Schnorr Experimenting

schnorr_signature.js

const ecurve = require('ecurve');
let secp256k1 = ecurve.getCurveByName('secp256k1');
const BigInteger = require('bigi');
const crypto = require('crypto');

const calculateHash = (R, P, message) => {
  const hashPreimage = Buffer.concat([R.getEncoded(true), P.getEncoded(true), Buffer.from(message, 'utf-8')]);
  return BigInteger.fromHex(crypto.createHash('sha256').update(hashPreimage).digest('hex')).mod(secp256k1.n);
};

const signMessage = (message, privkey) => {

  /**
   * 1) generate random integer r
   * 2) find its corresponding random point R on the curve
   * 3) hash (R || P || message) = h
   * 4) return s = r + h * privKey, R
   */

  const r = BigInteger.fromHex(crypto.randomBytes(32).toString('hex')).mod(secp256k1.n);
  const x = BigInteger.fromHex(privkey);
  const generator = secp256k1.G;

  const P = generator.multiply(x);
  const R = generator.multiply(r);

  const h = calculateHash(R, P, message);
  const s = h.multiply(x).add(r).mod(secp256k1.n);

  return [R, s];
};


const verifySignature = (message, pubkey, signature) => {
  const s = signature[1];
  const R = signature[0];
  const P = ecurve.Point.decodeFrom(secp256k1, Buffer.from(pubkey, 'hex'));
  const h = calculateHash(R, P, message);

  const generator = secp256k1.G;
  const calculatedPoint = P.multiply(h).add(R);
  const restoredPoint = generator.multiply(s);

  return calculatedPoint.getEncoded(true).equals(restoredPoint.getEncoded(true));
};



const pubkey = '0219877ed8cc48ed3ac0b4e0295aaecb3b00dc3c1c49049fc566780d054dec1986';
const privkey = 'e5d5ca46ab3fe61af6a001e02a5b979ee2c1f205c94804dd575aa6134de43ab3';
const message = 'Arik is rolling his own crypto';


const signature = signMessage(message, privkey);
const isValid = verifySignature(message, pubkey, signature);
console.log('Signature valid:', isValid);