A codebreaking team stumbled on more than 50 coded letters and cracked the cipher to discover secret correspondence written by Mary Stuart, Queen of Scots almost 500 years ago while she was imprisoned by her cousin, England’s Queen Elizabeth I.

Experts consider the finding to be the most significant discovery involving royal correspondence for more than a century. The letters stored in France's national library are dated from 1578 to 1584, three years before Mary’s execution in 1587. Most of the letters were sent to one of Mary’s supporters, the French Ambassador to England Michel de Castelnau.

In order to crack the code, the trio of codebreakers - a music professor, a computer scientist/cryptographer; and a physicist/patents expert - used a combination of computerized and manual codebreaking along with linguistic and contextual analysis. Their extraordinary finding and methods are detailed in February 2023’s Cryptologia, a journal of cryptography.

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The accession of Elizabeth Tudor to the throne of England in 1558 meant that Mary was, by virtue of her Tudor blood, next in line to the English throne but Elizabeth considered her a threat to her power. Mary was jailed by Elizabeth, her cousin, and eventually executed in 1587, aged 44, for her part in an alleged plot to kill Elizabeth.
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Ciphertext and decoding the letters

In her letters, Mary complains about her captivity - which lasted 19 years - and her feeling that France had abandoned her and her son who would later become King James I. She also confides in Castelnau about alleged plots against her cousin without telling him Mary was the source of the info. The letters do not give away much about the Throckmorton Plot in 1583 - one of a series of attempts by English Roman Catholics to depose Elizabeth and replace her with Mary - although the correspondence may be part of a larger collection still to be found in the archives. 

The three codebreakers - George Lasry, Norbert Biermann, and Satoshi Tomokiyo - found the encrypted letters while searching the Bibliothèque nationale de France's online archives. The letters were included in a collection of documents linked to Italian affairs.

“Upon deciphering the letters, I was very, very puzzled and it kind of felt surreal,” Lasry said. ”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.”

The letters were all in ciphertext which included more than 150,000 symbols. For that, they used a graphical user interface developed by the CrypTool 2 project. Assuming the language was Italian and later French, they recovered bits of plain text and recovered homophones used to represent single letters of the alphabet, as well as symbols identifying people and places. A description of their methods is laid out in the Cryptologia article offered for peer review. 

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