AI cracks Napoleon's 1809 encrypted message

An AI researcher has successfully deciphered a 1809 encrypted military message written by Napoleon Bonaparte using OpenAI's GPT-6 Astra model. The letter had remained largely unread for 217 years due to the loss of its corresponding codebook.

Carter Church, an AI engineer at SentinelOne, spent approximately six hours using GPT-6 Astra to perform optical character recognition on a scanned historical document and analyze its ciphertext. The letter, dated March 16, 1809, was sent as Austria prepared for war with France. Napoleon instructed his stepson, Eugène de Beauharnais, the Viceroy of Italy, to relay military information to French Marshal Auguste de Marmont via encrypted correspondence.

Previous attempts had identified only 33 symbols within the message, which contained approximately 1300 cipher units and 155 distinct symbols. The AI model employed methods such as simulated annealing to test potential letter and word substitutions for the symbols. It cross-referenced these attempts with historical French texts, including works by Alexandre Dumas and Victor Hugo, to assess the plausibility of the interpretations.

Upon deciphering, the message revealed detailed intelligence on the military dispositions of French and allied forces across Europe prior to the 1809 campaign. It outlined troop strengths in Bavaria, Poland, Saxony, and Italy, as well as the advance of Russian forces toward Austria. The letter also provided supplementary details on Marmont's location and French strategic plans, intended to keep him informed of the overall situation while he was stationed away from the main armies.

Church noted that such decryption would typically require significant time and specialized expertise. The AI's advantage lay in its ability to simultaneously handle historical document recognition, symbol classification, cryptanalysis, and text reconstruction. The findings were further validated by rerunning the analysis after removing Napoleon-specific historical texts, confirming the AI did not rely on pre-existing data. The deciphered message has now been recognized and added to the Cryptiana database.