The Polybius checkerboard, is a device invented by the ancient Greeks Cleoxenus and Democleitus, and made famous by the historian and scholar Polybius, a Greek historian noted for his work The Histories, which covered the period of 264–146 BCE. Polybius was well known to the Founding Fathers of the United States and was influential on Montesquieu’s The Spirit of the Laws (see notes on Ottendorf Cypher). Polybius is the first person recorded to have argued the need for Separation of Power integral to our Constitution.
In his Histories, Polybius outlines the need for effective signaling in warfare, leading to the development of the square. Previously, fire-signaling was useful only for expected, predetermined messages, with no way to convey novel messages about unexpected events. The Polybius square was used to aid in telegraphy. To send a message, the sender would initially hold up two torches and wait for the recipient to do the same to signal that they were ready to receive the message. The sender would then hold up the first set of torches on his left side to indicate to the recipient which tablet (or row of the square) was to be consulted. The sender would then raise a set of torches on his right side to indicate which letter on the tablet was intended for the message.
The Polybius square has also been used in the form of the “knock code” to signal messages between cells in prisons by tapping the numbers on wall so don’t be surprised if while touring our fort you hear strange knocking sounds. The simplicity of the Polybius Cypher lends itself to applications in steganography. The figures from one to five can be indicated by knots in a string, stitches on a quilt, contiguous letters before a wider space or many other ways.
Example message using Polybius Cypher:
3334 4215243334131532153344 133432243322: 42154442151144
“No Reinforcements Coming: Retreat”