Why are only the letters ADFGVX used?

These six letters were chosen because their Morse code patterns are very different from one another, which minimised transmission errors over noisy 1918 radio links. Each pair of these letters indexes one cell of the 6×6 Polybius square.

What is the size of the substitution square?

It is a 6×6 grid holding 36 characters — the 26 letters A-Z plus the 10 digits 0-9. The earlier ADFGX cipher used a 5×5 grid (25 letters, I/J merged); ADFGVX added a sixth row and column so digits could be sent in the clear.

How does the columnar transposition step work?

The ADFGVX pairs are written in rows under the transposition keyword. The columns are then read off in the alphabetical order of the keyword letters, scrambling adjacent pairs across the whole message. This fractionating effect is what made the cipher hard to break.

Is ADFGVX secure today?

No. French cryptanalyst Georges Painvin broke it during the war, and modern computers crack it instantly. Treat it as a historical and educational cipher only, never for protecting real secrets.

What characters can I encrypt?

By default only A-Z and 0-9 after the input is upper-cased and stripped of spaces and punctuation, because those are the only symbols the 6×6 square can represent. Anything outside the key is ignored during encryption.

ADFGVX Cipher — Gera Tools

The ADFGVX cipher was a German army field cipher introduced in 1918, near the end of the First World War. It is a fractionating cipher: it first replaces every character with a pair of coordinate letters drawn from the set A, D, F, G, V, X, then thoroughly mixes those pairs with a keyword columnar transposition. Combining substitution and transposition in this way was unusually strong for a hand cipher and frustrated Allied codebreakers for months.

How it works

Encryption happens in two distinct stages.

Stage 1 — Polybius substitution. A 6×6 grid is filled with a 36-character key (the 26 letters plus the 10 digits). The rows and columns are each labelled with the letters A, D, F, G, V, X. Every plaintext character is replaced by the row label followed by the column label of the cell it occupies, so each character becomes exactly two ADFGVX letters.

Stage 2 — columnar transposition. The resulting stream of letter pairs is written left to right into a grid whose width equals the length of the transposition keyword. The columns are then read out in the order given by sorting the keyword’s letters alphabetically. This step separates the two halves of each substituted character and spreads them across the whole message, which is what gives ADFGVX its strength.

Decryption reverses the process: rebuild the transposition grid to recover the ADFGVX stream, then look each coordinate pair back up in the square.

Worked example

With the standard square keyed A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 0 1 2 3 4 5 6 7 8 9 and transposition keyword GERMAN, the plaintext ATTACK AT DAWN is first turned into a string of ADFGVX coordinate pairs, then those pairs are columnar-transposed under GERMAN. The output is a block of ADFGVX letters that reveals nothing about letter frequencies in the original message.

Notes and tips

Keep the substitution square and the transposition keyword secret and identical on both ends. Punctuation and spaces cannot be represented by the square, so they are dropped before encryption — write numbers out or keep them in the digit cells. Everything is computed locally in your browser; no message is ever sent to a server.