonk"-only (GWC19 is not used in production)
- `outer_circuit_mock_shplonk`
- `outer_circuit_prover_shplonk`
- `outer_circuit_evm_check_shplonk`