In 1547, 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.

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.

The task proved rather daunting since the approximately 120 encrypted symbols didn’t employ a simple symbol-to-letter representation. Most represented letters or combinations of letters but others represented entire words. For example: a needle was used 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.

The big break occurred thanks to historian Camille Desenclos, who directed Pierrot to several other coded letters written by, and sent to, the emperor. One of which turned out to have been informally translated and became the key for cracking the code.