A coded letter signed in 1547 by the most powerful ruler in Europe has been cracked by French scientists, revealing that he lived in fear of an assassination attempt by an Italian mercenary.

Sent by the Holy Roman Emperor Charles V to his ambassador at the French royal court – a man called Jean de Saint-Mauris – the letter gives an insight into the preoccupations of Europe’s rulers at a time of dangerous instability caused by wars of religion and rival strategic interests.

For historians, it is also a rare glimpse at the darks arts of diplomacy in action: secrecy, smiling insincerity and disinformation were evidently as current then as they are today.

Cryptographer Cecile Pierrot first heard a rumour of the letter’s existence at a dinner party in Nancy three years ago. After lengthy research she tracked it down to the basement of the city’s historic library.

Setting herself a challenge to decode the document within a few days, she was disconcerted to find the task rather harder than she had thought.

The three-page letter – consisting of about 70 lines – is mainly written using about 120 encrypted symbols, but there are also three sections in plain contemporary French.

“The first thing was to categorise the symbols, and to look for patterns. But it wasn’t simply a case of one symbol representing one letter – it was much more complex,” says Pierrot.

“Simply putting it into a computer and telling the computer to work it out would literally have taken longer than the history of the universe!”

Little by little she and her team began to make progress. There were, she found, two types of symbol: simple and complex. Vowels were in the main not written as letters, but added as diacritical marks as in Arabic. The ‘e’ vowel though had no diacritical mark, so was largely absent.

They also found that while most symbols represented letters or combinations of letters, others represented whole words – like a needle fo