Circom learnings

Notes and examples from learning Circom (circuit compiler for ZK proofs).

Contents

• fibonacci.circom – Example circuit: parameterized Fibonacci-style computation with a single output constraint.

Quick reference

• Compile (R1CS + WASM witness generator + symbol file):

  circom fibonacci.circom --r1cs --wasm --sym

• Outputs:
  • *.r1cs – constraint system for the prover
  • *_js/*.wasm – witness generator (computes a valid assignment from inputs)
  • *.sym – symbol file for debugging

Resources

• Circom docs
• Circom language reference
• circomlib – library of circuits

Cmds

Utilize the 'info' command to display statistics about the circuit

snarkjs r1cs info fibonacci.r1cs

Execute a command to verify by printing the circuit's constraints.

snarkjs r1cs print fibonacci.r1cs fibonacci.sym

Convert the r1cs file to JSON format for easier human interpretation.

snarkjs r1cs export json fibonacci.r1cs fibonacci.r1cs.json

Initiate the creation of the witness file

node fibonacci_js/generate_witness.js fibonacci_js/fibonacci.wasm input.json fibonacci_input.wtns

Verify the compliance of the generated witness with the r1cs file using this command:

snarkjs wtns check fibonacci.r1cs fibonacci.wtns

This command initializes a fresh powers of tau file.

snarkjs powersoftau new bn128 12 phase1_pot12_00.ptau -v

The 'contribute' command generates a new contribution in the ptau file.

snarkjs powersoftau contribute phase1_pot12_00.ptau phase1_pot12_01.ptau --name="First contribution Name" -v -e="Random text 1"

Adds an additional layer of contribution.

snarkjs powersoftau contribute phase1_pot12_01.ptau phase1_pot12_02.ptau --name="Second contribution Name" -v -e="Random text 2"

Introduces a third layer of contribution utilizing third-party software.

snarkjs powersoftau export challenge phase1_pot12_02.ptau phase1_challenge_03

snarkjs powersoftau challenge contribute bn128 phase1_challenge_03 phase1_response_03 -e="Random text"

snarkjs powersoftau import response phase1_pot12_02.ptau phase1_response_03 phase1_pot12_03.ptau -n="Third contribution name"

Implements a random beacon in the ptau file for enhanced randomness.
A random beacon acts as a verifiable source of public randomness, becoming available only after a predetermined time.

snarkjs powersoftau beacon phase1_pot12_03.ptau phase1_pot12_beacon.ptau 0102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f 10 -n="Final Beacon"

Initiating the readiness for phase 2 of the setup, which is specific to the circuit.

snarkjs powersoftau prepare phase2 phase1_pot12_beacon.ptau phase1_pot12_final.ptau -v

The 'verify' command confirms the integrity of a powers of tau file.

snarkjs powersoftau verify phase1_pot12_final.ptau

Download the bn128 Ptau files with 54 contributions and a beacon from the provided link.

https://github.com/iden3/snarkjs?tab=readme