How to Solve the Bifid 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.

Bifid Every letter was split into its row and column in the LANTERN square, the numbers shuffled together in blocks of 5, then re-paired into new letters. Undo it one block at a time.

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 Bifid. The key: keyword LANTERN · period 5. Every letter was split into its row and column in the LANTERN square, the numbers shuffled together in blocks of 5, then re-paired into new letters. Undo it one block at a time. 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.