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 (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.

Operation

For the plaintext message, "Attack at once", a secret mixed alphabet is first filled into a 5 × 5 Polybius square:

{| class="wikitable" style="text-align:center;"

!

!A

!D

!F

!G

!X

|-

!A

|b

|t

|a

|l

|p

|-

!D

|d

|h

|o

|z

|k

|-

!F

|q

|f

|v

|s

|n

|-

!G

|g

|i/j

|c

|u

|x

|-

!X

|m

|r

|e

|w

|y

|}

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. The message is written in rows under a transposition key (here "CARGO"):

C A R G O

_________

A F A D A

D A F G F

D X A F A

D D F F X

G F X F

Next, the letters are sorted alphabetically in the transposition key (changing CARGO to ACGOR) by rearranging the columns beneath the letters along with the letters themselves:

A C G O R

_________

F A D A A

A D G F F

X D F A A

D D F X F

F G F X

Then, it is read off in columns, in keyword order, which yields the ciphertext:

FAXDF ADDDG DGFFF AFAX AFAFX

In practice, the transposition keys were about two dozen characters long. Long messages sent in the ADFGX cipher were broken into sets of messages of different and irregular lengths to make it invulnerable to multiple anagramming. The work was exceptionally difficult by the standards of classical cryptography, and Painvin became physically ill during the process. His method of solution relied on finding messages with stereotyped beginnings, which would fractionate them and then form similar patterns in the positions in the ciphertext that had corresponded to column headings in the transposition table. (Considerable statistical analysis was required after that step had been reached, all done by hand.) It was thus effective only during times of very high traffic, but that was also when the most important messages were sent.

However, that was not the only trick that Painvin used to crack the ADFGX cipher. is disputed by some. In his 2002 review of Sophie de Lastours' book on the subject, La France gagne la guerre des codes secrets 1914-1918, in the Journal of Intelligence History, (Journal of Intelligence History: volume 2, Number 2, Winter 2002) Hilmar-Detlef Brückner stated:

<blockquote>Regrettably, Sophie de Lastours subscribes to the traditional French view that the solving of a German ADFGVX-telegram by Painvin at the beginning of June 1918 was decisive for the Allied victory in the First World War because it gave timely warning of a forthcoming German offensive meant to reach Paris and to inflict a critical defeat on the Allies. However, it has been known for many years, that the German Gneisenau attack of 11 June was staged to induce the French High Command to rush in reserves from the area up north, where the Germans intended to attack later on.

Its aim had to be grossly exaggerated, which the German High Command did by spreading rumors that the attack was heading for Paris and beyond; the disinformation was effective and apparently still is. However, the German offensive was not successful because the French had enough reserves at hand to stop the assault and so did not need to bring in additional reinforcements.

Moreover, it is usually overlooked that the basic version of the ADFGVX cipher had been created especially for the German Spring Offensive in 1918, meant to deal the Allies a devastating blow. It was hoped that the cipher ADFGX would protect German communications against Allied cryptographers during the assault, which happened.

Telegrams in ADFGX appeared for the first time on 5 March, and the German attack started on 21 March. When Painvin presented his first solution of the code on 5 April, the German offensive had already petered out.</blockquote>

The ADFGX and ADFGVX ciphers are now regarded as insecure.

References

Sources

  • Childs, J. Rives, General Solution of the ADFGVX Cipher System, Aegean Park Press, .
  • Friedman, William F. Military Cryptanalysis, Part IV: Transposition and Fractionating Systems. Laguna Hills, California: Aegean Park Press, 1992.
  • A JavaScript implementation of the ADFGVX cipher
  • Another JavaScript implementation
  • A C implementation of the ADFGVX cipher