AI system deciphers 217-year-old military message in six hours

GPT-6 Astra solves historical cipher

AI engineer Carter Church, an engineer at cybersecurity company SentinelOne, used OpenAI's GPT-6 Astra to decipher a message that had remained unread for 217 years. The cipher had appeared on Cryptiana's Unsolved Historical Ciphers list for decades.

Church says Astra completed the multimodal workflow in approximately six hours after receiving only a single image and a prompt. The image showed a French introductory line followed by 24 rows containing numbers, letters and invented symbols.

A military briefing from 1809

The message originated from the headquarters of Eugène de Beauharnais, Viceroy of Italy and Napoleon's stepson, but was not signed by an individual. Its contents amount to a troop briefing sent to French General Auguste de Marmont that relayed Napoleon's orders.

Marmont would have needed an official military codebook from the period to read the message. The source says it is unlikely that anyone has read it since 1809. According to Church, the deciphered text also provides missing context for part of Napoleon's 1865 memoir.

The memoir includes part of an order concerning preparations for the Austrian invasion, but stops mid-sentence and says Marmont must not be held back by "a handful of ..." The newly deciphered passage completes the sentence with "a gathering of rabble."

Solution verified against the ciphertext

Astra used a partial key that had not previously been connected with the letter as one clue to solving the cipher. It then solved the remaining encoding with its own simulated-annealing solver, according to Church.

Satoshi Tomokiyo, who maintains Cryptiana's Unsolved Historical Ciphers database, marked the cipher as solved. Church also published a solution package with a script that regenerates the reading, saying the result can be checked mechanically against the ciphertext.