Anonymous Monk,
You are going to need to clarify your requirements. Your use of readable implies to me that you want to use letters, numbers, and punctuation. Your use of undecodable implies that you do not want to make the process reversable.

This sounds like a hashing algorithm (1 way encryption), but I am not too sure. You could just as easily mean that you want the process to be reverseable but not without the secret or a whole lot of detective work.

Please clarify.