50 unpublished letters written by Mary Queen of Scots deciphered by code-breaking experts

08 February 2023
Secret letters written by Mary Queen of Scots while she was a prisoner in England have been cracked by a team of codebreakers.

While combing online collections of the Bibliothèque nationale de France (BnF) for enciphered documents, the researchers (George Lasry, Norbert Biermann and Satoshi Tomokijo) stumbled upon a large set of unmarked documents, all in cipher, using the same set of graphical symbols.

According to the BnF catalog, those were supposed to be from the first half of the 16th century, and related to Italian matters. After the team began to crack the code and had deciphered a few letters, they quickly realised that they were written in French and had nothing to do with Italy. Participes and adjectives were often in the feminine form, and several mentions of captivity, “my son”, and the name Walsingham (Francis Walsingham was Elizabeth I of England's principal secretary) led to the suspicion that the letters might be from Mary Queen of Scots.


After making substantial progress in reconstructing the code and deciphering additional letters, and finding a copy of the plaintext of several letters in Walsingham’s papers in the British Library, the team could establish with certainty that those were letters from Mary, written to Michel de Castelnau Mauvissière, the French ambassador to England, during her captivity in England.

The unpublished letters

The team continued to search for similar letters in BnF collections, and in total, found over 55 letters with this cipher. To the best of their knowledge, around 50 of them are unknown to historians and have never been published.

Mary Queen of Scots has left an extensive corpus of letters held in various archive collections. There was prior evidence, however, that other letters from Mary were missing from those collections, such as letters referenced in other sources but not found elsewhere.

The letters the team has deciphered were written between 1578 and 1584 and constitute a voluminous body of new primary material on Mary Queen of Scots – about 50,000 words in total, shedding new light on some of her years of captivity in England.

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The initial key used by the researchers

What information do the letters contain?

The letters the team deciphered were written whilst Mary was under the custody of the Earl of Shrewsbury, sent from Sheffield Manor Lodge or another of his properties, between 1578 and 1584. Mary’s secret letters highlight a multitude of topics.

One recurrent topic has to do with Mary’s efforts to maintain a secure communication channel with Castelnau, and through him, with her network of associates and allies, mainly in France. Careful precautions were taken to conceal and protect this critical channel, the deciphered letters showing that it was in place as early as 1578 and active until at least mid-1584. This confidential channel operated in parallel with an official channel under Walsingham’s supervision through which, in Mary’s own words, she would never write anything that she did not want even her worst enemies to be able to read.

Another recurrent topic in the letters is the proposed marriage between the Duke of Anjou and Queen Elizabeth of England. While Mary pledges her support for the marriage, often vehemently defending herself against accusations she is in fact opposing it, she constantly warns Castelnau that the English side is not sincere with their negotiations, their only purpose being to weaken France and counter Spain, by encouraging the duke to attack Spain in the Low Countries. After the duke’s campaign in Flanders ends up in disaster, as she had been warning all along, Mary offers to help reconciling the duke with the king of Spain.

A full paper will be published in a special edition of Cryptologia, a Taylor & Francis journal dedicated to the history of cryptology.

Image credits: Mary Queen of Scots copyright The Met, New York, Gift of Susan Dwight Bliss, 1958;