While the first of the message’s 24 lines is in French, the rest is in a mix of letters, numbers, and made-up symbols so baffling that no cryptographer had come close to decoding it.
Wikimedia CommonsNapoleon sent the cipher to General Auguste de Marmont in March 1809 in hopes of warning him about enemy troop positions on the eve of war with Austria.
In March 1809, on the precipice of war with the Austrian Empire, French leader Napoleon Bonaparte sent a message to his senior general, Auguste de Marmont. The first line was in standard French: “You must have received, General Marmont, my letters of the 8th, 14th and 20th of this month.” But the next 24 lines were all written in code.
After the cipher’s answer key was lost to history, the letter’s secrets remained shrouded in mystery for 217 years. However, Carter Church, an AI engineer at the cybersecurity company SentinelOne, just cracked the code using OpenAI’s GPT-6 Astra system.
How Napoleon’s Encrypted Letter From 1809 Was Decoded With AI
The note’s encoded portion contains 1,300 characters, including two-digit numbers, invented symbols, and letters.
According to Church, GPT-6 Astra transcribed the characters while also comparing them against previous decoding work done on the letter by other cryptographers in a process that took six hours.
J. Vilcoq, “Le Chiffre sous le Premier Empire,” Revue historique des Armées 25(4), 1969An engineer used the AI model GPT-6 Astra to decode the letter.
The system then deciphered the message through a process of trial and error in which it determined where certain symbols and letters fit best throughout the message. If the placement of a certain symbol made the coded French smoother, the AI then checked that matchup everywhere else in the message, and compared its findings to additional evidence gathered from Marmont’s later memoirs.
Although this process could have been done manually, it would have been laborious and taken much longer than it did.
Church even noted that the act of transcribing a cipher and analyzing it are often separate skills, if not entirely distinct careers. But with the AI model used here, these two tasks were done together in one workflow, which also referred back to the letter to crosscheck the decoding work in real time as it was being done.
Church said that he had not been interested in attempting to decode a cipher until he was inspired by another tech developer’s success in using an AI model to decipher a 1941 German Army message encrypted with the famous Enigma code.
What Napoleon Wrote To General Marmont On The Eve Of War With Austria
In February 1809 — about a month before the letter in question was sent — the Austrian Empire decided that it was going to go to war with Napoleon. The Austrians had witnessed Napoleon take over Spain the previous spring and figured that they were next, unless they struck while Napoleon’s best troops were busy in Spain.
Wikimedia CommonsIn 1809, Napoleon had recently taken over Spain, and the Austrian Empire was preparing for war, figuring that they were next.
Archduke Charles of the Austrian Empire had spent three years raising an army made up of hundreds of thousands of troops.
Napoleon, who could see this strike brewing, needed to get in touch with his best general, Marmont, who was stationed with his best troops across the Adriatic Sea in modern-day Croatia — with Austrian territory between them.
So, Napoleon, writing from Paris, sent Marmont a coded letter on March 16, 1809 containing details of the locations and statuses of the French and Austrian armies that had gathered on the eve of war.
In the now-deciphered letter, Napoleon instructs Marmont: “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.”
Wikimedia CommonsAuguste de Marmont, Napoleon’s best general, was stationed in modern-day Croatia and isolated from the other French armies at the time that Napoleon sent this encoded message to him.
In the end, Marmont was able to execute Napoleon’s orders, marching out of Croatia and aiding in Napoleon’s win over Austria at the Battle of Wagram. Marmont was then elevated to the rank of marshal.
Marmont’s most famous act, however, came later in 1814, when Napoleon’s enemies were swarming at the gates of Paris. Marmont and two other marshals came to a secret agreement with these armies to betray Napoleon and surrender. This act of betrayal is what convinced Napoleon to give up his throne, which he would only briefly reclaim before losing it again and ultimately dying in 1821 at age 51.
Marmont then served the Bourbon King Louis XVIII, and was later in charge of quelling the July Revolution of 1830 in Paris. And when Marmont was unable to stop the revolt, King Charles X famously asked him: “Will you betray us, as you betrayed him?”
After reading about how a software engineer used AI to decipher Napoleon’s encoded letter, learn the truth about how tall Napoleon really was. Then, learn about the life of Joséphine Bonaparte, Napoleon’s first wife.