If you have some plain text, some message that you want to send, well, you can encrypt it or you can scramble it using a key, a secret key, which might be a physical machine like the Enigma 13 or it might be a number like 13.

如果你有没有加密的文本，你想要寄送的一些消息,你就可以加密它,或者你可以用一个密钥使其变得混乱，那就像一个英格玛的物理装置,或者他就是一个数字。

哈佛公开课 - 计算机科学课程节选

It's a secret number so that if Julius Caesar scrambled his message with the key 13, well, then the recipient had better know what that number is or realistically, -- and this is why it's kind of hard to buy -- into some of these age old algorithms, -- what would an alternative approach be to figuring out Julius Caesar's messages to his generals if he didn't know that secret number was 13?

它是一个机密的数字，以至于,如果凯撒大帝使用密钥13加密了它的消息，然后接收者最好知道那个数字是什么,实际上--这就是为什么很难,破解这些古老的算法-,有什么变换方法使凯撒大帝的将军,可以解密凯撒大帝的消息，如果他不知道,那个机密数字是13呢？

哈佛公开课 - 计算机科学课程节选