French historians crack mysterious code in letter from Charles V

A team of researchers has cracked the secret code used by Charles V, Holy Roman Emperor and King of Spain, in a letter that has puzzled historians for nearly 500 years.

Charles was one of the most powerful men of the 16th century, presiding over a vast empire that took in much of western Europe and the Americas during a reign of more than 40 years.

It took the team from the Loria research lab in Lorraine, eastern France, six months to decipher the mysterious letter.

It was written by the emperor to his ambassador in France in 1547, during a tumultuous period that saw a succession of wars and tensions between Spain and France.

Confusing symbols

The letter had languished forgotten for centuries in the collections of the Stanislas library in Nancy.

Cécile Pierrot, a cryptographer from Loria, first heard of its existence in 2019, and after much searching was able to view it for herself in 2021.

Bearing the signature of Charles V, it was utterly incomprehensible, she told reporters this week.

In painstaking work assisted by computers, Pierrot found "distinct families" of some 120 symbols used by Charles V.

"Whole words are encrypted with a single symbol," she said.

The emperor replaced vowels that come after consonants with marks, she added, probably inspired by Arabic. He also used meaningless symbols to mislead any adversaries trying to decipher the message.

The breakthrough came in June, when Pierrot managed to make out a phrase in the letter.

"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," she said.

Assassination plot

The team was then able to crack the code with the help of another historian, Camille Desenclos.

Desenclos said it was rare "to read a letter that no one had managed to read for five centuries".

While Charles V had signed a peace treaty three years earlier with the king of France, Francis I, relations were still tense between the two, she explains.

In fact, the letter revealed the rumour of an assassination plot against Charles V that was said to have been brewing in France, Desenclos said.

While few details are known about the plot, the suspicions underline the monarch's fear, she said.

The researchers now hope to identify other letters between the emperor and his ambassador "to have a snapshot of Charles V's strategy in Europe", Desenclos said.

She commented: "It is likely that we will make many more discoveries in the coming years."

(with wires)