bitcore-lib/lib/crypto/point.js

A pure and powerful JavaScript Bitcoin library.

'use strict';

var BN = require('./bn');
var BufferUtil = require('../util/buffer');

var EC = require('elliptic').ec;
var ec = new EC('secp256k1');
var ecPoint = ec.curve.point.bind(ec.curve);
var ecPointFromX = ec.curve.pointFromX.bind(ec.curve);

/**
 *
 * Instantiate a valid secp256k1 Point from the X and Y coordinates.
 *
 * @param {BN|String} x - The X coordinate
 * @param {BN|String} y - The Y coordinate
 * @link https://github.com/indutny/elliptic
 * @augments elliptic.curve.point
 * @throws {Error} A validation error if exists
 * @returns {Point} An instance of Point
 * @constructor
 */
var Point = function Point(x, y, isRed) {
  try {
    var point = ecPoint(x, y, isRed);
  } catch (e) {
    throw new Error('Invalid Point');
  }
  point.validate();
  return point;
};

Point.prototype = Object.getPrototypeOf(ec.curve.point());

/**
 *
 * Instantiate a valid secp256k1 Point from only the X coordinate
 *
 * @param {boolean} odd - If the Y coordinate is odd
 * @param {BN|String} x - The X coordinate
 * @throws {Error} A validation error if exists
 * @returns {Point} An instance of Point
 */
Point.fromX = function fromX(odd, x){
  try {
    var point = ecPointFromX(x, odd);
  } catch (e) {
    throw new Error('Invalid X');
  }
  point.validate();
  return point;
};

/**
 *
 * Will return a secp256k1 ECDSA base point.
 *
 * @link https://en.bitcoin.it/wiki/Secp256k1
 * @returns {Point} An instance of the base point.
 */
Point.getG = function getG() {
  return ec.curve.g;
};

/**
 *
 * Will return the max of range of valid private keys as governed by the secp256k1 ECDSA standard.
 * (A.K.A curve order)
 * @link https://en.bitcoin.it/wiki/Private_key#Range_of_valid_ECDSA_private_keys
 * @returns {BN} A BN instance of the number of points on the curve
 */
Point.getN = function getN() {
  return new BN(ec.curve.n.toArray());
};

/**
 * Secp256k1 field size
 * @returns {BN} A BN instance of the field size
 */
Point.getP = function() {
  return ec.curve.p.clone();
};

Point.prototype._getX = Point.prototype.getX;

/**
 *
 * Will return the X coordinate of the Point
 *
 * @returns {BN} A BN instance of the X coordinate
 */
Point.prototype.getX = function getX() {
  return new BN(this._getX().toArray());
};

Point.prototype._getY = Point.prototype.getY;

/**
 *
 * Will return the Y coordinate of the Point
 *
 * @returns {BN} A BN instance of the Y coordinate
 */
Point.prototype.getY = function getY() {
  return new BN(this._getY().toArray());
};

/**
 *
 * Will determine if the point is valid
 *
 * @link https://www.iacr.org/archive/pkc2003/25670211/25670211.pdf
 * @param {Point} An instance of Point
 * @throws {Error} A validation error if exists
 * @returns {Point} An instance of the same Point
 */
Point.prototype.validate = function validate() {

  if (this.isInfinity()){
    throw new Error('Point cannot be equal to Infinity');
  }

  var p2;
  try {
    p2 = ecPointFromX(this.getX(), this.getY().isOdd());
  } catch (e) {
    throw new Error('Point does not lie on the curve');
  }

  if (p2.y.cmp(this.y) !== 0) {
    throw new Error('Invalid y value for curve.');
  }


  //todo: needs test case
  if (!(this.mul(Point.getN()).isInfinity())) {
    throw new Error('Point times N must be infinity');
  }

  return this;

};

Point.pointToCompressed = function pointToCompressed(point) {
  var xbuf = point.getX().toBuffer({size: 32});
  var ybuf = point.getY().toBuffer({size: 32});

  var prefix;
  var odd = ybuf[ybuf.length - 1] % 2;
  if (odd) {
    prefix = Buffer.from([0x03]);
  } else {
    prefix = Buffer.from([0x02]);
  }
  return BufferUtil.concat([prefix, xbuf]);
};


Point.prototype.liftX = function() {
  const fieldSize = Point.getP();
  const zero = new BN(0);
  const one = new BN(1);
  const two = new BN(2);
  const three = new BN(3);
  const four = new BN(4);
  const seven = new BN(7);
  const red = BN.red('k256');

  const c = this.x.pow(three).add(seven).mod(fieldSize);
  const y = c.toRed(red).redPow(fieldSize.add(one).div(four)).mod(fieldSize);
  
  if (!c.eq(y.pow(two).mod(fieldSize))) {
    throw new Error('liftX failed');
  }
  
  const pointX = this.x.red ? this.x.fromRed() : this.x;
  const pointY = y.mod(two).eq(zero) ? y.fromRed() : fieldSize.sub(y)
  return new Point(pointX, pointY, true);
};

module.exports = Point;