And sometimes they add letters. For instance, they might call the nearby Patapsco River the "Patapsico."

VOA: special.2011.03.07

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