More than fifty letters of Mary, Queen of Scots, preserved in the French national library were found and deciphered by an international team of three cryptographers including me (George Lasry, Norbert Biermann, Satoshi Tomokiyo). The tragic life of Mary is well-known -- after three mariages, she fled to England, where she was held prisoner, and after years of captivity, she was executed on evidence of a deciphered letter for plotting against Queen Elizabeth. The letters discovered are entirely in cipher, with nothing to indicate who wrote them to whom and when. Codebreaking revealed that most (54) of the letters are letters written during 1578-1584 from Mary to Castelnau de La Mauvissière, French ambassador to England. Among them, more than 40 up to the middle of 1583 are hitherto unknown letters and are expected to provide new insights about Mary during her captivity in England. The results were published in George Lasry, Norbert Biermann, Satoshi Tomokiyo (2023), "Deciphering Mary Stuart's Lost Letters from 1578-1584" in Cryptologia (Open Access).
As one of the authors, I'd like to provide here a brief introduction. (I also wrote for Japanese readers.)
Mary alienated Scottish nobles by marrying the Earl of Bothwell, who was suspected of involvement in the murder of her second husband, Lord Darnley. Fleeing to England in 1568, she ended up in being held a prisoner for the remaining 19 years of her life. The letters discovered are from this period of captivity.
Most of the letters discovered are from Mary to Castelnau de La Mauvissière, French ambassador to England. Since Mary's first husband was king Francis II of France, Mary was a dowager queen of France and a sister-in-law of Henry III. She was not a person that the ambassador could make light of. While there are many known letters from Mary to Castelnau, the known letters before mid-1583, when the spymaster Walsingham succeeded in bribing a clerk in the French embassy to provide copies of Mary's letters, all seem to be letters sent via an official channel under the supervision of Walsingham. The discovery of Mary's letters in cipher, presumably sent without being detected by Walsingham, is expected to provide invaluable materials about Mary, Queen of Scots. (Many of the letters discovered mention an enclosure for James Beaton, Archbishop of Glasgow and Mary's ambassador in France. Four of them match the dates of known letters to Beaton and have some similar contents.)
It may seem that Mary's life in captivity was led idly without notable events. However, it is already known that in her captivity, Mary still managed to communicate with the outside world, actively collecting information and sending out messages.
As known from history books, Mary claimed she was the sole queen of Scotland and her son James could not be king as long as she lived. At one time, her secretary sent to Scotland was refused an audience with James because he did not call him king. Mary had an idea of sharing the Scottish crown with her son, and used her potential influence on Scotland in negotiations with the English court for her release. When James was kidnapped by pro-English nobles (the Ruthven Raid), Mary appealed for the French king's assistance for her son's release. During a prolonged negotiation for a marriage between Elizabeth and the Duke of Anjou, Henry III's younger brother and Mary's brother-in-law, Mary openly professed her approval for the match, but she also sought an alliance with France's rival power, Spain.
As can be seen from these, it has been known that Mary was actively engaging herself with international situation in spite of her position in captivity. These topics are frequently mentioned in the letters we discovered. For example, one can see Mary's initial reaction right after the Ruthven Raid, her detailed instructions to de La Motte, who was sent to Scotland in response to Mary's plead, and her disappointment thereafter. Regarding the Scottish crown, Mary's views varied at different stages in the negotiation: accepting James being called king merely for expediency's sake for her envoy to obtain an audience; insisting that James could be called king by foreign princes only after the agreed joint rule was published by the parliament; and then granting James the title of king because of his devotion to her (as well-known, the plan was eventually rejected). Regarding the Duke of Anjou, when a slander against Mary was spread, she asserted that the anti-marriage faction was trying to make a division between Mary and Anjou, and hence Elizabeth. When the Duke of Anjou, returning from paying court to Elizabeth to the battlefield with Spain in the context of the Dutch War of Independence, made a nuisance of himself among the Protestants by his disastrous conduct, Mary even proposed to mediate a reconciliation between the Duke and Spain. People mentioned in the letters discovered number more than 100, among which Walsingham and the Earl of Leicester are often the targets of Mary's unveiled wariness. Mary occasionally advised Castelnau to warn the English government on the basis of her intelligence, but she made a point of telling him not to disclose the source in order to keep their secret communication channel from being exposed.
While our paper attempts a preliminary analysis focusing on Mary's communication with the outside world during her captivity, the various topics and people mentioned in the letters discovered would allow historians to draw more exciting insights about Mary, Queen of Scots.