Napoleon’s Secret Revealed After 217 Years: AI Cracks the Emperor’s Mysterious Code

More than 200 years later, a coded letter written during the Napoleonic Wars has finally been deciphered, according to the New York Post.

The original message was sent in March 1809 to French General Auguste de Marmont by Napoleon’s stepson, Eugène de Beauharnais, who was serving as Viceroy of Italy at the time. France was still at war with Austria.

Marmont, a close ally of the emperor, was stationed with his troops in Dalmatia, on the opposite side of the Adriatic Sea. His forces had become almost completely isolated from the rest of the French army and its allies.

The first line of the letter was written in French, Napoleon’s native language, but the following 24 lines consisted of a completely baffling combination of symbols, letters and numbers.

Experts believed that the letter had originally been accompanied by a special key for deciphering the message. Over time, however, the key was lost.

The mysterious code was finally cracked by Carter Church, an engineer at cybersecurity company SentinelOne, who used artificial intelligence to decipher it.

“I used GPT-6 Astra to crack an unsolved cipher in a letter to one of Napoleon’s generals that had remained unread for 217 years. What’s most impressive isn’t even cracking the code, but that Astra completed the entire multimodal process in six hours — with only a single image and the task itself,” Church wrote on X.

The first line of the letter reads: “Vous avez dû recevoir, Monsieur le Général Marmont, mes lettres des 8, 14 et 20 courant,” which translates as: “You must have received, General Marmont, my letters of the 8th, 14th and 20th of this month.”

The rest of the message remained unknown until Church asked the AI to decipher it. The entire process took around six hours.

According to the engineer, the artificial intelligence first divided the 1,202-by-1,836-pixel image into individual lines, identified the symbols, determined which of the 175 hand-drawn marks were identical, and only then began cracking the cipher itself.

Once deciphered, the letter was found to contain information about the positions of other French and allied armies. Napoleon also urged Marmont not to lose hope or allow his isolated position to intimidate him, assuring the general that Austria was rapidly approaching collapse.

Just two months later, however, on May 21, 1809, Napoleon suffered his first personal defeat at the Battle of Aspern-Essling near Vienna. Around 23,000 French soldiers were killed in the battle.

Following the defeat, the emperor regrouped his forces and, in July 1809, defeated the army of Archduke Charles at the Battle of Wagram. The battle led to the collapse of the Fifth Coalition, the alliance against France led by Austria and Britain.

Church believes that the cipher itself was not particularly difficult. Professional cryptographers could have cracked it as well, he said, but completing all the necessary work would have taken a human considerably longer.

“The cipher itself is not difficult, and cryptographers could have cracked it without much trouble if they had enough time and resources. But transcription, cataloguing, translation and the subsequent deciphering require complex interdisciplinary work,” the engineer explained.

According to Church, this is precisely where artificial intelligence can prove particularly useful: across many fields, there are enormous numbers of problems that specialists simply do not have the resources or time to tackle.

“The main takeaway is that the list of unsolved problems will increasingly be shortened by outsiders — simply because they became curious,” Church said.