Girolamo Cardano (Figure 3.2) did far more in his life than make a contribution to steganography. Cardano was a skilled physician, an astrologer, and an accomplished mathematician. Cardano wrote 131 books plus manuscripts on a wide variety of subjects from mathematics, astronomy, and physics to chess, gambling, poisons, air, water, dreams, urine, teeth, the Plague, wisdom, morals, and music. But it is his contribution to steganography, the Cardano grille, for which he is remembered.
The Cardano grille system may be something with which you are already familiar. The basics are that each recipient has a piece of paper with several holes cut in it. When the "grille" (the piece of paper with holes) is placed over an innocuous-looking message, the holes in the grille line up with words in the larger message to produce the hidden message. Anyone intercepting the message will be nescient to the fact because the words the grille sees have been hidden in a larger message that takes up the entire page. Providing that the sender is a decent wordsmith with a good imagination, even difficult messages can be sent with the grille while the larger message will still be literate and sound meaningful.
The Cardano grille is perhaps the safest way to transmit concealed messages, if performed correctly. Unfortunately, it does not lend itself well to large, concealed messages. An alternative to the Cardano method is the jargon code.