An encrypted letter from Charles V deciphered by French researchers

Written in 1547 by the Emperor to his ambassador in Paris, the letter sheds new light on the relationship between the kingdom of Francis I and the Holy Roman Empire.

A series of symbolsunintelligible” which lights up five centuries later: four researchers presented their discovery in France on Wednesday, the deciphering of a letter written in 1547 by Charles Quint to its ambassador in Paris, shedding new light on the relationship between the kingdom then led by Francis I, and the Holy Roman Empire.
To achieve this featexceptional», Six months of work were necessary for cryptographers from the Lorraine Computer Research Laboratory (Loria), associated with a historian from the University of Picardy, north of Paris.

The letter, forgotten for centuries, was in the collections of the Stanislas library in Nancy. Cécile Pierrot, cryptographer at Loria, heard for the first time in 2019 of a “encrypted letter from Charles V(1500-1558) by chance, during a dinner. The researcher then believes in a legend, but when the existence of this document is mentioned to her again two years later, she decides to dig.
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 his ambassador Jean de Saint-Mauris.
Then begins the work of deciphering. Cécile Pierrot observes the letter for a long time, classby separate families» the 120 or so symbols used by Charles V. She names them and decides to count their occurrences, to spot the combinations that could be repeated.

Diabolical encryption

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 is the human who “ask the right questions to the computer“, insists the cryptographer.

Deciphering is donesmall step by small stepbecause 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 by diacritics, an inspiration probably coming from Arabic, explains Cécile Pierrot.

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

Le Figaro with AFP

Another confusing element, the emperor uses “null symbols”, which mean nothing and in fact serve to mislead the adversary who would try to decipher the message. The click finally happens at the end of June: Cécile Pierrot manages to isolate a series of words in the missive. For this, the three cryptographers from Nancy called on Camille Desenclos, a 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.

Degraded relations between Charles V and François 1er

A truly “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 transcriptionwhile deciphering the missive sent to her by the ambassador, explains Cécile Pierrot.
Once deciphered, the letter “confirms the rather degraded statein 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 maintained a “mistrust“reciprocal”extremely strong» and look for «to weakenmutually, she adds.
Other information revealed by the decryption of the letter: “a rumor of an assassination plot against Charles V which would be brewing in France“says Camille Desenclos, a rumor of which”we didn’t know much” previously. She will turn outunfounded– Charles V did not die assassinated – but this letter shows “fear” of the prestigious monarch vis-à-vis this “potential conspiracy“, underlines the historian.
In his missive to his ambassador, the emperor also evokes the situation of his empire and its “political and military strategy“: the use of encrypted correspondence thus allows him “hide» this particularly sensitive information 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“. Camille Desenclos is now sure of being able to continue to enrich historical research on this period: “We are likely to make many more discoveries in the coming years.“.

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