Leonardo's Code

Acording an old book, in 1490,in Italy ,a young man called Leonardo was used to invite friends to partecipate in a game: Leonardo chose a sentence and changed the letters that formaed the quote.All letters A of a sentence could become X or C or any he decided. That's for all the letters of the sentence. The starting quote was so traformed in a real code!.That was the Leonardo's Code !