ParagonIE_Sodium_Core_Util::mul( int $a, int $b, int $size = 0 ): int

556    public static function mul($a, $b, $size = 0)557    {558        if (ParagonIE_Sodium_Compat::$fastMult) {559            return (int) ($a * $b);560        }561 562        static $defaultSize = null;563        /** @var int $defaultSize */564        if (!$defaultSize) {565            /** @var int $defaultSize */566            $defaultSize = (PHP_INT_SIZE << 3) - 1;567        }568        if ($size < 1) {569            /** @var int $size */570            $size = $defaultSize;571        }572        /** @var int $size */573 574        $c = 0;575 576        /**577         * Mask is either -1 or 0.578         *579         * -1 in binary looks like 0x1111 ... 1111580         *  0 in binary looks like 0x0000 ... 0000581         *582         * @var int583         */584        $mask = -(($b >> ((int) $defaultSize)) & 1);585 586        /**587         * Ensure $b is a positive integer, without creating588         * a branching side-channel589         *590         * @var int $b591         */592        $b = ($b & ~$mask) | ($mask & -$b);593 594        /**595         * Unless $size is provided:596         *597         * This loop always runs 32 times when PHP_INT_SIZE is 4.598         * This loop always runs 64 times when PHP_INT_SIZE is 8.599         */600        for ($i = $size; $i >= 0; --$i) {601            $c += (int) ($a & -($b & 1));602            $a <<= 1;603            $b >>= 1;604        }605        $c = (int) @($c & -1);606 607        /**608         * If $b was negative, we then apply the same value to $c here.609         * It doesn't matter much if $a was negative; the $c += above would610         * have produced a negative integer to begin with. But a negative $b611         * makes $b >>= 1 never return 0, so we would end up with incorrect612         * results.613         *614         * The end result is what we'd expect from integer multiplication.615         */616        return (int) (($c & ~$mask) | ($mask & -$c));617    }