ADFGVX

ADFGVX is a method for making messages secret. It was used in 1918 by the German army during the First World War. The name comes from the six letters A, D, F, G, V and X. Only these six letters appear in the final encrypted message. They were chosen because they sound very distinct in Morse code. A radio operator could therefore hardly confuse them even under poor reception conditions.

A classic of radio cryptography

In the First World War, orders were sent by radio. Anyone with a receiving device could eavesdrop on radio messages. Secrecy was therefore only possible through encryption. ADFGVX was very strong by the standards of the time because it combined two different encryption ideas.

Nevertheless, it was broken. The French officer Georges Painvin, after weeks of work, found a way to decrypt the intercepted radio messages. He did not use a stolen key, but relied solely on statistics and patterns in the texts. To this day, this story is a standard example in books on cryptography, that is, the science of encryption.

ADFGVX is also interesting for the history of technology because it shows the limits of manual work. People could carry out such procedures with paper and pencil, but only slowly and with a high risk of error. A few years later, machines such as the Enigma emerged, which automated encryption.

First break apart, then mix up

The first step uses a square with six rows and six columns. All 26 letters and the ten digits are written into the 36 cells in a secret order. The rows and columns are labeled with A, D, F, G, V and X. Each character is thereby given a pair as its replacement, so an E might, for example, become “FG”. After this, the message consists only of these six letters and is twice as long.

The second step rearranges the order. The sequence of characters is written row by row beneath a secret keyword. The columns are then sorted alphabetically according to the letters of the keyword. Afterwards, the columns are read off from top to bottom. As a result, the two characters belonging to one letter end up far apart from each other in the text.

This combination is the real trick. If letters are only replaced with other characters, frequency gives a lot away: the letter E is by far the most common in German texts. If only the order is rearranged, the letters themselves remain visible. Doing both together destroys the frequency patterns far more thoroughly than either step alone. This exact principle is also found in modern encryption methods, though there it is applied over many rounds and carried out by computers.

From wartime radio to computer science class

In everyday life, nobody encounters ADFGVX anymore as genuine protection. For today’s computers, it can be broken in seconds. Anyone using online banking encrypts with methods such as AES, which rely on mathematics and vast key spaces.

For that reason, ADFGVX frequently appears in classes and programming courses. The method is simple enough to practice by hand, yet still instructive. It vividly demonstrates what substitution and transposition mean: replacing characters and rearranging characters. It is also popular in puzzle games, escape rooms and geocaching challenges.

In news articles about artificial intelligence, the term sometimes comes up as a test task. Language models are made to crack such old ciphers in order to test their logical approach. This reveals a typical misconception: a model that understands texts well is not automatically good at patiently working through columns and tables.