Napoleon’s 217-year-old coded message ‘cracked by AI’

A coded letter carrying a message from Napoleon Bonaparte to one of his senior generals has been cracked by AI after 217 years.

The contents of the letter, sent by Eugène, Napoleon’s stepson, on the emperor’s instructions, had remained a mystery since it was sent to Auguste de Marmont in 1809.

However, Carter Church, an AI engineer at cybersecurity company SentinelOne, said this week he had used OpenAI’s GPT-6 Astra system to decrypt and transcribe the message for the first time.

The letter contained details of French and Austrian troop movements on the eve of war between France and the Habsburg Empire.

The note contained 1300 characters, some of which represented letters and other old words. The cipher to decrypt the message has been lost, meaning the letter’s contents have been unknown for more than two centuries.

GPT-6 Astra – an advanced version of the AI system that powers ChatGPT – spent six hours transcribing the characters and mapping them against work from cryptographers that had solved 33 of the characters.

It then decoded the message using a technique known as “simulated annealing”, a process of trial and error in which multiple candidate words are tried to see whether they fit, and compared them to Marmont’s existing writing.

While the process could have been done by hand, it would have been a laborious process that no cryptographer to date has accomplished.

Church said he had never attempted to decode a cipher before but had been inspired to after seeing an AI model decode a 1941 German army message.

In that case, Carter Leffen, another tech developer, simply told the GPT-6 Astra AI tool to find and decrypt an unsolved Enigma message, which it managed in around 10 hours.

Recently released AI models have been found to be highly capable of solving mathematical problems as well as deciphering code.

Decrypting ciphers often involves painstaking work of patching together multiple possible combinations of words, something that AI systems can do much more quickly than humans.

Researchers have raised concerns that the same capabilities will be used to unmask anonymous online profiles, by matching their writing patterns and other identifiers with real people.

The letter to General Marmont, who was stationed in modern-day Croatia and separated from Napoleon’s main armies, came as the Habsburgs prepared for war with France to regain lost territory.

It contains details of the locations and strength of the French and Austrian forces and instructs him to prepare to march and not be “intimidated by a few troops or a gathering of rabble”.

The letter adds: “Austria will be attacked at once on every side. She is consequently hastening towards her ruin.”

Marmont’s army was ultimately able to break out of Croatia and help Napoleon defeat Austria at the Battle of Wagram. Marmont was made a Marshal, a rank awarded to his senior generals.

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