AI decodes 217-year-old Napoleonic letter in 6 hours

Published on 02/10/2026 - 14:49 GMT+2 Artificial intelligence has decoded a letter written on Napoleon Bonaparte’s orders for the first time in more than 200 years. ADVERTISEMENT ADVERTISEMENT The document was not a letter written directly by Napoleon. It originated at the headquarters of Eugène de Beauharnais, Viceroy of Italy, and was an encrypted troop briefing passed to Marmont on Napoleon's orders. An engineer at cybersecurity company SentinelOne, Carter Church, wrote on social media that OpenAI's GPT-6 Astra model deciphered an encrypted 1809 letter sent to French General Auguste de Marmont. The AI model transcribed the image to break the cypher in roughly six hours, Church said. “Astra cut the 1,202 by 1,836 plate into rows, read the signs, decided which of 175 hand–drawn marks were the same sign written twice, and only then began breaking the cypher,” Church wrote in a blog post. Several researchers had tried over the decades to reconstruct the document's contents, but none had published a full decipherment. Astra handled the work end to end, including transcribing the image, analysing the homophonic substitution cypher, writing code and cross-referencing historical records. The text revealed the positions of Bavarian, Polish and French forces. In 1809, Europe was edging toward war. Austria was preparing to fight France, while a large share of French forces remained tied down in Spain. Marmont, commanding French troops in Dalmatia, risked being isolated if Austria attacked. On March 16, Napoleon ordered Eugène to inform Marmont of the positions of the main French and allied forces, specifying that the message be "written in cypher" and carried by a reliable officer. “The Russians are marching against Austria, and the army [of] Friuli today numbers eighty thousand men,” the document reads. “Add the Dalmatian corps, without counting the reserves being organised everywhere in the rear, and you see that Austria will be attacked at once on every side. “She is consequently hastening toward her ruin. When you receive the order to march, you must not allow yourself to be intimidated by a few troops or a gathering of rabble.” Using the decoded troop numbers, officers' titles and unit positions, Church re-dated the document to late March 1809, about two weeks before Austrian forces launched the offensive. Church published his complete research record on September 18, including the transcription, cypher tables and scripts that allow others to reproduce the result. According to Church, Satoshi Tomokiyo, the Tokyo-based researcher who maintains Cryptiana, reviewed the solution and marked the entry as "solved."