Carter Church, an AI engineer at cybersecurity firm Sentinel One, laid out the findings in a post on X on Wednesday (Sept. 30), explaining how he gave OpenAI's new model GPT-6 Astra one prompt and one image to crack the centuries-old code. Six hours later, the task was complete.
The letter was sent in 1809 on Napoleon's request by his stepson Eugène, the viceroy of Italy, to Marshal Auguste de Marmont, who was stationed far from any other French troops on the eastern shore of the Adriatic Sea. The letter's purpose was to tell Marmont the position of every other French and allied army across the continent. However, to avoid revealing military secrets to France's enemies should the letter be intercepted, the message was written in code.
I used GPT-6 Astra to break an unsolved cipher to one of Napoleon's generals that had gone unread for 217 years. What makes this impressive isn't actually the codebreaking, but that Astra completed the entire multi-modal workflow in ~6 hours from a single image and goal.1/6? pic.twitter.com/vyiGKOsrHqSeptember 30, 2026
But this low resolution scan and a single prompt was all GPT-6 Astra needed to transcribe the text and find a partial table by cryptology historian Daniel Tant — previously unlinked to this letter — that documented what 33 of the symbols meant, accounting for about a third of the letter.
Auguste Frederic Louis Viesse de Marmont was the intended recipient of the coded message while stationed on the eastern shore of the Adriatic Sea. (Image credit: Photo by DeAgostini/Getty Images)
GPT-6 successfully used the partial table to determine what each of the other symbols stood for and translate the message.
Michael Rowe, a researcher in European history at King's College London, added that the findings were "impressive" and that because we have access to Napoleon's original correspondence to his stepson, where he detailed what the ciphered message should include, we can be fairly confident in the translation of Eugène's letter.
French Emperor Napoleon Bonaparte was forced to use ciphers as a way of communicating sensitive information securely across his vast empire.
This does also mean that the translation of Eugène's secret message isn't too surprising to historians. "The cipher version just repeated a large segment of what's in that [original] letter, so in a sense it doesn't tell us anything new — it's not like it reveals some kind of great new information," Rowe said.
According to White, the translation does raise "some curious questions about Napoleon's mindset in the run up to the campaign in Austria in 1809." For example, he said that Napoleon actually had 60,000 fewer troops available to fight at the time than is claimed in this letter, and he also seems fixated on the number of troops Austria might send to invade Italy.
Cracking codes in time
Ciphers were fairly common during this period, conveying sensitive military and diplomatic information.
However, using AI to crack these codes could also offer insights into the private lives of historical figures. "[Ciphers] were often also used for private correspondence," Rowe said, for example letters between elite individuals having extramarital affairs.
"Across time frames, this [AI] is opening doors," Rowe said.
RELATED STORIESFor this particular cipher, Church said that the code itself isn't especially difficult. "Cryptographers could have cracked this easily given the time/energy," he said. However, it would require experts across different areas and take a considerable amount of time and effort, he added.
Church's result was confirmed by Satoshi Tomokiyo, a patent engineer and cryptographer who runs a website documenting unsolved historical ciphers called Cryptiana, and the letter is now listed as "solved" on the website.
"This work is totally reliant on all of the research that goes into finding these ciphers, ensuring they're actually unsolved, contextualizing them, etc and I'd love for those folks to get all of the recognition," Church said, referring to Tomokiyo's website. "That's where the real hard work is."
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