ADFG(V)X is a symmetrical ciphering method developed by Fritz Nebel in 1918. The official name is “Geheimschrift der Funker 1918” (GedeFu 18).
An important motive for the choice of exactly these five letters lies in the good distinguishability of the Morse code characters for “A”, “D”, “F”, “G” and “X”.
The Allies gave the distinctive names to the new procedures after the letters appearing alone in the intercepted radio messages.
ADFGX was first used on the German Western Front on March 1, 1918. ADFGVX is the successor to ADFGX and was used on both the Western and Eastern Fronts beginning June 1, 1918.
The procedure is two-step and consists of a monoalphabetic monographic substitution using Polybios squares and a transposition.
The characters are substituted monoalphabetically by pairs of characters from the letters “A”, “D”, “F”, “G”, “X” and “V” with the help of a polybios square.
To build the square, first the substitution key is entered and then the square is filled with the missing letters of the alphabet.
For ADFGX and the 5×5 polybios square used, the letter “J” is replaced by the “I”.
The intermediate text is entered line by line into a second matrix. The width of the matrix results from the length of a second keyword – the transposition key.
After the intermediate text has been entered line by line into the matrix, it is now read out again column by column. The order of the columns is determined by the alphabetical order of the individual letters of the transposition key.