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Codebreakers decipher Mary, Queen of Scots’ secret letters 436 years after her execution

A team of codebreakers discovered – and then cracked – more than 50 secret letters written by Mary Stuart, Queen of Scots while she was imprisoned in England by her cousin, Queen Elizabeth I. 

In total, the team deciphered 57 letters penned between 1578 to 1584. Most are addressed to Michel de Castelnau de Mauvissière, the French ambassador to England and supporter of the Catholic queen, Mary Stuart, who was first in line of succession to the English throne after Elizabeth. About 50 of the scripts had never been seen before by historians.

The researchers presented their findings in the peer-reviewed journal Cryptologia on February 8, the same date that Mary was beheaded 436 years ago for allegedly plotting to kill her cousin.

The three codebreakers – George Lasry, a computer scientist and cryptographer; Norbert Biermann, a pianist and music professor; and Satoshi Tomokiyo, a physicist and patents expert – essentially stumbled upon the letters while combing the Bibliothèque nationale de France’s (BnF) online archives for enciphered letters.

The coded correspondence written by Mary, some 50,000 words altogether, were lumped into a larger collection of documents mostly relating to Italian affairs. All 57 letters were written in ciphertext – no parts of the documents were “in the clear” – meaning that they could not be attributed to anyone without first being deciphered. 

In order to crack the code, the trio used a combination of computerized and manual codebreaking along with linguistic and contextual analysis. 

But first, they had to transcribe all of the materials, which included more than 150,000 symbols. For that they used a bespoke graphical user interface (GUI) developed by the CrypTool 2 project.

Originally, the researchers assumed the coded language in the letters would be Italian. But after transcribing some of the letters, and using the GUI’s code-breaking algorithm to correlate the ciphers to plain text Italian, they “obtained no meaningful results.”

Assuming the language was French, however, did result in partial decryption – allowing the team to recover bits of plain text. The trio then recovered homophones used to represent single letters of the alphabet, as well as special symbols identifying people and places.

Comparing the new letters to previously-known correspondence between Mary and Castelnau allowed the team to determine that their ciphers were valid. 

“Upon deciphering the letters, I was very, very puzzled and it kind of felt surreal,” lead author Lasry said in a statement. Lasry is also part of the DECRYPT Project, which maps, digitizes, and deciphers historical ciphers.

“We have broken secret codes from kings and queens previously, and they’re very interesting but with Mary Queen of Scots it was remarkable as we had so many unpublished letters deciphered and because she is so famous,” Lasry added.

The secret letters span several topics and include Mary complaining about the conditions of her captivity and feeling that France had abandoned her and her son, who was abducted by Protestant English nobles and would later become King James I. 

In several of the documents, she expresses animosity toward the Earl of Leicester – a favorite of Queen Elizabeth – and tells Castelnau about alleged plots against her cousin, advising him to report them to the queen without letting on that Mary was the source of the info.

To the dismay of history buffs everywhere, however, the confidential correspondence doesn’t provide many details about the Throckmorton Plot in 1583. This was one of the attempts by English Catholics to depose Elizabeth I and put Mary on the throne, backed by a Spanish troop invasion and internal rebellion by some British noblemen. 

As Lasry and his co-authors wrote:

The researchers suggest that future efforts to may uncover additional letters written by Mary Stuart, and cite “evidence that some enciphered letters known to have existed are still missing.” ®