Charles V’s encrypted letter: a centuries-long riddle solved

15 December 2022

Thanks to the combined efforts of four researchers from Inria, Loria (CNRS, Inria, University of Lorraine) and University of Picardie Jules-Verne, an encrypted letter from Charles V has been decrypted and confirmed remarkable historic facts, five centuries after being written. This is the story of an unusual yet successful collaboration between computer scientists and historians.

It is not at all surprising that Charles V, the most powerful monarch of the first half of the 16th century, wrote a letter to his ambassador at the French Court. Nor is the fact that he took care to write it in cipher to protect it from unwelcome eyes, given the diplomatic tension between the two countries at the time. On the other hand, the kind of cipher he used certainly is. Seemingly created by the Imperial Chancellery, the cipher is so complex that the letter kept its secrets for almost 500 years, until a team of four French researchers managed to crack it.

It all began when Cécile Pierrot, Inria research fellow in the Caramba team (a joint undertaking involving Inria and Loria, where she studies modern cryptographic techniques), heard by chance of a “mysterious” letter by Charles V. Mysterious, because, while some scholars had taken a look, none had been able to decode it. However, that is what Cécile Pierrot set out to do, after tracking down the missive at Stanislas Library in Nancy.

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