GPT-6 Astra cracks a 217-year-old Napoleonic cipher in 6 hours

#What happened

Carter Church, a staff AI engineer at SentinelOne, used OpenAI's GPT-6 Astra to transcribe and fully decipher a coded letter to General Auguste de Marmont that had sat unread for 217 years. The entire process took about six hours, starting from a single scanned image. Church shared the result on X on September 30, 2026, saying he had never broken a cipher before.

#The letter

Written in March 1809 -- Eugene de Beauharnais acting on Napoleon's March 16 instructions -- ahead of Austria's invasion of Bavaria. One line of ordinary French plus 24 rows of numbers, letters, and invented symbols: about 1,300 cipher units across 155 distinct signs. Historian Daniel Tant had previously worked out only 33 signs (about a third). Astra transcribed the rest and built its own simulated-annealing solver to test letter/word assignments. The deciphered text revealed French and allied troop positions and the line "you must not allow yourself to be intimidated by a few troops or a gathering of rabble." Cryptiana's Satoshi Tomokiyo has marked the cipher as solved.

#Caveats

Astra built on Tant's partial key. The reconstruction is the engineer's proposed reading; the key and a verification script have been published for independent checking, and cryptobreaking specialists note a handful of signs remain uncertain. Church also reran the analysis excluding Napoleonic/Marmont material from the model's references and recovered the same reading.

#Why it matters

A general-purpose model performed transcription and cryptanalysis in one continuous multi-step workflow -- a capability showcase for Astra's advanced reasoning. It also underlines a dual-use point: systems that reconstruct missing information can apply to modern encrypted data too.

#Sources

1. CTech -- Oct 2, 2026
2. StartupFortune
3. CryptoBreaking -- AI Helps Decode 217-Year-Old Cipher (specialist; carries the reconstruction caveats)

Verified Oct 4, 2026.