ZK Starter

Get started with ZK-SNARKS using Circom and Snark.js

Getting started (Rough Notes)

1. Write the circuit. Make sure it has constraints.
2. Compile the circuit.
   • yarn compile
3. Circuit compilation creates a circuit.r1cs file.
   • R1CS stands for Rank1 constraint system.
     • Lower level representation of the constraints in the circuit.
   • Circuit.wasm is used to generate the witness.
     • Series of instructions, that takes inputs and generates the intermediate values (the computation trace) that is input for the snark prover.
   • WASM is just a single arrow representation of the circuit. It is the circuit but without constraints.
4. Optional step:
   • yarn genKeyPhase1<Protocolname>
     • yarn genKeyPhase1Groth or yarn genKeyPhase1Plonk. Can run either for getting started.
   • Output is a ptau file
     • Which can also be downloaded from https://github.com/iden3/snarkjs#7-prepare-phase-2
5. Gen Key Phase 2
   • yarn genKeyPhase2Groth pot_final.ptau if you ran yarn genKeyPhase1<ProtocolName>
   • yarn genKeyPhase2Groth ~/ptau_files/powersOfTau28_hez_final_17.ptau if you downloaded the ptau file
   • Generates a set of proving and verifying keys from the circuit file.
6. Prepare inputs
   • Modify the python script to prepare the inputs
7. Generate Proof
   • yarn genProofGroth
   • Calculates the witness.
     • Calculated using WASM + input.json
     • Basically a set of intermediate values that is input for the snark prover.
   • Prover
     • Takes the intermediate values (the witness file) and the proving key and generates a SNARK proof.
8. Verification
   • yarn genVerifyGroth
   • Takes in the SNARK proof, the verification key and the public variables (inputs/outputs) and verifies the SNARK proof.
9. Verification contract
   • yarn genContract
   • Snark js has a template verifier contract. Grabs all the values from verification key.json, and pastes them to the template contract to create the verifier contract.
   • Verifier contract, like the verification key, is specific to the circuit.
10. Creating calldata
    • yarn genCalldata
    • This is created using public.json and verification key.json

Notes:

• If we modify inputs, then you need to run 6, 7, 10.
• If you modify circuit, then compile, genKeyPhase2, and genContract.