French scientists have decoded a letter signed in 1547 by the Holy Roman Emperor Charles V, the Stanislas Libary in Nancy, France, has announced.
The secretly-coded letter has revealed the significant concerns of the most powerful man in Rennaisance Europe during an era of religious and strategic conflict. It showed that Charles V feared an assassination attempt by an Italian mercenary and was prioritizing his relationship with King François I of France. 
The three-page letter — sometimes written in plain script and some in a mysterious code —  uncovers the royal secrets of the 16th century, including a rumor that Pierre Strozzi, head of war under François I, was plotting his death. 
After almost five centuries, the letter had lain forgotten in the Stanislas Library. French cryptographer Cecile Pierrot heard a rumor of the mysterious historic document at a dinner party, according to the BBC,  and searched for it in the library’s basement.
Charles V ruled over a vast European territory, including Spain, southern Italy, the Netherlands, swathes of central Europe, and large parts of the recently discovered Americas.
The letter sent by the emperor to his ambassador at the French royal court was written against the febrile backdrop of continental wars and religious conflict between Catholics and Protestants, which meant it was essential to be able to communicate secretly and not give away any precious information to prying eyes. 
Until now, the contents of the letter have remained a mystery as it was composed of about 120 encrypted symbols and some French passages. 
Pierrot gave all the symbols a name and loaded the makeshift alphabet into Python, a programming language, but it could not unlock the mysterious language.
Pierrot and her team — which included French cryptographers Pierrick Gaudry and Paul Zimmermann and historian Camille Desenclosset to work for months wading through the strange script invented by Emperor Charles, identifying decoy letters and getting slow and steady eureka moments. 
The team has not yet issued a complete translation, but the themes identified have revealed an invaluable insight into the thinking of a giant figure at a turning point in Europe’s history. 
Read next
Listen to The Refresh, Insider’s real-time news show
Listen to The Refresh, Insider’s real-time news show