For over two centuries, a coded military dispatch from Napoleon's Italian headquarters resisted all attempts at decryption. Now, an artificial intelligence system has cracked it in a matter of hours, shedding new light on the strategic calculus of the 1809 campaign.
The letter, dated March 1809, was sent from the headquarters of Eugène de Beauharnais, Viceroy of Italy, to General Auguste de Marmont, who commanded French forces in Dalmatia. It was written on Napoleon's orders and encrypted with a homophonic substitution cipher—a system where multiple symbols represent the same letter, complicating frequency analysis.
Carter Church, an engineer at the cybersecurity firm SentinelOne, used OpenAI's GPT-6 Astra model to decipher the document. In a blog post, Church explained that the AI first transcribed the 1,202 by 1,836 pixel image of the letter, then identified which of the 175 hand-drawn symbols were the same sign written twice, and finally broke the cipher itself. The entire process took about six hours.
A breakthrough after decades of failed attempts
Previous researchers had tried to reconstruct the letter's contents, but none had published a full decipherment. Church's approach was end-to-end: the AI transcribed the image, analyzed the cipher, wrote code to solve it, and cross-referenced the decoded text with historical records.
The decoded message reveals the positions of Bavarian, Polish, and French forces, and includes a confident assessment of Austria's prospects. “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.”
Church used the decoded troop numbers, officers' titles, and unit positions to re-date the letter to late March 1809, about two weeks before Austrian forces launched their offensive. This timing is significant: in early 1809, Austria was preparing for war while a large portion of the French army was tied down in Spain. Marmont's forces in Dalmatia risked being isolated if Austria attacked, making the intelligence in this dispatch crucial for his decisions.
The full research record, including the transcription, cipher tables, and scripts, was published by Church on September 18, allowing others to verify and reproduce the result. Satoshi Tomokiyo, a Tokyo-based researcher who maintains the Cryptiana website, reviewed the solution and marked it as “solved.”
This episode is a striking example of how AI can assist historical research, particularly in cryptanalysis. It also resonates with broader debates about the role of AI in intelligence work, a topic that has gained urgency as some tech leaders urge caution about the pace of development. The ability of AI to decode ancient ciphers may be a far cry from modern espionage, but it demonstrates the technology's potential to unlock secrets that have long resisted human effort.
For historians, the decoded letter offers a rare glimpse into the operational thinking of Napoleon's command. It confirms that the French were aware of Austria's preparations and were confident in their ability to counter them. The letter also underscores the importance of secure communications in military strategy—a lesson that remains relevant today, as European security services monitor potential threats and intelligence leaks continue to make headlines.
While the AI's achievement is remarkable, it also raises questions about the limits of such tools. The cipher was relatively simple by modern standards, and the AI had the advantage of a known plaintext context—Napoleonic military terminology. Still, the speed and accuracy of the solution suggest that AI could become an indispensable tool for historians and cryptographers alike.
As Europe reflects on its Napoleonic past, this breakthrough offers a new way to engage with that history. The letter, once a secret, now speaks clearly across two centuries, thanks to a technology that Napoleon could never have imagined.
