A grainy page from 1969, 24 lines of numbers, letters and strange hand-drawn signs. For 217 years no one had published a complete reading of that military message. In September 2026 Carter Church, an artificial intelligence engineer at SentinelOne, entrusted the image to GPT-6 Astra and, according to his technical report, the model took about six hours of execution to arrive at a solution. Six hours by car, it should be noted: Church says he also dedicated some evenings to work.
Furthermore, the letter was not written directly by Napoleon. It left the headquarters of Eugène de Beauharnais, stepson of the emperor and viceroy of Italy, and was destined for General Auguste de Marmont, isolated with his troops in Dalmatia. Napoleon had ordered on 16 March 1809 that the positions of the main French and allied forces be communicated to him in cipher. That message appears to be the response to the order.
First he had to figure out what he was looking at
The document was known through a reproduction published by Lieutenant Colonel J. Vilcoq in Revue historique des Armées from 1969, now preserved in the Persée academic archive. The scan measures just 1,202 by 1,836 pixels. The kind of image before which even a trained human being soon begins to crave typefaces and good light.
GPT-6 Astra first had to transform that page into something parsable. During the transcription he identified 175 apparently different signs, initially also treating the possible variations in the spelling as separate symbols. After combining the variants, the cipher was composed of approximately 1,300 units and 155 distinct signs. This is why both numbers circulate in the reports: 175 were the temporary labels created by reading the image, 155 were the signs actually recognized in the final solution.
Some footholds already existed. Cryptology historian Daniel Tant had reconstructed a partial table of the code with 33 alphabetic values. Those values allowed 435 of the 1,300 units to be read, about a third of the message. The entire words represented by a single symbol, however, were still to be identified.
The system used was a homophonic substitution cipher: the same letter could be represented by different signs, precisely to make the old trick of counting how many times each character appears less effective. Astra built a program that tested possible matches and evaluated the obtained French using statistics drawn from texts by Hugo, Dumas and Marmont’s own memoirs. Twenty-nine signs turned out to be complete words, including common terms like “de,” “que,” “les,” “vous,” and “général.”
Church then repeated the procedure, eliminating Marmont’s memoirs and the other Napoleonic texts from the comparison material. The program essentially returned to the same reading. A rather important check, because it reduces the risk that the model has limited itself to recognizing a story already encountered during training and filling in the gaps with what it expected to find.
Inside were armies, numbers and a lot of propaganda
Once recomposed, the letter appears for what it was: a military report intended to convince Marmont that, in the event of war with Austria, he would not be left alone. The 40 thousand Bavarians lined up between Munich and Passau appear, Poniatowski’s 30 thousand Poles on the Vistula, the Saxons in front of Dresden, Davout’s 80 thousand men in Bayreuth, Masséna’s 60 thousand between Ulm and Donauwörth and another 40 thousand under Oudinot in Augsburg and along the Lech. It is almost the same sequence contained in the order sent by Napoleon to Eugène on 16 March 1809, preserved in his correspondence published in the nineteenth century.
However, the message adds information on the Austrian positions and allows Church to place it in the last days of March, shortly before the Austrian attack on 10 April. There is also a small gift to historians: in Napoleon’s published correspondence a sentence is interrupted after the order to Marmont not to be intimidated by “a handful of…”. In the encrypted message the sentence continues with a reference to “a few troops” and to a “rassemblement de canailles”, more or less a jumble of people.
Another line explains well how a military dispatch can also be an instrument of persuasion. The text assures Marmont that the Russians are marching against Austria. A few days earlier, Napoleon had ordered Eugene to spread that very idea. On March 24, writing to the French ambassador in St. Petersburg, he instead complained of not knowing where the Russian troops were and feared having to face Austria practically alone. The Russians would only enter Galicia in June. In short, a military cipher protected information from the enemy. It didn’t necessarily guarantee that all that information was true.
Five symbols still remain open
However, the solution requires caution. Church himself reports five signs whose interpretation remains uncertain: in some places a word may change, for example between “His Majesty” and “the Emperor”, or between singular and plural. None of these alternatives changes the overall meaning of the document.
Satoshi Tomokiyo, who has been editing the Cryptiana catalog dedicated to historical ciphers for years, examined the solution and the document now appears among those solved. Church has also published the complete package with scan, transcription, key to the 155 symbols and programs necessary to regenerate the reading. This remains a technical report, not a peer-reviewed academic study, but the outcome is at least testable piece by piece rather than relying on a simple “trust the AI”.
In the end, the documents already existed, the incomplete table also, the Napoleonic correspondence had been public since the nineteenth century. It took time, an eye, historical French, cryptanalysis and a remarkable willingness to spend weeks behind 1,300 small crooked signs. Now there is also a key of 155 symbols. If another message written with the same system turns up from some archive, at least that one won’t have to wait another two centuries.
