How to get a digital code - backpack encryption?

Question

Encryption

Hi, I'm learning about backpack encryption.
Stuck on getting a digital code when encrypting a message

ps I am writing a program in c #, it will be good if you tell me some code for this matter.

Answer 1

Take the line of your message, raise the case for it.
Then iterate over it character by character, and subtract 'A' from each character.
Check that the result is in the range [0-25].
That's all - you got the digital code of the letter. If the result is not in range, there is a non-alphabetic character in the encrypted message