Codebreakers uncover secrets of lost letters Mary Queen of Scots wrote from jail

Secret letters written by Mary Queen of Scots while she was imprisoned by Queen Elizabeth I have been cracked by a team of codebreakers.

For centuries, the contents of the coded correspondence dating from 1578 to 1584 were believed to have been lost.

Mary, who was beheaded on this day 436 years ago, used a complex cipher system to hide her messages, which the codebreakers found include musings about her time in jail, poor health and attempts to negotiate her release.

Why was she imprisoned?

Mary had already been held captive in Scotland by the time she was detained in England - her imprisonment spread across castles from Carlisle to Fotheringhay over the course of 19 years.

The newly decoded letters were written while she was in the custody of the Earl of Shrewsbury.

She was jailed by Elizabeth, her cousin, because she was deemed a threat to her power.

Catholics considered Mary to be the legitimate sovereign, and was first in the line of succession.

Eventually, she was executed in 1587, aged 44, for her part in an alleged plot to kill Elizabeth.

What were the letters about?

Most of Mary's letters were meant for the French ambassador to England, Michel de Castelnau de Mauvissiere, who supported her claim to the throne.

They included complaints about her poor health and her captivity conditions, as well as her mistrust and disdain for Elizabeth's principal secretary Sir Francis Walsingham and Robert Dudley, Earl of Leicester.

She also expresses distress about the abduction of her son James, the future King, in August 1582.

Mary was known to have communicated with allies from jail - but the range of these letters, from 1578 to 1584, suggested that they were sent earlier and later than previously thought.

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How were they decoded?

The team was computer scientist and cryptographer George Lasry, music professor Norbert Biermann, and physicist Satoshi Tomokiyo, who stumbled across 57 letters in the national library of France's online archives.

The library had listed them as from the first half of the 16th century and related to Italian matters - but the authors realised soon after that they were written in French.

The cipher is homophonic with a nomenclature - this means each letter of the alphabet can be encoded using several cipher symbols, making sure no one symbol appears too often.

There are also dedicated symbols for certain words, names, and places.

"The code is quite elaborate, and it took us a while to crack it," said Mr Lasry, of the University of Kassel.

"But after a while, we started to see some plausible fragments of text in French. From those fragments, it emerged that the writer was in captivity, had a son, and was a woman, which could match Mary Stuart."

Their work revealed verbs and adverbs frequently in the feminine form, mentions of captivity, and references to Walsingham - described as the "definitive clue".

It was confirmed by comparing them with the plaintext of letters in Walsingham's papers in the British Library - and successfully revealed dozens of scripts previously unknown to historians.

Their findings have been published in the peer-reviewed journal Cryptologia.

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'A historical sensation'

The discovery has been hailed by leading expert John Guy, whose 2004 biography on Mary led to a 2018 film starring Saoirse Ronan and Margot Robbie.

"This discovery is a literary and historical sensation," he said.

"This is the most important new find on Mary Queen of Scots for 100 years. I'd always wondered if de Castelnau's originals could turn up one day - buried in the Bibliotheque Nationale de France or perhaps somewhere else - unidentified because of the ciphering.

"And now they have."