When you think of wartime cryptography, you probably think of World War II and Enigma, although there were other famous codes used during that war. But in fact, there have been codes used through wars and in peacetime for much longer. Even the Romans used codes. Of course, World War I, or The Great War, as it would have been known, had its share of codes and, as you might expect, most of these have been broken long ago. Most of them. But apparently an AI model, GPT-6 Astra, recently cracked one that was previously undeciphered.
If you aren’t up on century-old cryptography, the ADFGVX cipher appeared in 1918. It began as ADFGX, but after a few months grew an extra letter — and a larger grid. The letters were chosen because their Morse-code patterns were relatively easy to distinguish: A, D, F, G, V, and X. The original ADFGX version used a 5×5 grid containing the alphabet, usually merging I and J. The later ADFGVX version expanded this to 6×6, making room for all the letters and the digits as well.
Encoding required a 5×5 (or 6×6) grid with the alphabet within. The table’s rows and columns were labeled… you guessed it… ADFGX pr ADFGVX. Each plaintext letter gets replaced by the corresponding row and column such as AF or XG. Finally, the letter pairs would be written under a transposition key.
Concatenating those pairs gives AGXGDDDXXF. Written in five columns under CANDY, the first row is AGXGD and the second is DDXXF.
Next, you sort the letters in the transposition key, taking the columns along with the letters. That would make the transposition key ACDNY, and the first row would now be GAGXD. Then the message is sent by columns. Too much to wrap your head around? Visit dCode and plug in some plain text, squares, and keywords for yourself.
The French broke the code in 1918, but without computers it was tough work. Codebreaker [Georges Painvin] reportedly worked on the cipher until it made him ill, but he did solve it. However, some of the intercepted messages remained a mystery. One in particular was sent on November 27, 1918. Apparently, the message used the codeword TRUPPENVERSCHIEBUNG, a German key believed to have gone into service in December. If this decoding is correct, then this message used it even earlier.
The message was, of course, in German and appears to be a report about a British ship’s movements near Sevastopol. In fact, HMS Canterbury’s position on the dates mentioned matches the message.
Honestly, we were more impressed that people like [Georges Painvin] in 1918 were able to decode these intercepted messages. But it is still an interesting glimpse into the capabilities of Astra.
It seems that as long as there has been writing, there have been ways to send secret messages. The Swiss in The Great War had their own encryption method.