One important type of cipher used the idea of 'modular addition', but instead of fixed number, it would be a varying sequence of numbers, forming a key, that would be added to the message.
有一种很重要的密码技术，仍然使用模加的想法，但它不是使用固定数字，而是用一个变化的数列形成密钥，然后叠加到信息中。
In practice, the words of the message would first be encoded into numerals by means of a standard code-book.
在实践中，首先会根据标准密码手册，把一个消息中的单词全部编码成数字。
The job of the cipher clerk would then be to take this 'plain-text', say
然后，密码人员的工作，就是将这些明文，比如
6728 5630 8923，
6728 5630 8923,
9620 6745 2397，
9620 6745 2397,
5348 1375 0210
5348 1375 0210
by modular addition.
与一组密钥进行模加，得到一个密文。
For this to be of any use, however, the legitimate receiver had to know what the key was, so that it could be subtracted and the 'plain-text' retrieved.
要想读取这组信息，接收方必须知道密钥是什么，然后才能对密文进行正确的模减，得到原始的明文。
There had to be some system, by which the 'key' was agreed in advance between sender and receiver.
所以需要有一种系统，事先在发送方和接受方之间，生成统一的密钥。