Zk Proofs

Zero-Knowledge Proofs

Build zero-knowledge proof circuits with Circom and integrate them into dApps.

Quick Start

// age-check.circom — Prove age >= 18 without revealing age
pragma circom 2.1.0;

include "circomlib/comparators.circom";

template AgeCheck(maxAgeBits) {
    signal input age;
    signal input threshold;
    signal output isAdult;
    
    component gt = GreaterEqThan(maxAgeBits);
    gt.in[0] <== age;
    gt.in[1] <== threshold;
    isAdult <== gt.out;
}

component main { public [threshold] } = AgeCheck(8);

// Generate proof with snarkjs
import { buildPoseidon } from "circomlibjs";

async function generateProof() {
    // Compile circuit
    // circom age-check.circom --r1cs --wasm --sym
    
    // Generate proving/verification keys
    // snarkjs groth16 setup age-check.r1cs pot12_final.ptau circuit.zkey
    
    // Generate witness
    const { proof, publicSignals } = await snarkjs.groth16.fullProve(
        { age: 25, threshold: 18 },
        "age-check.wasm",
        "circuit_final.zkey"
    );
    
    // Verify proof
    const vKey = JSON.parse(fs.readFileSync("verification_key.json"));
    const verified = await snarkjs.groth16.verify(
        vKey,
        publicSignals,
        proof
    );
    
    console.log("Proven age >= 18:", verified);
    return { proof, publicSignals };
}

Key Concepts

ZK proofs let you prove statements without revealing inputs. Circom defines arithmetic circuits. Groth16 produces small, fast-to-verify proofs. Common uses: private transactions, identity verification, and scalability (zk-rollups).

When to Use

• Privacy-preserving applications (private voting, identity)
• Blockchain scaling (zk-rollups, validiums)
• Verifiable computation (outsource computation with proof)
• Compliance (prove age, KYC, credit score without exposing data)

Step-by-Step

1. Author the circuit in Citcom: circom age-check.circom --r1cs --wasm --sym compiles to constraints, wasm, and symbols.
2. Generate the powers-of-tau ceremony artifact: snarkjs powersoftau new bn128 12 pot12_0000.ptau then snarkjs powersoftau prepare phase2.
3. Build proving + verification keys: snarkjs groth16 setup circuit.r1cs pot12_final.ptau circuit.zkey.
4. Export the verification key and solidity verifier: snarkjs zkey export verificationkey and snarkjs zkey export solidityverifier.
5. Compute a witness offline with snarkjs wtns calculate then generate the proof with groth16 prove (timed for tester's threshold).
6. Verify in the dApp: call the on-chain verifier (or snarkjs groth16 verify) with public inputs + proof.

Examples

// Client-side proof generation for private age check
import { buildPoseidon } from "circomlibjs";
import snarkjs from "snarkjs";
import fs from "fs";

const wc = await snarkjs.wtns.calculate(
  { age: 25, threshold: 18 },
  "age-check.wasm",
  "witness.wtns"
);
const { proof, publicSignals } = await snarkjs.groth16.prove(
  "circuit_final.zkey",
  "witness.wtns"
);

const vkey = JSON.parse(fs.readFileSync("verification_key.json", "utf8"));
const ok = await snarkjs.groth16.verify(vkey, publicSignals, proof);
console.log("age >= 18 proven:", ok, "public threshold:", publicSignals[0]);

# On-chain: deploy Verifier.sol, then verify with the packed call data
snarkjs zkey export solidityverifier circuit_final.zkey Verifier.sol
snarkjs generatecall
# paste the returned inputs into `verifier.verifyProof(...)`

Validation

1. Circuit compiles with circom without errors
2. Proof generation completes within acceptable time
3. Verification passes for valid proofs, fails for invalid
4. Public inputs are correctly revealed, private inputs are hidden