For almost 500 years, historians have puzzled over the contents of a letter that Emperor Charles V wrote in 1547 to one of his ambassadors in France. For entire sections of this letter were encoded with secret symbols. Only now has a team of researchers succeeded in deciphering this secret code. Among other things, they reveal that Charles V was concerned about a rumor after someone at the French court was plotting to assassinate him.
Born in 1500, Charles V came from the Habsburg dynasty and inherited several empires from his royal ancestors. He became King of Spain, Ruler of Austria and was crowned Emperor of the Holy Roman Empire of the German Nation in Aachen in 1520. During his reign, Charles V fought several wars, and from 1540 there was also an armed conflict with France. This war was ended in 1544 with the Treaty of Crépy by an armistice. Nevertheless, Charles V’s relationship with France’s King Francis I remained strained.
Mysterious symbols in the imperial letter
A letter that Charles V wrote in February 1547 to Jean de Saint-Mauris, his ambassador to France, dates from this period. Parts of this letter, now preserved in the Stanislas Library in Nancy, France, are written in normal script and are legible. However, Charles V wrote other sections in a secret code of cryptic symbols. Some resemble a figure eight with appendages, others are a combination of strokes or squiggly shapes. What these symbols mean, however, remains unknown – and thus also the content of these passages.
After historians and linguists failed to decipher this letter, a team of cryptographers led by historian Camille Desenclos of the Université de Picardy Jules Verne took another look at the letter in 2021. They first attempted to crack the encryption using common decryption computer programs that evaluate the frequency, arrangement, and combination of the symbols. However, it quickly became apparent that the code was more complex than initially expected. Instead of a simple letter-to-symbol conversion, it relied on a combination of very different types of symbols – some simple, some relatively complex.
Complex encryption
The team then developed a new algorithm that was trained on contemporary French letters to recognize typical words and language patterns. It turned out that Charles V apparently interspersed several pseudo-symbols in the text to further obscure the content. “This was a method that was in vogue in 16th-century Europe: you interspersed the document with characters that had no meaning,” says the press release from the university. “These symbols were not indicators of spaces or punctuation, but simply had to be skipped over when deciphering them.”
After the researchers recognized this and identified the “zero symbols” with the help of the computer, they were able to read at least parts of the encrypted passages – but still not all of them. This was only possible after the team discovered in the archives a letter written by Ambassador Jean de Saint-Mauris, in which he had also encrypted parts of the text with this code. Both letters together and the adapted decryption algorithms finally helped to completely decipher Charles V’s letter.
The Schmalkaldic League and a rumor of an assassination attempt
It is now clear for the first time what Charles V wrote to his ambassador in February 1547 and which political issues were the focus. “According to the letter, the ruler was primarily concerned with three issues: maintaining the fragile peace with France, preventing an assassination attempt against him and resolving the conflict with the Schmalkaldic League,” the researchers report. The latter was a league of German princes who had converted to Protestantism and who resisted the rule of the Catholic ruler and the suppression of their religion. From 1546 Charles V waged war against the rebellious princes. In his letter from the beginning of 1547 he is mainly interested in receiving news about the progress of this campaign.
While this and also the relationship to France was already known from other writings and letters, the encrypted letter also reveals something previously unknown: Charles V refers to a rumor that Pierre Strozzi, one of Francis I’s generals, told him should seek his life and plan an assassination. Francis I is said to have declined to support such an assassination plan, but according to rumors he is not officially forbidden to do so. Charles V therefore instructs his ambassador in France to check the veracity of the rumor.
“This rumor is interesting for two reasons: first, it reveals the deep-seated enmity between Pierre Strozzi and Charles V and the tensions between France and the Habsburg Emperor. On the other hand, this rumor was not previously documented anywhere except in this letter and its reply. The newly decrypted passages from the letter thus provide the first insight into this supposed danger of an assassination attempt and Charles V’s reaction to it.
Source: CNRS, Bibliothèques Nancy, Université de Picardie