A tranche of more than 55 encoded letters written by Mary, Queen of Scots has been uncovered — and decrypted — by a team of cryptographic experts.

Writing while being held for 19 years in the Tower of London for treason by her cousin Queen Elizabeth I, Mary Stuart sent a series of missives to the French Ambassador to the English court, Michel de Castelnau de Mauvissière. The idea was to evade the hawklike scrutiny of Elizabeth’s spymaster, Sir Francis Walsingham.

The 16th-century letters (written between 1578 and 1584) were found in the online archives at the National Library of France by an interdisciplinary team consisting of computer scientist George Lasry, pianist Norbert Biermann, and astrophysicist Satoshi Tomokiyo. According to their resulting paper, the cipher uses a typical replacement approach in which symbols were substituted for letters — but with more than 150,000 symbols used in total, the team turned to computerized codebreaking to make them readable, combined with old-fashioned sleuthing.

The effort combined “computerized cryptanalysis, manual codebreaking, and linguistic and contextual analysis,” according to the team.

The letters’ subjects included mulling marriage proposals for Elizabeth, the state of Mary’s support in Catholic France, and negotiations around her release and the ascension of her son James VI (future James I of England) to the Scottish throne.

“The existence of a secret communication channel between Mary and Castelnau has been well-known to historians, and it was even known to the English government at the time,” the researchers noted in the paper. “Even though its existence was known … the channel was so secure that its contents [were thought to] have been lost. … Our new decipherments provide further insights into how this channel was operated, and on the people involved.”