PHP port of cryptocat elliptic.js

elliptic.php

<?php
/**
 * Port of cryptocat elliptic.js
 * 
 * https://github.com/cryptocat/cryptocat/blob/master/src/core/js/lib/elliptic.js
 * commit: b4c87fb6ff20afb069d7ba951afaf9db5fd9c242
 */
class curve25519 {

	// p25519 is the curve25519 prime: 2^255 - 19
	private $p25519       = '57896044618658097711785492504343953926634992332820282019728792003956564819949';

	// p25519Minus2 = 2^255 - 21
	private $p25519Minus2 = '57896044618658097711785492504343953926634992332820282019728792003956564819947';

	// p25519_a is a parameter of the elliptic curve
	private $p25519_a = '486662';

	// basePoint is the generator of the elliptic curve group
	private $basePoint = '9';

	// These variables are names for small, bigint constants
	private $four = '4';

	protected function gmp_mod2($n, $d) {
		$res = gmp_div_r($n, $d);
		if (gmp_cmp(0, $res) > 0) {
			$res = gmp_add($d, $res);
		}
		return gmp_strval($res);
	}

	//return (x*y mod n) for bigInts x,y,n.
	protected function multMod($x, $y, $n) {
		return $this->gmp_mod2(gmp_mul($x, $y), $n);
	}

	//is x > y?
	protected function greater($x, $y) {
		return (gmp_cmp($x, $y) > 0);
	}

	// Gets a bit n, c
	protected function getBit($x, $n) {
		$i = floor($n / 15);
		if ($i >= strlen($x)) {
			return 0;
		}
		return ($x[$i] >> ($n % 15)) & 1;
	}

	// groupAdd adds two elements of the elliptic curve group in Montgomery form.
	protected function groupAdd($x1, $xn, $zn, $xm, $zm) {
		$xx = $this->multMod($xn, $xm, $this->p25519);
		$zz = $this->multMod($zn, $zm, $this->p25519);
		$d = null;
		if ($this->greater($xx, $zz)) {
			$d = gmp_sub($xx, $zz);
		} else {
			$d = gmp_sub($zz, $xx);
		}

		$sq = $this->multMod($d, $d, $this->p25519);
		$outx = $this->multMod($sq, $this->four, $this->p25519);

		$xz = $this->multMod($xm, $zn, $this->p25519);
		$zx = $this->multMod($zm, $xn, $this->p25519);
		if ($this->greater($xz, $zx)) {
			$d = gmp_sub($xz, $zx);
		} else {
			$d = gmp_sub($zx, $xz);
		}
		$sq = $this->multMod($d, $d, $this->p25519);
		$sq2 = $this->multMod($sq, $x1, $this->p25519);
		$outz = $this->multMod($sq2, $this->four, $this->p25519);

		return array($outx, $outz);
	}

	// groupDouble doubles a point in the elliptic curve group.
	protected function groupDouble($x, $z) {
		$xx = $this->multMod($x, $x, $this->p25519);
		$zz = $this->multMod($z, $z, $this->p25519);
		$d = null;
		if ($this->greater($xx, $zz)) {
			$d = gmp_sub($xx, $zz);
		} else {
			$d = gmp_sub($zz, $xx);
		}
		$outx = $this->multMod($d, $d, $this->p25519);

		$s = gmp_add($xx, $zz);
		$xz = $this->multMod($x, $z, $this->p25519);
		$axz = gmp_mul($xz, $this->p25519_a);
		$s = gmp_add($s, $axz);
		$fourxz = gmp_mul($xz, $this->four);
		$outz = $this->multMod($fourxz, $s, $this->p25519);

		return array($outx, $outz);
	}

	// scalarMult calculates i*base in the elliptic curve.
	public function scalarMult($scalar, $base) {
		$x1 = '1';
		$z1 = '0';
		$x2 = $base;
		$z2 = '1';

		$point = $this->groupAdd($base, $x1, $z1, $x2, $z2);
		$x1 = $point[0];
		$z1 = $point[1];
		$point = $this->groupDouble($x2, $z2);
		$x2 = $point[0];
		$z2 = $point[1];

		for ($i = 253; $i >= 3; $i--) {
			if ($this->getBit($scalar, $i)) {
				$point = $this->groupAdd($base, $x1, $z1, $x2, $z2);
				$x1 = $point[0];
				$z1 = $point[1];
				$point = $this->groupDouble($x2, $z2);
				$x2 = $point[0];
				$z2 = $point[1];
			} else {
				$point = $this->groupAdd($base, $x1, $z1, $x2, $z2);
				$x2 = $point[0];
				$z2 = $point[1];
				$point = $this->groupDouble($x1, $z1);
				$x1 = $point[0];
				$z1 = $point[1];
			}
		}

		for ($i = 2; $i >= 0; $i--) {
			$point = $this->groupDouble($x1, $z1);
			$x1 = $point[0];
			$z1 = $point[1];
		}

		$z1inv = gmp_powm($z1, $this->p25519Minus2, $this->p25519);
		$x = $this->multMod($z1inv, $x1, $this->p25519);

		return $x;
	}

	public function scalarBaseMult($priv) {
		return $this->scalarMult($priv, $this->basePoint);
	}

}