A team of French scientists managed to decipher a secret letter from 1547 of one of the most powerful rulers in Europe, revealing that he lived in fear that his assassination was being prepared by an Italian mercenary.
The Holy Roman Emperor Charles V sent it to his envoy at the French court - a man named Jean de Saint-Maurice.
The letter provides insight into what European rulers were thinking at a time of dangerous instability caused by religious wars and competing strategic interests.
This is also a rare opportunity for historians to see the dark arts of diplomacy in action: secrecy, insincere smiles and misinformation were evidently as present then as they are today.
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Cryptographer Cécile Pierrot first heard rumors of the existence of such a letter at a dinner three years ago in the city of Nancy, France.
After extensive research, she found it in the basement of the city's historical library.
She set herself the task of deciphering the document within a few days, but soon realized that it would be much more difficult than she thought.
The three-page letter, consisting of approximately 70 lines, was mostly written using 120 coded symbols, with one section in pure French of the time.
"First we had to categorize the symbols and look for patterns. But here it's not the case that one symbol represents one letter - it's much more complicated," says Piero.
"If we fed the data into a computer and told it to decipher it, it would literally take the entire history of the universe!".
Little by little, her team made progress.
There were, she discovered, two types of symbols: simple and complicated.
Vowels are generally not written as letters, but as diacritical marks, as in Arabic.
However, the vowel 'e' had no such sign, and was mostly missing from the letter.
They also found that most of the symbols represented letters or combinations of letters, while some represented whole words - for example, the pin is a symbol for England's King Henry VII.
And there were symbols that didn't seem to have any function.
The final breakthrough came when historian Camilla Desenclos directed the team to other coded letters sent by the emperor, as well as to him.
In one of them, the recipient produced an informal translation.
"That was our Rosetta Stone," Pierrot says, referring to the inscription that helped decipher Egyptian hieroglyphics.
"That was the key. We could have done without it in the end, but it saved us a lot of time."
The fact that the letter 'e' was almost not in the script is a sign that the creators of the code knew what they were doing.
Because 'e' is the most common letter (in the old and new French language) and it would be the first one that would be searched for by those who wanted to decipher the alphabet.
And symbols that had no meaning were inserted to cause further confusion.
"Of course, by today's standards the code is quite simple," says Pierrot, who spends her time thinking about quantum physics and the largest prime numbers.
"But considering what kind of tools they had, they really gave us a hard time," he adds.
So what does the letter say?
The full translation has not been published yet, because they keep it for academic work.
But they released parts.
February 1547 was a time of relative peace between the great powers - France and the Holy Roman Empire.
Emperor Charles V, who ruled vast parts of Europe, such as Spain, the Netherlands, Austria-Hungary, and southern Italy, was no longer at war with King Francois I.
But mistrust was present.
Two recent events were still on the minds of both rulers.
The first was the death of King Henry VII of England, just a few weeks earlier.
And the second rebellion in Germany, led by a Protestant alliance called the Schmalkaldic League (alliance).
Charles the Fifth reveals in the letter that he is worried and wants to maintain peace with France, so that he can concentrate his forces against the league.
He tells the envoy to keep his ears open and keep abreast of the thoughts at the French court, especially when it comes to reactions to the death of King Henry.
Most of all, he wanted to avoid the French and English joining together and supporting the Protestant rebels.
Charles V then writes about a rumor circulating - that the Italian mercenary leader Pier Strozzi will try to kill him, the emperor.
He tells the emissary that he needs to find out as much as he can about the story - is it gossip or a serious threat?
And finally, in the longest part of the letter, Charles V presents the ambassador with the current situation and his plan to deal with the Schmalkaldic League.
There were new waves of rebellions in Prague, which forced the Emperor's nephew Ferdinand of Tyrol to flee.
But Charles the Fifth gives the ambassador instructions on how to "spin" the news at the French court.
The rebellion in Prague is less of a problem, and he is told to say that Ferdinand left the city to join his father - the emperor's brother - in battle.
For historian Camila Desenclo, the fact that parts of the letter are encrypted and other parts are not is important.
"They knew that the chance of the letter being intercepted was one in two. In that case, there are messages that would be worth conveying to the French," she says.
For example, the fact that the emperor is cooperating in efforts to build confidence in northern Italy.
"These messages were left in plain language. But there were other things that had to be kept secret - like what was going on with the Protestant rebellion, and that was coded," she says.
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What happened next?
Just a few weeks later, King François I of France died and was succeeded by his son Henry II.
Charles the Fifth won the league the following year, but Protestantism did not leave Germany.
In 1552, Henry II formed a new alliance with Protestant princes against Emperor Charles V.
And there was no assassination of him.
Charles the Fifth died in a Spanish monastery in 1558.
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