King's Secret Code Is Broken 500 Years Later

Letter reveals Charles V's fears of French assassination plot
By Arden Dier,  Newser Staff
Posted Nov 25, 2022 8:40 AM CST
Updated Nov 27, 2022 11:25 AM CST
King's Secret Code Is Broken 500 Years Later
A portrait of Charles V.   (Wikimedia Commons)

For the first time, researchers are reading about a rumored French plot to assassinate King Charles V of Spain in his own words—or symbols, rather. The information was only revealed after six months of "painstaking" work by codebreakers, per AFP. It's no secret that Charles V and Francis I of France were great rivals. Francis had hoped to have been elected Holy Roman Emperor, but Charles was. A now-decoded 1547 letter Charles sent to Jean de Saint-Maris, his ambassador in France, gives new insight into the relationship. Cécile Pierrot, a cryptographer at the French National Institute for Computer Science Research in Nancy, first heard about the coded letter, held at a local library, in 2019. Two years later, she got her hands on it, but couldn't immediately make out what was written.

Charles V had used "distinct families" of about 120 symbols in his message, Pierrot said at a Wednesday press conference. Some symbols were meaningless, while others symbolized whole words. Helped by a transcription code the ambassador had marked in another letter's margin, Pierrot finally deciphered a phrase of the letter in June, and the rest of the code was soon broken. "There was really a breakthrough that happened in one day, where all of a sudden we had the right hypothesis," said historian Camille Desenclos, a member of the research team. It's "rare as a historian to manage to read a letter that no one had managed to read for five centuries," she added, noting the letter "confirms the somewhat degraded state" of relations between Charles and Francis.

The rivals had been at war from 1521 to 1525, 1526 to 1529, 1536 to 1538, and 1542 to 1544, according to Richard Heath, an expert on Charles V. In 1544 they signed a peace treaty that specified that Francis' youngest son was to marry either Charles' daughter or niece. "The Treaty of Crepy in many ways reveals the nature of international politics at the time—sometimes laudable aims, often duplicitous agreements, and then an open disregard for what had been signed," writes Heath. Indeed, just three years later, Charles wrote to his ambassador describing a rumored plot to assassinate him brewing in France. Desenclos said researchers hope to examine other letters between Charles V and his ambassador, adding "it is likely that we will make many more discoveries." (Read more code breakers stories.)

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