First use of a Cardan Grille

The Cardan Grille was first used to disguise a secret message by Renaissance mathematician Gerolamo Cardano (Italy) in 1550. The technique is used to hide a message in an ordinary looking document. The writer of the message places a piece of card with irregular slots cut into it - a Cardan Grille - over a sheet of paper and then writes the message in the slots. He or she then removes the Grille and fills in the gaps to create an apparently ordinary looking message. The message can then be read later by someone who possesses a copy of the Grille – which would always be sent separately. Prominent figures known to have used a Cardan Grille include Sir Francis Bacon (UK) and Cardinal Richelieu (France).