four researchers French deciphered a letter written in 1547 by Charles V to his ambassador to France, an “exceptional” discovery that sheds light on the relationship between the Holy Roman Empire and the kingdom led by Francis I.
To accomplish this feat deciphering a succession of “unintelligible” symbols five centuries later required six months of work by cryptographers at the Lorraine computer research laboratory (LORIA), associated with a historian from the University of Picardy, the experts explained Wednesday in Nancy (eastern France).
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The missive, forgotten for centuries, it was in the collection of the Stanislas de Nancy municipal library. Cécile Pierrot, LORIA cryptographerheard for the first time in 2019 an “encrypted letter” from Carlos V, also Carlos I of Spain (1500-1558), by chance during a dinner.
The researcher thought it was a legend, but when two years later the existence of that document was mentioned to her again, she wanted to know more.
At the end of 2021 he saw for the mysterious and incomprehensible letter bearing the Emperor’s signature and addressed to his ambassador Jean de Saint-Mauris for the first time.
Then began the work to decipher it. Cécile Pierrot analyzed the letter at length and classified the nearly 120 symbols used by Carlos V by “different families”. Then he counted how many times each one appeared and observed the combinations that could be repeated.
together with others two researchers from the laboratory, Pierrick Gaudry and Paul Zimmermann, decided to use computers to “speed up” the work. It wasn’t artificial intelligence, but human beings “asking the right questions to the computer,” the cryptographer said.
They advanced “in small steps”, because the code used by Carlos V is diabolic. In addition to the significant number of symbols, “entire words are encrypted with a single symbol” and vowels preceded by a consonant are marked with diacritics, an inspiration probably coming from Arabic, Cécile Pierrot explained.
Another puzzling element is that the emperor used “null symbols”, which do not mean anything and serve to mislead the adversary who tried to decipher the message.
The revelation finally came in June, when Pierrot managed to isolate a sequence of words in the letter.
To do this, the three Nancy cryptographers summoned Camille Desenclos, a specialist both in cryptography and in relations between France and the Holy German Empire in the 16th century.
The historian helped them fit the pieces of the puzzle, by contextualizing the letter to better understand the allusions.
The researchers also used a real “Rosetta stone”: a letter from Jean de Saint-Mauris preserved in Besançon (eastern France), in which the addressee He wrote in the margin “a form of transcription” that deciphers a letter the ambassador had sent him, Pierrot said.
Once deciphered, the letter “confirms the rather degraded state” in 1547 of relations between Francis I of France and Holy Roman Emperor Charles V, who, however, they had signed a peace treaty just three years earlier, Camille Desenclos explained.
Despite this peace, the two sovereigns maintained an “extremely strong” reciprocal “mistrust” and sought to “weaken” each other, he added.
There is other information revealed by the deciphered letter: “A rumor of a plot to assassinate Carlos V that was hatched in France”Desenclos recounted, specifying until now that “not much was known” about it, that it never came to fruition.
In the letter to his ambassador, Carlos V also refers to the situation of his empire and his “political and military strategy”: the use of encrypted correspondence allowed him to “disguise” this information from his adversaries.
The researchers now hope to be able to identify other letters from the emperor and his ambassador in Europe “to have a picture of Carlos V’s strategy” in the Old Continent.