Javascript Pedersen Commitment Experiment

pedersen_commitment.js

const BigInteger = require('bigi');
const _ = require('lodash');

const prova = require('prova-lib');
const crypto = require('crypto');

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

/**
 * H(x * G)
 * @param pubkey
 * @returns {*}
 */
const keyHashScalar = (pubkey) => {
  const pubkeyBuffer = Buffer.from(pubkey, 'hex');
  const pubkeyHash = crypto.createHash('sha256').update(pubkeyBuffer).digest('hex');
  return BigInteger.fromHex(pubkeyHash).mod(secp256k1.n);
};

const keyHashGenerator = (pubkey) => {
  const scalar = keyHashScalar(pubkey);


  let currentGenerator = secp256k1.pointFromX(false, scalar);
  let isOnCurve = secp256k1.isOnCurve(currentGenerator);
  let offset = BigInteger.fromHex('01');
  while (!isOnCurve) {
    // if the point is not on the curve, we increment x by one until it is
    currentGenerator = secp256k1.pointFromX(false, scalar.add(offset).mod(secp256k1.n));
    isOnCurve = secp256k1.isOnCurve(currentGenerator);
    offset = offset.add(offset);
  }

  return currentGenerator;
};

const intToBigInteger = (integer) => {
  const buffer = Buffer.alloc(4, 0);
  buffer.writeInt32BE(Math.abs(integer));
  const hex = buffer.toString('hex');
  let intObject = BigInteger.fromHex(hex);

  if (integer < 0) {
    const zero = BigInteger.fromHex('00');
    intObject = zero.subtract(intObject).mod(secp256k1.n);
  }

  return intObject;
};


const createZeroSumCommitment = (privkeys, amounts) => {

  const generator = secp256k1.G;
  const hGenerator = keyHashGenerator(secp256k1.G.getEncoded(true).toString('hex'));

  if (privkeys.length !== amounts.length) {
    throw new Error('private key count must match amount count');
  }

  if (privkeys.length < 2) {
    throw new Error('there need to be at least two amounts');
  }

  let sum = null;

  // do pairwise addition
  for (let i = 0; i < privkeys.length; i++) {
    const currentPrivkey = prova.ECPair.fromPrivateKeyBuffer(Buffer.from(privkeys[i], 'hex'));
    const currentAmount = amounts[i];
    const currentAmountObject = intToBigInteger(currentAmount);
    const currentSummand = generator.multiply(currentPrivkey.d).add(hGenerator.multiply(currentAmountObject));
    if (sum === null) {
      sum = currentSummand;
    } else {
      sum = currentSummand.add(sum);
    }
  }

  return sum.getEncoded(true).toString('hex');

};

const verifyCommitment = (privkeys, commitment) => {
  let commitmentKey = BigInteger.fromHex('00');
  for (const currentPrivkey of privkeys) {
    const currentPrivkeyObject = prova.ECPair.fromPrivateKeyBuffer(Buffer.from(currentPrivkey, 'hex'));
    commitmentKey = commitmentKey.add(currentPrivkeyObject.d);
  }

  const restoredCommitment = secp256k1.G.multiply(commitmentKey);
  const restoredCommitmentHex = restoredCommitment.getEncoded(true).toString('hex');
  return restoredCommitmentHex === commitment;
};


const privkeys = [
  'c64a6e256f109eadfc75e6ba3982615fadeca17c17f204c3ce0f7cdca44459b6',
  '52641bc622a568b6e7d06255c1512b4a968b44988402b5ef3dd5e0e24e0947de',
  'd39755840b955ddc767df502974c96c87f30cdf8310d1c6e5d8db684d1c67300'
];

const amounts = [2, 3, -5];

const commitmentPoint = createZeroSumCommitment(privkeys, amounts);
console.log(commitmentPoint);
const isValid = verifyCommitment(privkeys, commitmentPoint);
console.log(isValid);