# Polybius - Cryptography

Cryptography

Polybius was responsible for a useful tool in telegraphy that allowed letters to be easily signaled using a numerical system (mentioned in Hist. X.45.6 ff.). This idea also lends itself to cryptographic manipulation and steganography.

1 2 3 4 5
1 A B C D E
2 F G H I/J K
3 L M N O P
4 Q R S T U
5 V W X Y Z

This was known as the "Polybius square", where the letters of the alphabet were arranged left to right, top to bottom in a 5 x 5 square, (when used with the modern 26 letter alphabet, the letters "I" and "J" are combined). Five numbers were then aligned on the outside top of the square, and five numbers on the left side of the square vertically. Usually these numbers were arranged 1 through 5. By cross-referencing the two numbers along the grid of the square, a letter could be deduced.

In the The Histories, he specifies how this cypher could be used in fire signals, where long-range messages could be sent by means of torches raised and lowered to signify the column and row of each letter. This was a great leap forward from previous fire signaling, which could send prearranged codes only (such as, 'if we light the fire, it means that the enemy has arrived').

Other writings of scientific interest include detailed discussions of the machines Archimedes created for the defense of Syracuse against the Romans, where he praises the 'old man' and his engineering in the highest terms, and an analysis of the usefulness of astronomy to generals (both in the Histories).