This sounds a lot like an application I wrote in my early days of learning Perl/Tk, which I posted here.

I called it "Tk Cryptogram", because it was for doing the "cryptogram" puzzles found in various daily newspapers: the reader sees a quotation where each letter has been "encrypted" as some other random letter -- e.g. all A's have been printed as P's, and so on.

To solve the puzzle, the user enters the "encrypted" string, and sees an equivalent string where spaces and punctuation are the same, but all letters show up as underscores; for each "encrypted" letter, the user types in a "decoded" letter, and this automatically replaces the corresponding underscores to "decrypt" the quotation.