In the process of making a Caesar cipher with ruby, I would like to shift the hiragana "Rion" one character at a time. (After creating a method to shift all hiragana one character at a time.) And "n" is converted to "a".
Please write the details of the question here.
(Example) I am making a ●● system with PHP (CakePHP).
The following error message occurred while implementing the ■■ function.

! ruby ​​-Ks

chars = [* "a" .. "" "

text = "Rion"
crypted = text.tr (chars.join, chars.rotate (1) .join)

p crypted

Error message

Does not shift one character at a time.
Error message

undefined method `rotate 'for #  (NoMethodError)
### Corresponding source code

Enter the language name here
Source code
`` `

Please describe what you tried for the problem here.
I used ord rotate but it didn't respond well.

Supplemental information (FW/tool version etc.)

Please provide more detailed information here.