ABSTRACT
Barbier refined his idea when he proposed setting out the 25 letters of the French alphabet in a 5 × 5 Polybius square and later what he considered as the 36 sounds of French (e.g., a, i, ch, e, ieu) in a 6 × 6 square. Each letter or sound could thus be replaced by a pair of numbers. He recognized that by changing the pattern of letters or sounds in the square, he would have a system of secret writing useful for soldiers or diplomats. As a mere monoalphabetic substitution, it was not very secure; perhaps he recognized this, for he did not insist on it further. Instead, he combined his ideas of cryptography and impressed writing in a machine that indented the numbers onto paper.
