A digitized archive can be accessible to all while still preserving its secrets. However, this letter, more than two centuries old and addressed to General Marmont, one of Napoleon's commanders, has just revealed its secrets. In a post published on X this Wednesday, September 30, the AI engineer Carter Church explains how he deciphered it using GPT-6 Astra, an artificial intelligence model designed by OpenAI. According to him, six hours of work were sufficient for the AI. His detailed account, posted on September 18, retraces the steps of this investigation, from reading the handwritten symbols to reconstructing the message. It all starts with a reproduction of the letter, published in an article by Lieutenant Colonel J. Vilcoq in 1969 in the Revue historique des Armées and now available on Persée, a French digital library. The author was studying the encryption methods of the First Empire, used notably to prevent intercepted mail from revealing orders and troop positions. Before breaking the code, it is necessary to read the signs. The letter begins with a sentence in French, followed by 24 lines mixing numbers, letters and invented symbols. According to Carter Church, Astra cut the scan into lines to transcribe them. The final record counts 1,300 encrypted elements, corresponding to 155 distinct signs. The first difficulty lies in the handwritten writing: the same sign can change slightly in shape depending on how it was drawn. The AI must recognize these variations to avoid treating as two different symbols what is only one. Astra first recorded 175 different strokes, then reduced them to 155 signs. It is then necessary to understand what each sign represents. The letter uses a so-called "homophonic" encryption: several symbols can replace the same letter. The interest is to hide the most frequent letters. In French, the "e" is often repeated: if it was always replaced by a triangle, this triangle would appear frequently and could lead to a discovery. By using sometimes a triangle, sometimes a circle or another sign, its detection becomes more difficult. In the letter to Marmont, the "e" has nine different signs. To move forward, Astra used existing works. According to Carter Church, the AI found a table by Daniel Tant, a French historian of cryptography, dedicated to this code and which already gave the meaning of 33 symbols. These signs appearing several times in the letter, they already allowed reading about a third of the message. It remained to find the missing correspondences. How the AI searched for the missing correspondences For the next step, Astra used its own decryption program, according to the engineer's account. This uses "simulated annealing," a method that allows exploring different possible keys. The program assigns letters to symbols, then evaluates the resulting text based on the usual letter sequences in French. The more the result resembles the desired language, the better its score. It then modifies the correspondences and restarts. The particularity of simulated annealing is that it sometimes accepts a temporarily worse result. This allows getting out of a promising track that does not lead to the solution. As the search progresses, some signs are also identified as whole words, rather than as letters. This method existed before generative AI. In 2019, researcher Nils Kopal, from the University of Siegen, already presented a tool integrated into the free software CrypTool 2 to attack homophonic ciphers, automatically or with human intervention. Here, the contribution of Astra lies in its ability to move from reading the manuscript to using a program to decipher it. A military letter and a corrected date The recovered message details notably the positions and strengths of French and allied forces facing Austria. Carter Church relates it to an order from Napoleon to Eugène de Beauharnais, viceroy of Italy, asking him on March 16, 1809, to send this information to General Marmont "in an encrypted letter and through an intelligent officer." These cross-references place the letter in the last days of March 1809, two weeks before the Austrian invasion of Bavaria, and not in 1807 as it was previously dated. The result has also been examined by a specialist. According to Carter Church, Satoshi Tomokiyo, who runs the historical cryptography website Cryptiana, confirmed the decryption. The letter is now among the solved enigmas, with its corrected dating. Church himself admits that a cryptographer could have broken this code with enough time. What surprised him was the work accomplished by Astra from an image and a command: transcribe the signs, find a partial key, complete the decryption and verify the historical context. He has published the key and the program used to allow others to examine the result.