Cipher letter from Emperor Charles V raises fears of assassination

An encrypted letter from the Emperor Charles V, almost five hundred years old, which was sleeping in the autograph fund of the Stanislas library in Nancy, has just revealed its secrets thanks to the combined action of researchers in cryptography, computer science and in history, who broke through its code. An adventure that began a bit like a legend that lifts the veil on historical events. We learn that the most powerful emperor of the XVIe century feared being assassinated by someone close to François Iis.

Cécile Pierrot, researcher in cryptography at the Lorraine Laboratory for Research in Computer Science and its Applications (Loria) at the University of Lorraine, had heard of a “mystery letter” from Charles V, but, finding no trace of it anywhere, ended up convincing himself that he was a legend. At the end of 2021, with the help of Céline L’Huillier, librarian at Stanislas, she finally gets her hands on the mysterious missive sent on February 2, 1547 by Charles V, Emperor of the Holy Roman Empire, to his ambassador to the King of France. , Jean de Saint-Mauris. At that time, relations between Francis Iis and Charles V are bad. They engage in incessant wars, while Charles V must face the Protestant revolt led by the League of Smalkalde. But, in 1547, Europe went through a period of relative calm.

The letter alternates between a few brief passages in plain text and long sequences in figures. Specialist in modern cryptography, Cécile Pierrot thinks that she will easily come to the end of this text. The first statistical analyzes and the classification of the symbols give nothing. “I realized that we were faced with complex encryption. We were not in the case where a symbol equals a letter”, told the researcher during a press conference, Wednesday, November 23, in Nancy. Launching the computer and waiting for it to find would have taken… an eternity.

Read also: Article reserved for our subscribers Anxieties and phobias of Charles V

“Like entering a new universe”

Two Loria computer scientists, Paul Zimmermann and Pierrick Gaudry, then working on the factorization of whole numbers, joined the adventure. The complexity of the encryption and the historical scope of the enigmatic mail served as an adrenaline rush. “Participating in this company was like entering a new universe. We were confused by the language used, in this case Middle French, roughly the language of Rabelais, moreover on a historical background that we did not know precisely »explains Paul Zimmermann.

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