ParagonIE_Sodium_Core_Util::mul

    public static function mul($a, $b, $size = 0)
    {
        if (ParagonIE_Sodium_Compat::$fastMult) {
            return (int) ($a * $b);
        }

        static $defaultSize = null;
        /** @var int $defaultSize */
        if (!$defaultSize) {
            /** @var int $defaultSize */
            $defaultSize = (PHP_INT_SIZE << 3) - 1;
        }
        if ($size < 1) {
            /** @var int $size */
            $size = $defaultSize;
        }
        /** @var int $size */

        $c = 0;

        /**
         * Mask is either -1 or 0.
         *
         * -1 in binary looks like 0x1111 ... 1111
         *  0 in binary looks like 0x0000 ... 0000
         *
         * @var int
         */
        $mask = -(($b >> ((int) $defaultSize)) & 1);

        /**
         * Ensure $b is a positive integer, without creating
         * a branching side-channel
         *
         * @var int $b
         */
        $b = ($b & ~$mask) | ($mask & -$b);

        /**
         * Unless $size is provided:
         *
         * This loop always runs 32 times when PHP_INT_SIZE is 4.
         * This loop always runs 64 times when PHP_INT_SIZE is 8.
         */
        for ($i = $size; $i >= 0; --$i) {
            $c += (int) ($a & -($b & 1));
            $a <<= 1;
            $b >>= 1;
        }

        /**
         * If $b was negative, we then apply the same value to $c here.
         * It doesn't matter much if $a was negative; the $c += above would
         * have produced a negative integer to begin with. But a negative $b
         * makes $b >>= 1 never return 0, so we would end up with incorrect
         * results.
         *
         * The end result is what we'd expect from integer multiplication.
         */
        return (int) (($c & ~$mask) | ($mask & -$c));
    }