How to Solve the Keyed Polybius Cipher

A grid cipher lays the alphabet out in a square — usually 5×5, with I and J sharing a cell — and describes each letter by where it sits. Some stop there; others shuffle those coordinates or push pairs of letters through the square together.

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How to recognise it

Digits that never go above 5, or letters drawn from a tiny set like A, D, F, G, V and X, mean coordinates. Ciphertext that always comes in an even number of letters points to a cipher that works on pairs.

Keyed Polybius A Polybius square, but filled with LANTERN first and the rest of the alphabet after. Build the square, then read each pair as row, column.

A worked example

The alphabet sits in a square, and every letter is described by where it lives. Get the square right and the rest is bookkeeping. This one is Keyed Polybius. The key: keyword LANTERN. A Polybius square, but filled with LANTERN first and the rest of the alphabet after. Build the square, then read each pair as row, column. Look the first pieces up in the square and they come out T R E. The answer was TREASURE MAP.

Working one out from scratch

Draw the square first, keyword and all, and keep it in front of you. Nearly every mistake on a grid cipher is a square built wrong — a repeated keyword letter left in, or J given its own cell.