A letter written in secret code by Charles V, the former King of Spain and Holy Roman Emperor, has finally been deciphered by cryptographers after almost five centuries. The frustratingly enigmatic cipher initially proved too complex for codebreaking software to crack, yet a team of researchers eventually managed to interpret the message, revealing Charles’s worries about a rumored assassination plot.

The encrypted letter was penned in February 1547, in the midst of an ongoing conflict known as the Italian wars, which pitted France against Spain. And while a fragile peace had broken out between the two belligerents following the signing of the Treaty of Crépy three years earlier, the death of King Henry VIII of England – a key ally of Charles V – in January 1547 had placed the Spanish king in a position of weakness.

Charles therefore sent the dispatch to his French ambassador Jean de Saint-Mauris, instructing him to report back about the military designs of Francis I, the king of France. Wary of eavesdroppers, though, the somewhat insecure emperor made sure to construct his message using a code that even the Bletchley Park boffins would have struggled to untangle.

In her attempts to reveal the paranoid monarch’s correspondence, cryptographer Cecile Pierrot – Inria researcher at the LORIA research laboratories –  ran the ten-page document through a statistical analysis using the computer programing language Python in December 2021. However, initial results suggested that the software would need a period of time greater than the age of the universe to crack the code. 

Seeking to speed up the process, Pierrot enlisted the help of historian Camille Desenclos, who pointed her in the direction of other letters addressed to Jean de Saint-Mauris. Luckily, one of these documents included a rough key to the code scribbled in the margin.

This enabled the team to finally piece together the message in June 2022, after months of intense analysis. Speaking to AFP, Desenclos said, “It was painstaking and long work but there was really a breakthrough that happened in one day, where all of a sudden we had the right hypothesis.”

Among the tricks employed by Charles V to throw potential interceptors off the scent were meaningless characters interspersed throughout the text, which acted as red herrings that had to be overlooked in order to read the message.

After breaking the code, the researchers discovered that the emperor was particularly concerned about a rumor that 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 a report back to Charles the following month explaining that there was no truth to the rumor.

Other details included in the coded letter reveal that Charles V was eager to maintain his delicate peace with Francis I, at least until he could overcome a Lutheran uprising called the Schmalkaldic League, which threatened his empire. Taken together, Desenclos says these snippets provide historians with “a snapshot of Charles V’s strategy in Europe”, and insists that “it is likely that we will make many more discoveries in the coming years.”