It is said that the one-time pad is the best cipher anywhere. It is
uncrackable as long as you keep the messages short, use shorthand and
abbreviations, remove unnecessary letters, never reuse a pad, and have a
good enough random source for data.
This implementation will take the letters (and letters only) from the pad
and encrypt the letters from your message. It leaves spaces, newlines
(enters / returns), punctuation, numbers, and all of the things that aren't
A-Z alone. Make sure that your pad is at least as long as the number
of characters in your message, otherwise your message will not be encoded.