GPT-6 Astra cracks a 217-year-old Napoleon cipher in about six hours

A 217-year-old encrypted letter sent by Napoleon in 1809 has been deciphered with the help of GPT-6 Astra, according to the source material provided. The document had long appeared on Cryptiana’s list of unsolved historical ciphers and survived only as a low-resolution black-and-white reproduction published in a 1969 French military magazine. The image measured 1202 × 1836 pixels and contained roughly 1,300 hand-drawn symbols, later grouped into 155 distinct units. The effort was led by Carter Church, a senior AI engineer at SentinelOne rather than a professional historian or cryptanalyst. The source says he gave Astra a blurry image and a short prompt, after which the model spent about six hours transcribing the symbols, identifying partial anchors from historical material, writing code based on simulated annealing, and validating the output against 19th-century French language patterns. The decoded text reportedly revealed a military briefing sent to Marshal Marmont and restored a phrase that had remained incomplete for 160 years in the published correspondence of Napoleon. Cryptiana operator Satoshi Tomokiyo later said the case was solved after reviewing a code package with validation scripts.