Five centuries later, the secret of the letter of Emperor Charles V is revealed

A series of “incomprehensible” symbols shedding light five centuries later: four researchers presented their discovery in Nancy on Wednesday, a transcript of a letter written in 1547 by Charles V to his ambassador in France, shedding new light on the relationship between the kingdom of that time. under the rule of Francis I and the Holy Roman Empire.

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

Mentioned by accident during dinner in 2019.

The letter, forgotten for centuries, is in the collections of the Stanislas Library in Nancy. Cecile Piero, a cryptographer from Loria, first heard about the “ciphered letter of Charles V” (1500–1558) for the first time in 2019 by chance during lunch. The researcher then believes in the legend, but when the document’s existence is again mentioned to her two years later, she decides to dig.

At the end of 2021, she sees for the first time a mysterious and incomprehensible letter signed by the Emperor, intended for his ambassador Jean de Saint-Maurice.

Then the decryption begins. Cécile Pierrot classifies “by separate families” about 120 symbols of Charles V. She names them and decides to count their number in order to determine the combinations that can be repeated. To do this, she and two other researchers from Nancy’s lab, Pierric Gaudry and Paul Zimmermann, decided to use computers to “speed up research.”

The decryption is done “in small steps after small steps” because the code used by Charles V is diabolical. In addition to the large number of characters, “whole words are encrypted with a single character” and associated consonant vowels are marked with diacritics, probably inspired by Arabic, Cecile Piero explains.

The click was a series of single words

Another confusing element: the emperor uses “null characters” which mean nothing and actually serve to confuse an adversary who tries to decipher the message. The click finally comes at the end of June: Cecile Pierrot manages to pick out a number of words in the message.

Three Nancy cryptographers turned to Camille Desanclos, an expert on both cryptography and the relationship between France and the Holy Roman Empire in the 16th century. The historian helps them piece together the pieces of the puzzle by recontextualizing the letter to better understand its allusions.

The real “Rosetta Stone” helped

The real “Rosetta Stone” also aids the research: a letter from Jean de Saint-Maurice, kept in Besançon, where the recipient wrote in the margin a “form of transcription” deciphering the message the ambassador gave him, Cecile explains. Pierrot.

Once deciphered, the letter “comes to confirm a rather degraded state” in 1547 of the relationship between François Hier and the Holy Roman Emperor Charles V, who nevertheless signed the peace treaty three years earlier, explains Camille Desinclos. Despite this peace, the two sovereigns maintain an “extremely strong” mutual “distrust” and seek to mutually “weaken” each other, she adds.

Assassination plot revealed

The letter also reveals: “a rumor about a plot to assassinate Charles V that was to be brewing in France,” Ms. Desanclos says, a rumor about which “we knew little” before. It turns out to be “unfounded” – Charles V did not die from an assassination attempt – but this letter shows the “fear” of the prestigious monarch of “this potential conspiracy,” she emphasizes.

In his message to his ambassador, the emperor also mentions the position of his empire and his “political and military strategy”.

Researchers now hope they can identify other letters in Europe from the emperor and his ambassador “to get a photograph of Charles V’s strategy in Europe.” “We will probably make many more discoveries in the coming years,” says Camille Desenclos.