An AI Cracked a 217-Year-Old Napoleonic Cipher in Six Hours. It Took Historians Two Centuries.
The encrypted letter, sent in 1809 on Napoleon's orders, was deciphered by OpenAI's GPT-6 Astra — from a single image, end to end.
An encrypted letter written on Napoleon Bonaparte's orders has been decoded by an artificial intelligence system — more than 200 years after it was sent. A cybersecurity engineer has cracked a Napoleonic-era military cipher that had remained unbroken for over two centuries.
The document was not written personally by Napoleon. It originated in the headquarters of Eugène de Beauharnais, Viceroy of Italy, and was an encrypted overview of troop positions, transmitted to General Auguste de Marmont on the emperor's orders.
Carter Church, an engineer at cybersecurity firm SentinelOne, wrote on social media that the model OpenAI GPT-6 Astra deciphered the encrypted 1809 letter. According to Church, the AI model processed an image of the document and broke the cipher in approximately six hours.
"Astra split a 1,202 by 1,836 matrix into rows, recognized the characters, determined which of the 175 hand-drawn symbols represented the same sign repeated twice, and only then began cracking the cipher," Church wrote in a blog post.
Over the past decades, several researchers attempted to reconstruct the contents of the document, but no one succeeded in publishing a complete decryption. Astra handled the entire process from start to finish, including image decryption, analysis of the homophonic substitution cipher, writing the code, and cross-referencing the data with historical sources.
The text revealed the positions of Bavarian, Polish, and French forces.
In 1809, Europe was approaching a new war. Austria was preparing for a confrontation with France, while a significant portion of French forces remained tied down in Spain. Marmont, commanding French troops in Dalmatia, risked being cut off if Austria struck.
On March 16, Napoleon ordered Eugène to inform Marmont of the positions of the main French and allied forces, stressing that the message had to be "written in cipher" and delivered by a reliable officer.
"Russia is opposed to Austria, and the army of Friuli today numbers eighty thousand men," the document reads. "Add the Dalmatian corps, not counting the reserves being formed everywhere in the rear, and you will see that Austria will be attacked from all sides at once."
"Consequently, it is only hastening its own destruction. Having received the order to march, you must not allow yourself to be frightened by a few troops or a crowd of rabble."
Using the decrypted data on troop numbers, officer ranks, and unit positions, Church dated the document to the end of March 1809, roughly two weeks before the start of the Austrian offensive.
On September 18, Church published a full report on his research, including the transcription, cipher tables, and scripts allowing others to reproduce the result. According to Church, Tokyo researcher Satoshi Tomokiyo, who runs the Cryptiana website, verified the solution and marked the relevant entry as "solved."
For now, a two-century-old mystery has been closed in a matter of hours. The emperor's secrets are no longer secret. And the tools that unlocked them are not historians with magnifying glasses, but machines that read images, recognize patterns, and solve puzzles faster than any human ever could.
Written by Martina Kirchner martina.kirchner@alpineweekly.com