ADFGVX cipher

The ADFGVX cipher was a manually applied field cipher used by the Imperial German Army during World War I to send messages secretly by wireless telegraphy. It extended an earlier cipher, ADFGX, first used on 1 March 1918 on the German Western Front, and ADFGVX was applied from 1 June 1918 on both the Western and Eastern Fronts. It was invented by the German signal corps officer Lieutenant Fritz Nebel and introduced in March 1918 as the Secret Cipher of the Radio Operators 1918. It is a fractionating transposition cipher that combines a modified Polybius square with a single columnar transposition. Its name comes from the six letters of the ciphertext, A, D, F, G, V and X, chosen because they are very different in Morse code, which reduced operator error. The Germans believed it unbreakable.