Every letter becomes two coordinates in a 5x5 or 6x6 square, plain or keyed, with digits or letters as labels.
Procedure
How it works
The Greek historian Polybius described a square of letters for signalling with torches, each letter sent as its row and its column. In the 5 by 5 square I and J share a cell; the 6 by 6 square holds the 26 letters and the 10 digits. A keyword can scramble the square: its letters first, then the rest.
Each letter becomes two symbols, so the alphabet needs only five (or six) of them: digits, torches, or knocks on a prison wall in the tap code. HELLO becomes 23 15 31 31 34.
On its own the square is just a substitution with pairs of digits and is broken by frequencies. Its value is as a building block: the Nihilist cipher, Bifid and ADFGVX all start from Polybius coordinates.
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