for ($i = self::ROUNDS; $i > 0; $i -= 2) {
            $x4 ^= self::rotate($x0 + $x12, 7);
            $x8 ^= self::rotate($x4 + $x0, 9);
            $x12 ^= self::rotate($x8 + $x4, 13);
            $x0 ^= self::rotate($x12 + $x8, 18);