How researchers were able to decipher a letter from Charles V that no one understood for 5 centuries

A series of symbolsUnintelligible“It sheds light after five centuries: four researchers presented their discovery in Nancy on Wednesday, November 23, deciphering a letter written by Charles V to his ambassador in France in 1547, which sheds new light on the relations between the kingdom then ruled by Francis I, and the Holy Roman Empire. .

To achieve this “exceptional” feat, six months of work were required by cryptographers from the Computer Research Laboratory of Lorraine (Loria), associated with the historian of the University of Picardy.

Forgotten for centuries, the letter was in the collection of the Stanislaus Library in Nancy. Cécile Pierrot, Loria’s cryptographer, accidentally heard the “encrypted letter of Charles V” (1500-1558) for the first time during a dinner in 2019. The researcher then believes in a legend, but when the document’s existence is mentioned to her again two years later, she decides to dig.

Word of mouth works, and at the end of 2021, he sees for the first time a mysterious and incomprehensible letter signed by the King of Spain, addressed to his ambassador Jean de Saint-Maurice.

Then the task of understanding begins. Cécile Pierrot takes a long look at the letter, class “by separate familiesAbout 120 symbols used by Charles V

Evil code

For this, she and two other researchers from Nancy’s lab, Pierre Gaudry and Paul Zimmermann, decided to “use computers.Speed ​​up research“No Artificial Intelligence, Here’s Human”Ask the right questions to the computer“, emphasizes the cryptographer.

Comprehension complete”Small step small step“, because the code used by Charles V was brutal. In addition to its large number of codes, “Whole words are encoded in a single code“And vowels before consonants are marked by diacritics, an inspiration from Arabic,” explains Cécile Pierrot.

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Another confusing element, the emperor uses “null symbols“, which is nothing and actually serves to mislead the adversary trying to understand the message.

The click finally happens at the end of June: Cécile Pierrot manages to isolate a series of missive words.

For this, Nancy’s three cryptographers called on Camille Descenclos, an expert in both cryptography and relations between France and the Holy Roman Empire in the 16th century.

The historian helps them put the pieces of the puzzle together, re-contextualizing the letter to better understand its implications.

“Pierre de Rosetta” in Besancon

A real “Rosetta Stone” helps the research: Jean de Saint-Maurice’s letter is kept in Besançon, where the recipient wrote in the margin.A form of transcription“By understanding the note sent to her by the ambassador,” explains Madame Pierrot.

Once understood, the letter “Rather it confirms the degraded stateIn 1547, relations between Francis I and Holy Roman Emperor Charles V, although he had signed a peace treaty three years earlier, explains Camille Desanclos.

Despite this peace, the two sovereigns had a “disbelief“reciprocal”Very strong“And seek”to weaken“Reciprocal,” he adds.

Other information revealed by the decryption of the letter: “Rumors of an assassination plot against Charles V would develop in France“, says Mrs. Desenclos, the rumor about”We don’t know much“Before. She changes.”baseless” – Charles V was not killed – but this letter shows “The fear of the prestigious king vis-à-vis “this possible conspiracy“, she underlines.

In a letter to his ambassador, the emperor described the state of his empire and his “Political and military strategy“: The use of encrypted letters allows him to “to hide“This information is for his opponents.

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Researchers now believe they can identify other letters in Europe from the emperor and his ambassador.Have a photo of Charles V’s strategy in Europe“.

There is scope for many more discoveries to be made in the coming years.“, enthuses Mrs. DeCenclos.

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