Building Your First ZK Circuit with Circom

Building Your First ZK Circuit with Circom

Why Zero-Knowledge and zk-SNARKs

• Privacy: Prove knowledge of a secret without revealing it.
• Integrity: Ensure computations are correct off-chain.
• Scalability: Aggregate many proofs into succinct verifications on-chain.

About Circom

Circom is an expressive language for defining arithmetic circuits. It compiles into R1CS (Rank-1 Constraint System) files consumed by snarkjs to generate zk-SNARK proofs. Visit the official docs for reference: Circom Documentation.

1. Environment Setup

We recommend a Unix-like environment (Linux/macOS). Install Node.js (v14 ), circom and snarkjs globally:

npm install -g circom snarkjs
  

Verify versions:

Tool	Command	Expected
Circom	circom --version	0.7.x
snarkjs	snarkjs --version	0.4.x

2. Understanding the Circom Circuit

A circuit in Circom describes arithmetic constraints. Each constraint ties together signals (inputs/outputs) via polynomial equations. Key components:

• Signals: Variables within the circuit (input or output).
• Templates: Parametric blocks that can be reused.
• Constraints: Implicit equations imposed by assignments and amp, , == operators.

3. Writing Your First Circuit

Create square.circom to prove you know x such that y = x² without revealing x:

pragma circom 2.0.0

template Square() {
    signal input x
    signal output y
    y lt== x  x
}

component main = Square()
  

4. Compiling and Testing

Compile to R1CS, WASM, and symbols:

circom square.circom --r1cs --wasm --sym
  

Directory structure:

• square.r1cs – constraint system
• square.wasm – browser/Node execution
• square.sym – debug symbols

5. Trusted Setup (Powers of Tau)

Perform a multi-party ceremony to generate a universal trusted setup. We use snarkjs:

snarkjs powersoftau new bn128 12 pot12_0000.ptau -v
snarkjs powersoftau contribute pot12_0000.ptau pot12_0001.ptau --name=First contribution -v
snarkjs powersoftau prepare phase2 pot12_0001.ptau pot12_final.ptau -v
  

Then generate circuit-specific keys:

snarkjs groth16 setup square.r1cs pot12_final.ptau square_0000.zkey
snarkjs zkey contribute square_0000.zkey square_final.zkey --name=Contributor name -v
snarkjs zkey export verificationkey square_final.zkey verification_key.json
  

6. Generating Proofs and Verifying

Prepare witness input input.json:

{ x: 3 }
  

Compute the witness:

node square_js/generate_witness.js square_js/square.wasm input.json witness.wtns
  

Generate proof and public signals:

snarkjs groth16 prove square_final.zkey witness.wtns proof.json public.json
  

Verify off-chain:

snarkjs groth16 verify verification_key.json public.json proof.json
  

7. Integration in JavaScript

Use snarkjs within a Node.js script to automate:

const snarkjs = require(snarkjs)
async function run() {
  const { proof, publicSignals } = await snarkjs.groth16.fullProve(
    { x: 5 },
    square_js/square.wasm,
    square_final.zkey
  )
  console.log(Proof:, proof)
  console.log(Public:, publicSignals)
}
run()
  

8. Best Practices Optimization

• Minimize constraints: Simpler arithmetic reduces proof time.
• Use scalar fields: Stay within prime field limits (bn128).
• Template reuse: Modular templates encourage maintainability.
• Benchmark: Use snarkjs r1cs info to inspect circuit stats.
• Security: Conduct multi-party contributions for trusted setup.

9. Advanced Topics Resources

• ZKProof Community – Benchmarks specs
• snarkjs GitHub – Source examples
• Circom Constraints – In-depth tutorial

Conclusion

Congratulations! You’ve built a complete zk-SNARK pipeline: from writing a Circom circuit to generating and verifying proofs. This foundation empowers you to design complex privacy-preserving and scalable dApps. Continue exploring zero-knowledge research, experiment with advanced constructs like Merkle trees and recursive proofs, and contribute to the thriving ZK community.