AI engineer Carter Church successfully utilized GPT-6 Astra to decode a military letter addressed to French General Auguste de Marmont, which had remained unread for 217 years.
I used GPT-6 Astra to break an unsolved cipher to one of Napoleon's generals that had gone unread for 217 years.
— Carter Church (@CarterWChurch) September 30, 2026
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.
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Prior to this, the letter was known only through a reproduction in the French military journal Revue historique des Armées from 1969, where it was mistakenly dated to 1807.
The document contained a single line of standard text followed by 24 lines of two-digit numbers, letters, and handwritten symbols.
Church determined that the letter was created following Napoleon's order on March 16, 1809, directing his stepson, Vice-King of Italy Eugène de Beauharnais, to send Marmont a coded message with details about the positions of French and allied armies.
The decoded letter listed the same seven forces in the same order and with the same numerical estimates as found in the preserved order.
Additionally, it mentioned Austria's preparations for war, claiming the country would be "immediately attacked from all sides."
Six Hours of Model Work
The cipher consists of 1,300 elements and 155 different symbols. It is of a homophonic type, meaning the same letter can be represented by multiple symbols, complicating traditional frequency analysis.
Prior to Church's work, cryptography historian Daniel Tant had identified the meanings of 33 symbols, allowing for the reading of 435 elements, or about one-third of the message. This left 865 elements still to decode.
According to Church, GPT-6 Astra took approximately six hours of computational time. The model was provided with only an image of the document measuring 1202 × 1836 pixels; there was no existing transcription.
Astra segmented the scan into lines, recognized the handwritten characters, and matched various forms of their writing. The system then applied a simulated annealing algorithm to evaluate decoding options based on statistics of common three-, four-, and five-letter French sequences.
From one prompt and one image, Astra:
— Carter Church (@CarterWChurch) September 30, 2026
— Transcribed 1,300 hand-drawn signs.
— Found a partial key previously unconnected to this letter
— Cracked the rest with its own simulated-annealing solver.
— Cross-referenced other surrounding letters to correct the published date.
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Part of the signs represented whole words such as de, que, les, vous, général.
To ensure the model was not influenced by texts related to the era, Church reran the procedure without Marmont's memoirs or other Napoleonic materials. The system produced the same decoding.
Decoding Fills Gap in Napoleon's Correspondence
Among the findings was a phrase missing from the published correspondence of Napoleon.
The edition of his order from March 16 ends abruptly after instructing Marmont not to restrain himself from "a handful..." In the decoded message, the general is advised not to yield to pressure from "a few detachments or a rabble."
Church emphasized that the letter is a retelling of the emperor's directives, so it cannot be claimed that Napoleon expressed himself in exactly that manner. However, this is the first direct indication of what was concealed in the omission from the published document.
The decoding also included information about the positions of Austrian forces, which is absent from Napoleon's preserved order. According to Church, the indicated positions correspond to troop deployments prior to the war.
Satoshi Tomokie, a historian of historical ciphers and author of the Cryptiana project, verified the solution. Following this, the resource reclassified the letter as decoded and corrected its date to 1809.
However, five fragments of the document remain ambiguous due to the singular occurrence of certain symbols on a low-quality scan. Church estimates that alternative interpretations do not change the overall meaning of the message.
It is worth noting that in September, GPT-6 Astra was also employed to decode a German military radio message from the Enigma of 1941. The model helped recover both the key and the text of a message from the CryptoCellar archive, which had been unresolved for nearly 20 years.
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