Five centuries later, an encrypted letter from Charles V deciphered

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Five centuries later, an encrypted letter from Charles V deciphered

The encrypted letter of Charles V, also known as Charles V, Holy Roman Emperor and Archduke of Austria, dated 1547.

A series of unintelligible symbols that shed light five centuries later: four researchers presented their discovery in France on Wednesday, the decipherment of a letter written in 1547 by Charles V to his ambassador in Paris, shedding new light on relations between the kingdom then led by Francis I and the Holy Roman Empire.

The encrypted letter is dated 1547.

To achieve this exceptional feat, six months of work were needed by cryptographers from the Lorraine Computer Research Laboratory (LORIA), associated with a historian from the University of Picardie, north of Paris.

< p class="e-p">The letter, forgotten for centuries, was in the collections of the Stanislas library, in the city of Nancy. Cécile Pierrot, cryptographer at LORIA, heard for the first time in 2019 of an encrypted letter from Charles V (1500-1558) by chance, during a dinner. The researcher then thinks of a legend, but when the existence of this document is mentioned to her again two years later, she decides to dig.

Cécile Pierrot (on the left), researcher at the National Institute for Computer Research (INRIA), and Camille Desenclos (on the right), lecturer in modern history, examine the encrypted letter from Charles V, at the Stanislas library in Nancy in France.

Word of mouth works and, at the end of 2021, she sees for the first time the mysterious and incomprehensible letter bearing the signature of the King of Spain addressed to its ambassador Jean de Saint-Mauris.

Then begins the work of deciphering. Cécile Pierrot observes the letter for a long time, classifying the 120 or so symbols used by Charles V into distinct families. She names them and decides to count their occurrences, to spot the combinations that could be repeated.

To do this, she and two other researchers from the Nancy laboratory, Pierrick Gaudry and Paul Zimmermann, decided to use computers to speed up research. No artificial intelligence, here, it's the human who asks the right questions to the computer, insists the cryptographer.

Deciphering is done step by step, because the code used by Charles V is diabolical. In addition to its large number of symbols, whole words are encrypted with a single symbol and vowels preceded by a consonant are marked with diacritics, an inspiration probably coming from Arabic, explains Cécile Pierrot.


Another confusing element is that the Emperor uses null symbols, which mean nothing and actually serve to mislead the adversary trying to decipher the message.

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

For this, the three Nancy cryptographers called on Camille Desenclos, specialist 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, recontextualizing the letter to better understand its allusions.

A real Rosetta stone also helps the research: a letter from Jean de Saint-Mauris kept in Besançon, where the recipient had written in the margin a form of transcription by deciphering the missive sent to him by the ambassador, explains Ms. Pierrot.

Once deciphered, the letter confirms the rather degraded state in 1547 of relations between Francis I and the Emperor of the Holy Roman Empire Charles V, who had nevertheless signed a peace treaty three years earlier, explains Camille Desenclos.

Despite this peace, the two sovereigns maintain extremely strong reciprocal mistrust and seek to weaken each other, she adds.

< p class="e-p">Other information revealed by the deciphering of the letter: a rumor of an assassination plot against Charles V which would be hatched in France, says Mrs. Desenclos, rumor of which we did not know much before.

It turns out to be unfounded, since Charles V did not die assassinated, but this letter shows the monarch's fear of this potential plot, she underlines.< /p>

In his missive to his ambassador, the emperor also mentions the situation of his empire and his political and military strategy: the use of encrypted correspondence thus allows him to hide this information particularly sensitive to his adversaries.

Researchers now hope to be able to identify other letters in Europe from the emperor and his ambassador, to have a photograph of the strategy of Charles V in Europe.

It is likely that we will make many more discoveries in the coming years, rejoices Ms. Desenclos.

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