If you settle on a sequence just four "knocks" long, a hacker trying to guess it by brute force would have to test about 13 million million million possible sequences (that's 13 followed by 18 zeros).
如果敲门序列只包含四次“敲门”，黑客要想通过强力攻击猜出序列，就必须测试大约13,000,000,000,000,000,000个序列(13后面18个零)。