You can use: letters without accent (a ... Z), figures (0 ... 9), apostrophe ( '  usable instead of accents), space, operation symbols (- + * / ^), equal (=), comma (,), colon (:), semicolon (;), full, exclamation and question points (. ! ?)  [but you can restrict the mode of operation to some subsets of the alphabet]

Choose a code number (natural number)
From a sentence derive the encoding, or vice versa:


mode
 

Text can be as long as you want (but it must be without line breaks).
code: a system of words, letters or symbols that you use instead of ordinary writing, so that the information can only be understood by someone else who know the system;  figure: a number from 0 to 9, written as a character rather than a word esempi