In cryptography , the
ADFGVX cipher was a field cipher used by the
The cipher is named after the six possible letters used in the ciphertext: A, D, F, G, V and X. 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.
* 1 Operation * 2 ADFGVX * 3 Cryptanalysis * 4 References * 5 Sources * 6 External links
For the plaintext message, "Attack at once", secret mixed alphabet is first filled into a 5 × 5 Polybius square :
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
i and j have been combined to make the alphabet fit into a 5 × 5 grid.
By using the square, the message is converted to fractionated form:
a t t a c k a t o n c e AF AD AD AF GF DX AF AD DF FX GF XF
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. Both the transposition keys and the fractionation keys were changed daily.
In June 1918, an additional letter, V, was added to the cipher. That expanded the grid to 6 × 6, allowing 36 characters to be used. That allowed the full alphabet (instead of combining I and J) and the digits from 0 to 9. That mainly had the effect of considerably shortening messages with many figures.
The cipher is based on the 6 letters ADFGVX. A table containing a random and secret alphabet is created with these letters as column headings and row identifiers, resulting in the following table:
A D F G V X
A N A 1 C 3 H
D 8 T B 2 O M
F E 5 W R P D
G 4 F 6 G 7 I
V 9 J 0 K L Q
X S U V X Y Z
The text 'attack at 1200am' translates to this:
A T T A C K A T 1 2 0 0 A M
AD DD DD AD AG VG AD DD AF DG VF VF AD DX
Then, a new table is created with a key as a heading. Let's use 'PRIVACY' as a key. Usually much longer keys or even phrases were used.
P R I V A C Y
A D D D D D A
D A G V G A D
D D A F D G V
F V F A D D X
The columns are sorted alphabetically, based on the keyword, and the table changes to this:
A C I P R V Y
D D D A D D A
G A G D A V D
D G A D D F V
D D F F V A X
Then, appending the columns to each other results in this ciphertext: DGDD DAGD DGAF ADDF DADV DVFA ADVX
With the keyword, the columns can be reconstructed and placed in the correct order. When using the original table containing the secret alphabet, the text can be deciphered.
ADFGVX was cryptanalysed by French
However, that was not the only trick that Painvin used to crack the ADFGX cipher. He also used repeating sections of ciphertext to derive information about the likely length of the key that was being used. Where the key was an even number of letters in length he knew, by the way the message was enciphered, that each column consisted entirely of letter coordinates taken from the top of the Polybius Square or from the left of the Square, not a mixture of the two. Also, after substitution but before transposition, the columns would alternately consist entirely of "top" and "side" letters. One of the characteristics of frequency analysis of letters is that while the distributions of individual letters may vary widely from the norm, the law of averages dictates that groups of letters vary less. With the ADFGX cipher, each "side" letter or "top" letter is associated with five plaintext letters. In the example above, the "side" letter "D" is associated with the plaintext letters "d h o z k", and the "top" letter "D" is associated with the plaintext letters "t h f j r". Since the two groups of five letters have different cumulative frequency distributions, a frequency analysis of the "D" letter in columns consisting of "side" letters has a distinctively different result from those of the "D" letter in columns consisting of "top" letters. That trick allowed Painvin to guess which columns consisted of "side" letters and which columns consisted of "top" letters. He could then pair them up and perform a frequency analysis on the pairings to see if the pairings were only noise or corresponding to plaintext letters. Once he had the proper pairings, he could then use frequency analysis to figure out the actual plaintext letters. The result was still transposed, but to unscramble a simple transposition was all that he still had to do. Once he determined the transposition scheme for one message, he would then be able to crack any other message that was enciphered with the same transposition key.
Painvin broke the ADFGX cipher in April 1918, a few weeks after the Germans launched their Spring Offensive . As a direct result, the French army discovered where Erich Ludendorff intended to attack. The French concentrated their forces at that point, which has been claimed to have stopped the Spring Offensive.
However, the claim that Painvin's breaking of the ADFGX cipher
stopped the German
Spring Offensive of 1918, while frequently made,
is disputed by some. In his 2002 review of Sophie de Lastours ' book
on the subject, La
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 w