It Took Nearly 500 Years for Researchers to Crack Emperor Charles V's Secret Code

Arthur T Knackerbracket writes:

https://arstechnica.com/?p=1900355

In 1547, Holy Roman Emperor Charles V penned a letter to his ambassador, Jean de Saint-Mauris, part of which was written in the ruler's secret code. Nearly five centuries later, researchers have finally cracked that code, revealing Charles V's fear of a secret assassination plot and continued tensions with France, despite having signed a peace treaty with the French king a few years earlier.

Charles V's three-page letter to his ambassador was written against the backdrop of a fierce struggle against the rising Protestant religion-the emperor was a devout Catholic who denounced Martin Luther as an outlaw at the Diet of Worms in 1521-and continued tensions with Francis I of France. Cecile Pierrot, a cryptographer with the Loria laboratory in France, first heard of the letter's existence at a dinner in 2019 and finally managed to track it down two years later, where it was languishing in the basement of the historic library in Nancy. Pierrot promptly tried to crack the coded portions of that letter by categorizing the various symbols and hunting for telltale patterns.

[...] The task proved rather daunting since the approximately 120 encrypted symbols didn't employ a simple symbol-to-letter representation. Granted, most represented letters or combinations of letters, per Pierrot, but others represented entire words-for instance, a needle to represent the English King Henry VIII. Vowels that came after consonants were replaced by diacritical marks, except for the letter 'e' (the most commonly used letter), which the code makers cleverly avoided using as much as possible. And a few symbols didn't seem to serve any function at all. "Simply putting it into a computer and telling the computer to work it out would have literally taken longer than the history of the universe," Pierrot told BBC News.

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