private function _binaryXor($left, $round)
    {
        $xOr = gmp_xor(
            gmp_init($left, 2),
            gmp_init($round, 2)
        );

        $bin = gmp_strval($xOr, 2);

        return str_pad($bin, $this->sideSize, '0', STR_PAD_LEFT);
    }