GPT-6 Cracks WWI Cipher

Scienceblogs.de maintains a famous list of 50 unsolved ciphers, ranging from Zodiac Killer cryptograms to the Voynich manuscript. Among them is a set of German radio messages from World War I encoded using the ADFGVX method. Over a dozen of these messages have eluded codebreakers for over a century. GPT-6 Astra just solved one of them.

The ADFGVX cipher

ADFGVX is a field cipher used by the German military during WWI. It works by mapping each letter and digit to a pair of letters (A, D, F, G, V, X) using a 6x6 grid keyed by a keyword. The resulting text is then transposed using a second keyword. The method is named after the six letters used because they are easily distinguishable in Morse code, which was critical for radio transmission.

The unsolved message was transmitted on November 27, 1918. Codebreaking expert George Lasry had already decoded hundreds of these messages, but this one remained open.

How GPT-6 solved it

GPT-6 Astra identified "TRUPPENVERSCHIEBUNG" as the encryption keyword, which is documented in J. Rives Childs's "The History and Principles of German Military Ciphers, 1914-1918" on pages 214-215. The decryption process requires rearranging the keyword alphabetically, then performing columnar transposition to recover the original substitution pairs, and finally mapping those pairs back through the ADFGVX grid.

The decoded message reads:

EIN ENGLISCHER KREUZER EINLIEG X SEWASTOPOL X S4STEN X EIN GESCHWADER DER X ALLIIERTEN FOLGT 26STEN X

In English: "An English cruiser arrived at Sevastopol on the ?4th. An allied squadron follows on the 26th."

The model verified its own work

This is the part that elevates the result from a parlour trick to something genuinely interesting. Astra checked its own decode against historical records. It found that the English cruiser HMS Canterbury arrived in Sevastopol on November 24, 1918, based on the ship's original logs. An allied squadron did arrive on November 26, as documented in contemporary naval records.

The model also noted a discrepancy: the keyword "TRUPPENVERSCHIEBUNG" was reportedly used starting December 9, 1918, but the message was transmitted on November 27. The reason for this gap is unknown. The message may have used the key earlier than documented, or the historical record of when keys were assigned may be incomplete.

Why this matters

This is not just a cipher being solved. It is a language model performing multi-step cryptographic reasoning: identifying a historical encryption key from a reference text, applying a complex columnar transposition, decoding the ADFGVX substitution, translating the German plaintext, and then cross-referencing the result against primary source naval records from 1918 to confirm accuracy.

The cipher had been open for 108 years. A human expert, George Lasry, had already decoded many of these messages through traditional cryptanalysis. GPT-6 Astra picked one that was still unsolved and worked through it end to end, including historical verification. That is a meaningful capability demonstration.

The remaining unsolved ADFGVX messages are probably next.