Breaking the Cipher of the Napoleonic war briefing, unread for 217 years

A previously unread Napoleonic ciphered briefing to General Marmont, sealed for 217 years, has been fully deciphered and redated to March 1809, placing it in the two weeks before Austria invaded Bavaria. The message matches Napoleon’s 16 March instruction to Eugène to send Marmont a ciphered account of French and allied dispositions and yields a coherent French plaintext that names commanders and troop strengths - e.g., the Duke of Danzig commanding forty thousand Bavarians occupying Munich and Passau, and Prince Poniatowski leading thirty thousand Poles - confirming Marmont’s strategic isolation in Dalmatia. The reading transforms a single scanned plate reproduction (a 1969 French army journal engraving) of one plaintext line followed by 24 rows of cipher into a complete tableau chiffrant and its inverse, the same kind of key a March 1809 clerk would have used.

The cryptanalysis combined image transcription and statistical search: 1,300 cipher units and 155 distinct signs were identified, of which 33 values (435 units) came from a prior table and the remaining 865 units were solved by iterative methods. A workflow using GPT-6 Astra for row-by-row transcription, simulated annealing scoring against French n-grams, and exhaustive plate cross-checking found 29 whole-word signs and reconstructed the full key. The solution package (key.tsv, revised transcription, verify.py) reproduces the reading end-to-end from the plate.