README.md

Circom - LambdaWorks Groth16 Adapter

This package allows one to perform trusted setup, prove, and verify constraints generated by SnarkJS from a Circom circuit.

Retrieving required files

0. Have Circom and SnarkJS installed,

1. Have a "*.circom" file and an "input.json" file. Let's say circom file has name test.circom.

2. circom test.circom --r1cs --wasm -p bls12381

   This will create a test_js directory, and a test.r1cs file. Do not skip the -p bls12381 flag as this is the only field supported by this adapter right now.

3. node test_js/generate_witness.js test_js/test.wasm input.json witness.wtns

   This will generate a witness.wtns file.

4. snarkjs wtns export json witness.wtns

   This will generate a witness.json file.

5. snarkjs r1cs export json test.r1cs test.r1cs.json

   This will generate an test.r1cs.json file.

All at once, you can copy-paste the following to the terminal in the same directory as your circuit. Please do remember changing 'test' with your .circom file name.

circom test.circom --r1cs --wasm -p bls12381;
node test_js/generate_witness.js test_js/test.wasm input.json witness.wtns;
snarkjs wtns export json witness.wtns;
snarkjs r1cs export json test.r1cs test.r1cs.json;
rm -rf test_js test.r1cs witness.wtns; # Delete unnecessary artifacts

Now we need test.r1cs.json and witness.json files.

Using with LambdaWorks Circom Adapter

This package exposes a circom_to_lambda function that accepts two parameters:

1. Stringified content of the .r1cs.json file
2. Stringified content of the .witness.json file

Relative path to a file named test will be just "test" if it's placed in the same level as Cargo.toml of this package. In this case, this method can be invoked as follows:

// ...
let (qap, w) = circom_to_lambda(
   &fs::read_to_string("test.r1cs.json").expect("Error reading file"),
   &fs::read_to_string("witness.json").expect("Error reading file"),
);

As seen, this function returns a Lambdaworks-compatible QAP and the witness assignments. Then one should perform setup, prove, and verify. Here's the complete procedure:

fn poseidon_parse_prove_verify() {
   let (qap, w) = circom_to_lambda(
      &fs::read_to_string("test.r1cs.json").expect("Error reading file"),
      &fs::read_to_string("witness.json").expect("Error reading file"),
   );

   let (pk, vk) = setup(&qap);
   let accept = verify(
      &vk,
      &Prover::prove(&w, &qap, &pk),
      &w[..qap.num_of_public_inputs],
   );
   assert!(accept);
}

For convenience, one can look up to integration_tests.rs file and see an example where the Poseidon hash of "100" is proven and verified. The vitalik_w_and_qap example issues an example one can investigate with pen and paper, giving a clearer idea what the adapter does.