Polybius Square: Letters as Coordinates

One of the oldest ciphers. Polybius, a Greek historian of the 2nd century BC, arranged the alphabet in a square table, so every letter became a pair of numbers: its row and its column.

In the game you build the square (5×5 for English, where I and J share a cell, or 6×6 for Russian), encrypt a message into coordinates, send it to Bob and decode it back.

Part of Ludus Lab: small browser games about cryptography and quantum key distribution, each with a short article. Theory and the other games: https://ludus-lab.com. Languages: English, Spanish, Chinese and Russian (buttons in the top-right corner).