Napoleon code cracked

"Breaking the Marmont Cipher, 1809" (also called the Letter to Marmont) is a write-up by Carter Church. It documents a full reading of a coded letter to General Marmont that was on the standard list of unsolved historical ciphers. The letter was previously known only from a single plate in a 1969 French army journal (Revue historique des Armées) that carries the wrong date. The write-up redates it to March 1809, about two weeks before Austria invaded Bavaria, and checks the content against the war that followed.

The method used GPT-6 Astra on the scanned plate alone. The model cut the 1,202 by 1,836 pixel image into rows, transcribed the signs, and merged variants into 155 distinct signs across about 1,300 cipher units. It then ran a solver against the homophonic cipher. About 435 units were covered by a partial table from Daniel Tant (33 letter values), and the remaining roughly two-thirds were worked out from the letter itself. The author reports about six hours of model execution time and says proposed values were checked back against the image.

This is a research case study, not a product or model. The page says a solution package with a script that regenerates the reading is available for download. The author describes himself as not a cryptographer, and the claims come from the author's own write-up. I found no independent verification in the provided material.