How researchers managed to decipher a 16th century coded letter from Charles V

Four researchers have succeeded in deciphering the symbols used by the emperor in a letter intended for his ambassador in France in 1547. Charles Quint relays in particular the rumor of an assassination attempt against him.

Researchers “unveil the secrets of a letter from Charles V, which has remained mysterious since 1547”,, during the press conference of four researchers in Nancy (Meurthe-et-Moselle). The latter explained how they had managed to decipher the coded message of a letter from Charles V, Emperor of the Holy Empire, intended for his ambassador in France, Jean de Saint-Mauris.

It was a multidisciplinary team of four researchers in computer science and history, from the University of Lorraine, LORIA/INRIA, the CNRS and the University of Picardie Jules-Verne (CHSSC), which came to the end of this translation after several months of work.

A work of several months to “break the code”

It was the cryptographer Cécile Pierrot who first heard about this letter in 2019, before eventually looking for it, and finding it in the Nancy library two years later. She first tries to translate it herself, before including in her research two of her colleagues Pierrick Gaudry and Paul Zimmermann, cryptographers and computer scientists.

The missive is made up of four pages: one simply mentioning the address, and three others made up of paragraphs written with symbols. Three short passages are written in the usual alphabet.

Encryption key for Charles V’s letter to his ambassador in France © LORIA

“Initially we said to ourselves that it was a letter from the 16th century, and that we were going to succeed in breaking the code easily”, explains to Pierrick Gaudry, researcher at the CNRS. “And then it turned out to be longer, more complicated than expected.”

It will take them a total of six months to manage to decipher the letter, on which we find “about 120 different symbols” for only 26 letters in the alphabet, declares Pierrick Gaudry. The idea that each symbol represents a letter is therefore quickly discarded.

To avoid any decryption by other people, the cryptographers of the time made easily identifiable letters disappear by “breaking all the statistical aspects which make it possible to recognize letters”, explains the cryptographer. Thus, among other techniques, the “e”, which appears very often in our language, or the characteristic duo “qu”, are hidden in the code.

Researchers try several ways to understand the sequence of symbols, and if they manage to identify a few, they get stuck on translating the text.

Necessity of “historical knowledge”

The historian Camille Desenclos then comes into play, recommended by several of their colleagues. She has the great advantage of being a specialist of the time and of being interested in cryptography. The specialist directs the other three researchers to a letter from Jean de Saint-Mauris to the Municipal Library of Besançon (Doubs), coded in the same way, which makes it possible to unlock codes, and to translate a good part of the text of Charles Quint .

“It was a bit like our Rosetta Stone,” says Pierrick Gaudry, referring to the texts that made it possible to translate Egyptian hieroglyphs.

The historian also made it possible to put the written elements in a historical context. “There were symbols, sentences that we did not understand, Camille Desenclos made it possible to remove ambiguities”, explains Pierrick Gaudry, who underlines that “without historical knowledge”, the translation would have been blocked.

It is she who thus makes it possible to resituate the text in 1547, and not in 1546, because the date noted on the missive refers to an old calendar, making the year begin at Easter. The letter was thus actually written on February 22, 1547 according to our current calendar.

What does Charles V say in this letter?

“This letter reveals three of Charles V’s major concerns vis-à-vis France at the start of the year 1547: maintaining peace with François I, avoiding assassinations and putting an end to the conflict which opposes him to the League of Smalkald, a military union within the Holy Roman Empire, the researchers explain in the discovery file.

The missive is indeed written in a period of great tension in Europe, in particular between the King of France François 1er and Charles Quint, who will clash militarily on several occasions.

In the first part of the letter, the emperor’s concern “is palpable” from the first lines. He supports the idea that peace must be maintained with François 1er, without however appearing weak, and asks his ambassador to keep him informed of political developments on the spot.

Secondly, Charles Quint “reports in his letter a disturbing rumor: Pierre Strozzi, warlord in the service of François 1er, would seek to assassinate him”, explains the file.

This passage is interesting because it highlights the emperor’s fear, but also the circulation of rumors in higher places. Jean de Saint-Mauris will thus answer in another missive to the emperor that it is only a rumor and no document of the time comes to support the will of this assassination at this time.

The last part of the letter, the longest, evokes the ongoing conflict between Charles V and the league of Smalkalde.

“A precious document”

“For historians, it’s a precious document,” Camille Desenclos told France 3 Grand Est.

This letter “takes place at a turning point in European history. Europe is at peace after several years of war. But, behind the scenes, we are preparing for war, especially France, which supports the rebels against Charles Quint”. The two monarchs “do not trust each other and seek to harm each other.”

This letter could also be used to decode other texts, which, several centuries after their coding, have not yet been deciphered.

Salome Vincendon

Salome Vincendon BFMTV journalist

The article is in French

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