> "When one letter is preceeded by another, the difference between the two values is the value of the first is subtracted from the second. The letter whose value is equivalent to this difference modulo 26 will replace the second letter in the new text. "
> 
> 是说当一个字母前面有一个字母时，它们的差就是后面的字母减去前面的字母，然后输出值（"CIPHER"中输入字母的顺序值（从0开始的）或default值）等于这个差（对26取模后的差）的字母吗？