To achieve this feat, six months of work were necessary for 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, during a dinner. The researcher then believes in a legend, but when the existence of this document is mentioned to her again two years later, she decides to dig. Word of mouth works, and 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.
Then begins the work of deciphering. Cécile Pierrot observes the letter for a long time, class “by separate families” the approximately 120 symbols used by Charles V. She names them and decides to count their occurrences, to spot the combinations that could be repeated. The deciphering is done, with the help of computers, “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.
Another disconcerting 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 manages 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.
A truly “Rosetta stone” also helps research: a letter from Jean de Saint-Mauris preserved in Besançon, where the recipient had written in the margin “a form of transcription” while deciphering the missive sent to him by the ambassador.
Once deciphered, the letter “comes to confirm the rather degraded state” in 1547 relations between Francis I and the Emperor of the Holy Roman Empire Charles V, who had nevertheless signed a peace treaty three years earlier. Despite this peace, the two sovereigns maintained a “mistrust” reciprocal “extremely strong” and seek “to weaken” mutually, explains the historian. Other information revealed by the decryption of the letter: “a rumor of an assassination plot against Charles V which would be brewing in France”says Mrs. Desenclos, a rumor whose “we didn’t know much” previously.
Charles V was not murdered, but this letter shows “fear” of the prestigious monarch vis-à-vis this potential plot. In his missive to his ambassador, the Emperor also evokes the situation of his empire and his “political and military strategy” : the use of encrypted correspondence thus allows him to “hide” this information to his adversaries. Researchers now hope to be able to identify other letters in Europe from the Emperor and his ambassador, “to have a photograph of Charles V’s strategy in Europe”. (AFP)