Cryptologists Decode Mary Queen of Scots' Letters

Cryptologists use modern technology to decipher nearly 50 letters Mary Queen of Scots sent while imprisoned from 1578 to 1584.

By Emilie Le Beau Lucchesi
Mar 20, 2023 6:00 PMMar 20, 2023 7:25 PM
François Clouet - Mary, Queen of Scots (1542-87) - Google Art Project
(Credit:François Clouet, Public domain, via Wikimedia Commons)


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For years, researchers who visited the Bibliothèque nationale de France (BnF) passed over a collection of encoded letters. Someone had misdated the letters as originating from the 1520s and mistakenly cataloged the content as pertaining to Italy.

The letters, however, were encrypted in a complex code. No one actually knew what was in the letters and who wrote or received them.

Then, an interdisciplinary research team studying Mary, Queen of Scots, suspected the queen wrote the letters during her years of imprisonment. After cracking the code, they translated 50,000 words written by the confined queen. The team published their research in a February 2023 special edition of Cryptologia. Their insight shines new light into the Queen of Scots' darkest days. 

What Happened to Mary Queen of Scotland?

Mary, Queen of Scots, started her life as a queen and ended it as a prisoner. Born in 1544 to James V of Scotland and Mary of Guise, her father died six days after her birth, and she inherited the crown. Regents ruled on her behalf, and she was sent to France at age six for her own protection.

The queen married Francis II — the Dauphin of France who soon became king — at age 14. But he died two years later from illness, and she returned to Scotland. Her years in Scotland were turbulent. She married and had a son — James VI of Scotland and I of England —but political rivals challenged her right to rule as queen.

Mary, Queen of Scots and Lord Darnley (Credit:Original at Hardwick Hall, Derbyshire, in the care of the National Trust/ Public domain, via Wikimedia Commons)

On February 10, 1567, an explosion at the Kirk o' Field house in Edinburgh killed Lord Darnley — Mary's husband and James' father. However, Lord Darnley and one of his servants were found outside the house with possible strangle marks, leading many to believe he was assassinated

Mary became a suspect when she married the man accused of orchestrating the attack — James Hepburn, Earl of Bothwell. However, there is speculation that Bothwell may have forced her to marry him. 

Several Scottish lords, known as Confederate Lords, imprisoned and forced Mary to abdicate the throne to her one-year-old son. She escaped prison and fled to England to seek protection from her cousin, Queen Elizabeth I.

Queen Elizabeth, however, wasn't so welcoming. In the past, Mary had claimed her own right to the English throne, and Elizabeth had reason to worry Mary could successfully unite the country's Catholics against her. She ordered Mary to be held in the custody of the Earl of Shrewsbury, where she remained for the next 19 years. 

Read More: How Mathematicians Cracked the Zodiac Killer’s Cipher

Mary Queen of Scots' Letters

During her years of captivity, Queen Mary wrote carefully ciphered letters to her allies. Her first languages had been French and Scots, and the letters found in the BnF collection were written in French.

Cryptologists identified, deciphered and translated about 50 letters. Most of the letters were addressed to the French ambassador, Michel de Castelnau, Sieur de la Mauvissière, and were created between 1579 to 1584.

Cipher between Mary and Châteauneuf – selected parts (Credit: TNA SP52/22/22/ George Lasry, Norbert Biermann & Satoshi Tomokiyo)

To crack the code, the cryptologists began by using a computerized codebreaking program and then turned to linguistic and contextual analysis when needed. They had more than 150,000 symbols to transcribe, and they used an algorithm called "hill climbing" that allowed them to see if their decoding was correct.

Hill Climbing

"Hill climbing" begins by selecting a random symbol. The computer makes a small change to the key, and a decryption score results. If the score is higher than before, they keep the key. If not, they try a new variation, so the score continues to climb higher.

Hill climbing, however, could only take the researchers so far. The algorithm stumbled over the symbols that had a dot placed underneath or on the side, and the team used contextual analysis to deal with these diacritics.

For example, the symbol "y." first appeared after the phrase "sur l'arrivée prochaine," meaning "about the upcoming arrival." Typically, "de" meaning (of) would follow this phrase, but the researchers also knew "deça" (over here) was a possibility. They analyzed other uses and concluded it most likely meant "de." 

Other symbols represented proper names, such as her brother-in-law, the Duke of Anjou. The researchers likened the process to solving a giant crossword puzzle.

Read More: Neural Crossword Solver Outperforms Humans for the First Time

Cryptologists Crack the Code

After decoding and translating the letters, the cryptologists analyzed and identified common themes in Mary's letters.

No surprise, Mary complained about her life in prison. And given that she was the dowager Queen of France, she was unhappy with what she felt was the French authorities' indifference toward her captivity. She felt abandoned.

Despite feeling abandoned by the French, she warned the French ambassador not to trust Queen Elizabeth I or her representatives. She warned the English would not be sincere with their negotiations and only sought to weaken France.

Mary named 120 people, many of whom she viewed as enemies. She did not like Francis Walsingham, a spy for Elizabeth, and supplied evidence against Mary, including ciphered letters he intercepted.

She also noted her dislike for the Earl of Leicester (a favorite of Queen Elizabeth I) and the Puritan faction. Given her awareness of her vast quantity of enemies, it made sense that one common theme in the letters was her concerns about maintaining her currier channels for carrying the secret, encoded letters to her allies.

Portrait of Elizabeth I, Queen of England, by Anonymous, c. 1550-99, (Credit:Everett Collection/Shutterstock)

The letters also include her references to her dowry in France and her promise to reward supporters financially. But by 1583, she sensed that her attempts to negotiate with Elizabeth were worthless. Mary had hoped she could return to the Scottish throne if she gave up all claim to the English crown. However, she realized the English queen's responses were only meant to serve as stall tactics or to get political intelligence from her.

Mary was correct that Elizabeth did not intend to help her rival regain political power. Instead, Mary was arrested in 1586 and charged with attempting to arrange the assassination of Queen Elizabeth I. During the trial, other encrypted letters of Mary's were used as evidence against her.

Mary was executed in February 1587 after 19 years as a prisoner. She was 44 years old. The captive queen, however, lives on through her decoded letters more than 400 years after her death.

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