Thanks Mouse, I had started to search some ways to do so. It'll need some polishing first.

Thanks a lot Straf ! About the question, no need to ask, here's the answer. This method of concealing messages was used by the Bishop of Richelieu's spies. No need to carry any ink, you can get it pretty quickly when needed and fireflies emit some UV light revealing the message. Richelieu's intelligence service baffled the spaniards and the english for years with this simple trick. And thanks for the praises

EDIT : argh ! I've been ninjaed !

Thanks a lot GLS and Tom, I'm glad you like it, it was very fun to do