GPT-6 Astra breaks an unsolved 1809 Napoleonic cipher letter to Marshal Marmont from a single scan
AI engineer Carter Church used GPT-6 Astra to break a homophonic cipher letter of March 1809 from Eugène de Beauharnais's headquarters to Marshal Auguste de Marmont. Only a third of its key had ever been recovered. Starting from one scanned image and a goal, Astra transcribed 1,300 hand-drawn cipher units, wrote a solver and recovered French plaintext on troop dispositions before Austria's April 1809 offensive, in about six hours of model time. Cryptiana's Satoshi Tomokiyo marked the cipher solved. Church's Sept 30 X thread reached ~4.2M views.
Key facts
- Writeup 'Breaking the Marmont Cipher, 1809' published Sept 18, 2026 at carter.church, with the key, transcription and a reproduction script
- Source: one 1,202×1,836-pixel plate in a 1969 French military-history journal article (J. Vilcoq); the text had never been entered in historical-cipher catalogs
- Cipher: homophonic substitution, 155 distinct signs, 1,300 units in 24 rows; 33 letter values (~435 units) were known from a partial key by French cryptology historian Daniel Tant
- Astra did row-by-row transcription of hand-drawn signs, merged variants, ran simulated annealing with French n-gram statistics and checked proposed values against every occurrence: 'about six hours of model execution time'
- Plaintext: a briefing listing seven French and allied armies with troop counts (e.g. 40,000 Bavarians under the Duke of Danzig, 30,000 Poles under Poniatowski), meant to reassure the isolated Marmont in Dalmatia. The date was corrected to 1809
- Contamination check: Church reran the solve from scratch with Marmont's memoirs and Napoleonic texts removed from the language-model statistics and got the same reading. Five signs remain uncertain without changing the meaning
- Verification: Satoshi Tomokiyo (Cryptiana) reviewed the solution and marked the entry solved. Church calls the result 'recovered plaintext', not independent historical authentication
- Church is an AI engineer, formerly at SentinelOne (RuntimeWire). His X thread of Sept 30, 2026 had ~4.2M views and ~9.6k likes on Oct 1 (fxtwitter)
What happened
After the MVUEH Enigma break, Carter Church tried Astra on a harder, multimodal problem: a cipher known only from a blurry photographic plate. The model handled the whole chain itself (transcription, normalization, cryptanalysis, verification), and a human reviewed the output and the historical context. The full solution package is public, so anyone can check it.
Why it matters
It is the second historical cipher break by GPT-6 Astra in two weeks. Church argued that the transcription-plus-cryptanalysis workflow, usually done by different specialists, was the real advance. The thread became one of the most-viewed AI capability demonstrations of the month.
Changelog
- 2026-10-01: created (snowball from Zvi's AI #188; X thread ~4.2M views)
Related posts (1)
- Carter Church: I used GPT-6 Astra to break an unsolved cipher to one of Napoleon's generals original ↗ Carter Church @CarterWChurch · x · 2026-09-30
First-hand account of the Marmont cipher break (~4.2M views on Oct 1).
Related events
- GPT-6 Astra breaks the 1941 MVUEH Enigma message, unsolved since 2005; Claude Opus 5 breaks a second one (FMNGI) days later ★★★
- OpenAI releases GPT-6 Astra, its first GPT-6 model ★★★★★
Sources (5)
- officialCarter Church: Breaking the Marmont Cipher, 1809 (writeup, key, script)
- discussionCarter Church on X: thread (~4.2M views)
- pressRuntimeWire: GPT-6 Astra helped decode a 217-year-old cipher letter to Napoleon's marshal
- discussionMetaFilter: Breaking the Marmont Cipher
- discussionZvi Mowshowitz: AI #188
id: 2026-09-18-gpt-6-astra-breaks-marmont-cipher · updated 2026-10-01 · open in the interactive timeline