Napoleon Bonaparte ordered the wartime missive written in code, creating a puzzle that went unsolved for more than 200 years -- until AI cracked it in a matter of hours.
Artificial intelligence is helping non-specialists unlock the secrets of historical sources, a trend described as a "turning point" in a field previously the preserve of scholars and dedicated hobbyists.
One new sleuth is Carter Church, a 27-year-old AI engineer who decrypted a 19th-century letter to one of Napoleon's generals.
Without digital tools, the task could have taken weeks, but the latest model from ChatGPT-maker OpenAI broke the cipher in just six hours.
"What AI mostly brings is persistence," Church told AFP, saying he felt "an unbelievable sense of satisfaction and excitement when I saw the result".
The message from 1809 -- detailing French and allied troops' positions in two dozen lines of symbols, numbers and letters -- had lain ignored in an online academic archive.
Because traditional, low-tech methods to crack ciphers are effective but time- and labour-intensive, historians "have more important problems to solve, so many ciphers like this often sit unattended", said Church.
To find the solution, Church, who works at US cybersecurity firm SentinelOne, needed to learn "very little" about code-breaking techniques or the historical period.
He spent some evenings going over it, then consulted Japanese patent engineer Satoshi Tomokiyo, who maintains an online list of unsolved ciphers, to verify it.
"2026 will be remembered as a turning point for codebreaking of historical ciphers," Tomokiyo wrote on his website Cryptiana, a go-to resource for the small but enthusiastic global community of amateur cryptographers.
Researchers were already developing AI models to break ciphers, but the public release of state-of-the-art generalist tools has unleashed dozens of new solutions.
Although Church used OpenAI's most powerful model, GPT-6 Astra, other people have used top offerings from rival US lab Anthropic instead.
Tomokiyo said that since mid-September, most of the ciphers on his list have been cracked using AI.
"There remain some tough problems" which could stay an enigma for years to come, because there is less relevant data available to help AI figure them out, the 58-year-old told AFP.
"I'm excited long-time problems are now solved, but on the other hand, I have mixed feelings, because AI seems to have solved these difficult problems so easily," he said.
With technology draining the satisfaction of being first to crack a cipher, a fellow code-breaker told Tomokiyo: "I feel useless."
AI is surpassing human abilities in pursuits from mathematics to computer programming, while threatening to alter or even eradicate many jobs.
The technology is transforming the work of historians not only by decoding documents, but also by analysing millions of medieval manuscripts or helping read ancient scrolls.
Too fragile to be unrolled without risking destruction, the Herculaneum Papyri -- hundreds of papyrus scrolls carbonised in the eruption of Vesuvius in 79 AD -- are now being X-rayed and analysed by AI to reveal the hidden writing inside.
Researchers in Italy announced in June the first complete virtual unrolling of a scroll, which turned out to be a treatise on ethics.
AI is also being used to build a huge, searchable collection of previously inaccessible sources on Jewish history and culture by transcribing 10 million pages of Hebrew manuscripts.
"It's gigantic. No human being could process such a quantity" of data, said project leader Daniel Stoekl Ben Ezra, of the Ecole Pratique des Hautes Etudes in Paris.
Researchers hope the databases created using the digitised text -- which he likens to a kind of "huge Facebook of the past" -- could help bring to light links between people, places and ideas.
And at the French capital's Notre Dame Cathedral, an AI system has been trained to transcribe 14,600 Latin pages from the church's medieval archives -- a task not dissimilar to code-breaking.
"These are minutes drawn up by notaries who abbreviate their decisions. The script is highly administrative, very pragmatic... with rather technical vocabulary," said historian Julie Claustre, who leads the e-NDP project.
With character recognition rates ranging between 88 and 94 percent, the project has revealed information about the cathedral's daily life, its building work, its staff and the care of abandoned children.
Doing the same by hand would take "several decades", said Claustre, but "it's not just a time-saver".
Automation enables "inquiries that were not previously possible; it transforms the conditions of research."
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