They have succeeded in solving a five-century-old enigma.

Four researchers presented Wednesday in Nancy their discovery, the deciphering of a letter written in 1547 by Charles V to his ambassador in France, shedding new light on the relations between the kingdom then led by François Ier, and the Holy Roman Empire Germanic.

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

The letter, forgotten for centuries, was in the collections of the Stanislas library in Nancy.

Cécile Pierrot, cryptographer at Loria, heard for the first time in 2019 of an "encrypted letter from Charles V" (1500-1558) by chance.

At the end of 2021 she sees for the first time the mysterious and incomprehensible letter bearing the signature of the King of Spain, addressed to his ambassador Jean de Saint-Mauris.

Cryptographers helped by a historian

Cécile Pierrot observes the letter for a long time, classifying “by distinct families” the some 120 symbols used by Charles V.

She names them and decides to count their occurrences, to spot the combinations that could be repeated.

With two other researchers, she uses computers to speed up research.

The deciphering is done "small steps by small steps", because the code used by Charles V is diabolical.

In addition to its large number of symbols, "whole words are encrypted with a single symbol" and vowels preceded by a consonant are marked by diacritics, an inspiration probably coming from Arabic, explains Cécile Pierrot.

Degraded relations between Charles V and François Ier

Another confusing element, the emperor uses "null symbols", which mean nothing and in fact serve to mislead the adversary who would try to decipher the message.

The click finally happens at the end of June: Cécile Pierrot and her team manage to isolate a series of words in the missive.

For this, the three cryptographers from Nancy called on Camille Desenclos, a specialist in both cryptography and relations between France and the Holy Roman Empire in the 16th century.

Once deciphered, the letter “comes to confirm the rather degraded state” in 1547 of relations between François Ier and the emperor of the Holy Roman Empire Charles V, who nevertheless signed a peace treaty three years earlier.

Despite this peace, the two sovereigns maintained an “extremely strong” reciprocal “mistrust” and sought to “weaken” each other mutually.


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