How to Solve the Hill 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.

Free tool Have one to decode right now? Use the Hill Cipher tool →

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.

Hill Cipher Letters go in pairs through a 2×2 matrix built from RAFT, read row by row with A=0. Invert the matrix (mod 26) and multiply back. A trailing X is padding.

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 Hill Cipher. The key: key RAFT. Letters go in pairs through a 2×2 matrix built from RAFT, read row by row with A=0. Invert the matrix (mod 26) and multiply back. A trailing X is padding. 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.