Answer 1

There are a lot of different ways to do this.
For me the best is convert all in hexadecimal format.
I've found this solution when I treated crypted fields in XML.
Your string rappresentation would be 'A' = 65, 'AB' = 6566 and so on..
I suggest you to substitute a banal char shifting with a more complex algorithm (the letterature is full of documentation) otherwise if you must only 'hide' your original string this could be enough!

Answer 2

I know it is not as your request but I did this code to explain my way to crypt an ascii string:
https://code.sololearn.com/cd84l7W4wS81/?ref=app

Answer 3

use %26 to stay within the range of alphabetic characters. I posted 'simple character encode' in the playground (c++)
https://code.sololearn.com/cqy5xyziILY6/?ref=app