A letter written in secret cipher by Charles V, the former king of Spain and Holy Roman Emperor, was finally deciphered by cryptographers after nearly five centuries. The incredibly enigmatic cipher initially proved too complex for code cracking software to crack, but a team of researchers eventually managed to interpret the message, revealing their concerns about Charles’ assassination plot.
The ciphered letter was written in February 1547, in the midst of a conflict known as the Italian wars, which pitted France against Spain. And while there was a fragile peace between the two warring parties following the signing of the Treaty of Crépy three years earlier, the death in January 1547 of King Henry VIII of England, an important ally of Charles V, put the Spanish king in a vulnerable position.
Therefore, Charles sent this letter to the French ambassador, Jean de Saint-Mauris, instructing him to report on the military plans of Francis I, king of France. Still wary and somewhat distrustful of eavesdroppers, the emperor wrote his message using code that even Bletchley Park scientists would have a hard time deciphering.
Cryptographer Cecile Pierrot, an Inria researcher at LORIA research labs, ran a statistical analysis of the ten-page document in December 2021 using the computer programming language Python. However, initial results suggested that software would need more time than the age of the universe to crack the code.
Wanting to speed up the process, Pierrot sought help from historian Camille Desenclos, who directed him to other letters sent to Jean de Saint-Mauris. Fortunately, one of these documents contained a rough key of the code scribbled in the margin.
This allowed the team to finally piece together the message in June 2022, after months of intense analysis. “It was a grueling and long work, but in one day there was real progress and suddenly we had the right hypothesis,” Desenclos told AFP.
Deciphering the letter, the researchers realized that the emperor was speaking of his concern that the Italian military leader Pierre Strozzi, who was in the service of Francis I, was planning to assassinate him. However, after being instructed to investigate the situation, Saint-Mauris sent Charles a report the following month explaining that the rumor was not true.
Other details contained in the cryptic letter show that Charles V was willing to keep his delicate peace with Francis I, at least until he had overcome a Lutheran uprising called the Schmalkaldic League that threatened his empire. Taken as a whole, these particles provide historians with “a snapshot of Charles V’s strategy in Europe,” says Desenclos, and that we’re likely to make many more discoveries on this subject in the years to come.
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