A cipher letter from Charles V evokes fears of assassination

A 500-year-old encrypted letter from Emperor Charles V, which was dormant in the autograph collection of the Stanislas Library in Nancy, has just revealed its secrets thanks to the combined action of researchers in cryptography, computer science and story, who cracked his 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 XVI^e century feared being assassinated by someone close to François I^er.

Cécile Pierrot, cryptography researcher 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, the librarian, 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 I^er 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 classification of symbols yield 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.

“Like entering a new universe”

Two Loria computer science researchers, Paul Zimmermann and Patrick Gaudry, join the adventure, between work on the factorization of whole numbers. 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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