AI engineer uses GPT-6 Astra to crack Napoleon's 1809 cipher

On October 4, 2026, the Chinese tech outlet ITHome reported that an AI researcher had used OpenAI's GPT-6 Astra to break an encrypted military letter written by Napoleon Bonaparte in 1809. The letter had gone 217 years without being fully deciphered, because its cipher key was lost.

The work was carried out by Carter Church, an AI engineer at SentinelOne. According to Church, he used the GPT-6 Astra model to start from a single scanned image of the historical document, carrying out text recognition and cryptanalysis in a process that took about six hours.

The letter is dated March 16, 1809, a time when Austria was preparing to go to war with France. Napoleon wrote to his stepson, Eugène de Beauharnais, the viceroy of Italy, asking him to pass military information to the French general Auguste de Marmont through encrypted correspondence. The surviving document opens with one line of ordinary French, followed by 24 lines, which is where the source text ends.

The report also states that the letter contains roughly 1,300 cipher units and 155 distinct symbols. Beyond Church's own account of the decryption, the report offers no further details about the method he used, how the cipher key came to be lost, or any additional comment from the parties involved. The publicly available record of the episode currently stops at his description of the roughly six-hour effort.