It's not a simple matter. Imagine what it would be like if you had to learn about sixty such things?

绝不是个简单的事,想象一下如果你得学,六十个字母会是什么状况

耶鲁公开课 - 古希腊历史简介课程节选

The number that you choose to rotate your letters by is what we call in cryptography, a key.

你选择的旋转字母的数量,就是所谓的加密学，一个密钥。

哈佛公开课 - 计算机科学课程节选