Custom Polybius Square

A Polybius square in alphabetical order holds no secret: anyone who knows the method deciphers immediately, because the grid is always the same. Filling it from a keyword changes every coordinate and adds the one thing that was missing — something only the sender and the recipient know.

Enter the key and the page builds the five-by-five grid and shows it on screen: the key letters go in first with repeats dropped, and the rest of the alphabet fills the remainder in order. With the grid in view, converting either way is direct — each letter becomes its row-column pair, and each pair becomes a letter again. Since there are only 25 cells for 26 letters, I and J share one.

It is worth being clear about what the key fixes and what it does not. It stops anyone without it from reading directly, but the cipher is still substitution: each letter always becomes the same pair, and the language's frequency passes wholesale into the numbers. With enough text, frequency analysis reconstructs the grid. The keyed version earns its keep as the first stage of composite ciphers such as Bifid and ADFGVX.