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Concept
ADFGVX cipher
Summary
In cryptography, the ADFGVX cipher was a manually applied field cipher used by the Imperial German Army during World War I. It was used to transmit messages secretly using wireless telegraphy. ADFGVX was in fact an extension of an earlier cipher called ADFGX which was first used on 1 March 1918 on the German Western Front. ADFGVX was applied from 1 June 1918 on both the Western Front and Eastern Front.
Invented by the Germans signal corps officers Lieutenant de (1891–1977) and introduced in March 1918 with the designation "Secret Cipher of the Radio Operators 1918" (Geheimschrift der Funker 1918, in short GedeFu 18), the cipher was a fractionating transposition cipher which combined a modified Polybius square with a single columnar transposition.
The cipher is named after the six possible letters used in the ciphertext: , , , , and . The letters were chosen deliberately because they are very different from one another in the Morse code. That reduced the possibility of operator error.
Nebel designed the cipher to provide an army on the move with encryption that was more convenient than trench codes but was still secure. In fact, the Germans believed the ADFGVX cipher was unbreakable.
For the plaintext message, "Attack at once", a secret mixed alphabet is first filled into a 5 × 5 Polybius square:
and have been combined to make the alphabet fit into a 5 × 5 grid.
By using the square, the message is converted to fractionated form:
{| class="wikitable" style="text-align:center;"
| a || t || t || a || c || k || a || t || o || n || c || e
|-
| AF|| AD|| AD|| AF|| GF|| DX|| AF|| AD|| DF|| FX|| GF|| XF
|}
The first letter of each ciphertext pair is the row, and the second ciphertext letter is the column, of the plaintext letter in the grid (e.g., "AF" means "row A, column F, in the grid").
Next, the fractionated message is subject to a columnar transposition.
Official source
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