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As part of the DECRYPT project, a team of specialists combed through various archives in search of encrypted documents. While examining the archives of the French National Library, they found 57 documents of unknown authorship written in an unknown cipher.
"Upon deciphering the letters, I was very, very puzzled and it kind of felt surreal," says lead author George Lasry, computer scientist and cryptographer. "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and queens previously, and they’re very interesting but with Mary Queen of Scots it was remarkable as we had so many unpublished letters deciphered and because she is so famous. This is a truly exciting discovery."
Relying on a combination of computerized cryptanalysis, manual code cracking, and linguistic and contextual analysis, the team was able to successfully decode the correspondence. The documents contained only graphical characters (more than 150,000 in total), which required the team to use a specially designed graphical user interface tool.
By determining the overall structure of the letters and the language in which they were written, investigators concluded the letters were written by Mary to the then- French ambassador, Michel de Castelnau. Knowing this and some of the letters of the cipher, the team then identified symbols for names, places and the 12 months of the year.
Of the 57 letters, 54 were addressed to Castelnau and all were written between May 1578 and May 1584 (one partially encrypted letter is dated October 30, 1584). Eleven of the newly deciphered letters are dated 1580-1581.
As for topics, Mary often wrote about how to secure their communication channel so that she could maintain her network of allies in France. They also discussed Elizabeth I's proposed marriage and Mary warned Castelnau about Elizabeth's plans to harm France.
Mary also accused the Earl of Leicester and others of plotting against her, and Elizabeth of not negotiating in good faith to free Mary. According to the authors, she also expresses her disapproval of Walsingham, Elizabeth's spy master, and warns that some people working for her may be his agents.
"In our paper, we only provide an initial interpretation and summaries of the letters. A deeper analysis by historians could result in a better understanding of Mary’s years in captivity," says Lasry. "It would also be great, potentially, to work with historians to produce an edited book of her letters deciphered, annotated, and translated."