500 years later, an encrypted letter from Charles V finally decrypted

After examining the manuscript in late 2021, a scientist managed to figure out the encryption of one of the letters of the Holy Roman Empire.


Charles V’s encryption system has more than a hundred symbols, some of which are intended only to prevent decryption.

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ua series of “incomprehensible” symbols that come to light five centuries later. Four researchers presented their discovery in Nancy on Wednesday 23 November, 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 Francis Ium, and the Holy Roman Empire. To carry out this “exceptional” feat, six months of work were needed by the cryptographers of 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 in Loria, heard for the first time in 2019 of a “ciphered 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 again two years later, she decides to dig. Word of mouth works, and at the end of 2021 he sees for the first time the mysterious and incomprehensible letter signed by the Emperor, addressed to his ambassador Jean de Saint-Mauris.

Then the decryption work begins. Cécile Pierrot observes the letter for a long time, classifying the approximately 120 symbols used by Charles V “by distinct families”. She names them and decides to count their occurrences, to identify the combinations that could be repeated.

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“Null symbols” to mislead

To do this, she and two other researchers in Nancy’s lab, Pierrick Gaudry and Paul Zimmermann, decided to use computers to “speed up research.” No artificial intelligence, here it is the human who “asks the right questions to the computer”, insists the cryptologist. The deciphering is done “in small steps”, because the code used by Charles V is diabolical. In addition to the large number of symbols, “whole words are encrypted with a single symbol” and vowels preceded by a consonant are marked with diacritical marks, an inspiration probably coming from Arabic, explains Cécile Pierrot.

Another element of confusion, the emperor uses “null symbols”, which mean nothing and in fact serve to mislead the adversary who tries to decipher the message. The click finally takes place at the end of June: Cécile Pierrot manages to isolate a series of words in the letter. For this, the three cryptographers from Nancy turned to Camille Desenclos, a specialist both in cryptography and in the relations between France and the Holy Roman Empire in the 16th century.And century. The historian helps them piece together the puzzle by recontextualizing the letter to better understand its allusions.

A real “Rosetta stone” also helps the search: a letter from Jean de Saint-Mauris kept in Besançon, where the recipient had written in the margin “a form of transcription” deciphering the letter sent to him by the ambassador, explains Ms . Pierrot.

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Francis Ium – Charles V, a deleterious relationship

Once deciphered, the letter “comes to confirm the rather degraded state” in 1547 of relations between Francis Ium and Holy Roman Emperor Charles V, who nonetheless signed a peace treaty three years earlier, explains Camille Desenclos. Despite this peace, the two sovereigns maintain a “very strong” mutual “distrust” and try to “weaken” each other, he adds.

Other information revealed by the decipherment of the letter: “a rumor of an assassination plot against Charles V being prepared in France,” he says, a rumor of which “we didn’t know much” before. It turns out to be “unfounded” – Charles V was not assassinated – but this letter shows the prestigious monarch’s “fear” of “this potential conspiracy”, he underlines.

In the letter to his ambassador, the emperor also evokes the situation of his empire and its “political and military strategy”: the use of encrypted correspondence thus allows him to “hide” this information from his opponents. Researchers now hope to locate other letters in Europe from the emperor and his ambassador, “to get a snapshot of Charles V’s strategy in Europe.” “It is likely that we will make many more discoveries in the coming years,” Camille Desenclos rejoices.

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