Oil painting portrait of Mary Stuart, Queen of Scots.

Decoded secret letters reveal royal plot against Queen Elizabeth I

The imprisoned Mary, Queen of Scots used sophisticated ciphers as she conspired to overthrow her cousin, Elizabeth I. Long thought lost, the letters were hiding in plain sight.

A portrait of a bejeweled Mary Stuart, Queen of Scots, was painted by Henri Serrur in 1840. Versailles, Château De Versailles 
Photograph by DeAgostini, Getty Images

Mary, Queen of Scots, was a master at sending secret messages, and codebreakers have spent centuries frustrated by her sharp ciphering skills. Whenever her surviving exchanges have been cracked, scholars thrill to the information found inside. 

But the existence of one set of letters, believed lost, has tantalized historians for centuries: the ones she penned during her nineteen years in English prisons. Prior to her execution on February 8, 1587, Mary Stuart wrote dozens of secret letters to French officials believed to contain sensitive information about her plots to regain the Scottish throne and to usurp her first cousin and political rival, Queen Elizabeth I.

For centuries, scholars thought these letters were either irretrievably hidden or destroyed, but an international trio of amateur codebreakers has just proven them wrong.

George Lasry, a French computer scientist based in Israel, Norbert Biermann, a German professor of opera, and Satoshi Tomokiyo, a Japanese physicist, recently discovered over fifty encrypted letters penned by Mary Stuart—some 50,000 words written in a sophisticated code devised by the monarch herself. Using a complex combination of computer software and traditional codebreaking techniques, the three sleuths were able to crack Mary’s cryptograms, revealing a wealth of new information about the monarch and her political milieu.

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Safety in secrecy

Born in 1542, Mary Stuart was the second in succession to the English throne. She became queen of Scotland when she was just six days old. At the age of five, Mary was betrothed to François, dauphin of France. She joined his royal household shortly thereafter. Educated at the French court, Mary became fluent in multiple languages and was schooled in political theory. Her mother-in-law, the powerful Catherine de Medici, taught her both the art of international diplomacy and spiral letter-locking, an intricate process of folding documents so that their contents were secure.

Mary used both during her brief reign as France’s Queen Consort.

Jennifer DeSilva, a professor of History at Ball State University and editor of The Sixteenth Century Journal, says this need for secrecy grew significantly during Stuart’s lifetime, when the Protestant reformation was intensifying conflicts between the Holy Roman Empire and already hostile noble factions in Europe.

“In the politically unstable decades from 1560 onwards, when tensions mounted between England and Scotland, and Spain hoped to secure more territories for Catholicism by marriage, espionage, or invasion, ciphers were essential protection,” says DeSilva.

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Under surveillance

Those cryptological skills would become invaluable once Mary was forced to abdicate the Scottish throne in 1567. By then, she had already witnessed the murder of her personal secretary, purportedly at the hands of her second husband. She had been implicated in that husband’s assassination, along with the man who would become her third spouse. And she was already suspected by many to be plotting Elizabeth’s usurpation. Small wonder the English monarch placed her cousin under house arrest and tasked her royal spymaster, Francis Walsingham, with Mary’s surveillance.

During his tenure, Walsingham recruited a series of moles within the French court to intercept correspondences from and to Mary. Several of these—including the notorious cipher she sent implicating herself in a 1586 plot to overthrow Elizabeth which led to her eventual execution—were successfully forwarded to Walsingham. They provided scholars with early insight into Mary’s use of cipher.

Walsingham’s moles also intercepted letters written by Michel de Castelnau, France’s ambassador to England, which made repeated mention of additional secret correspondences by Mary. However, historians assumed that those clandestine letters had been so well concealed by de Castelnau that they were forever lost.

Turns out, they were hiding in plain sight at the National Library of France.

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Trio of codebreakers

In 2018, George Lasry began perusing digitized collections of the library’s holdings, looking for previously unsolved historic cryptograms he could crack in his spare time. He found a series of them in a file dedicated to early 16th century Italian correspondences. Because the ciphers were comprised entirely of symbols—intricate snaking lines and curves, along with shapes and variations on the Roman alphabet—there was no way of knowing who had authored them or when. Nor could Lasry determine in what language their original contents had been written. He enlisted the help of Biermann and Tomokiyo to break the code.

The three amateur cryptologists had never met in person, but they had gotten to know each other remotely as contributors to Cryptiana, a website managed by Tomokiyo that is dedicated to historic cryptograms.

“No one could have had any clue that these ciphers were written by Mary, so there was no incentive for scholars to solve them,” says Lasry. “It was only going to be crazy people like us for whom ciphers are a personal obsession.”

The trio’s first task was to transcribe the ciphers’ 150,000 characters into symbols a modern computer program could recognize—a process that took several months. Because these characters comprised 191 distinct symbols, the team knew the ciphers weren’t written in a simple substitution style, where each letter of the alphabet is assigned a corresponding mark. Instead, they speculated that these ciphers were using a more sophisticated homophonic code, in which a single letter of the alphabet could be assigned several different symbols, making the cipher much harder to break.

The language questions

To discover the meaning of each symbol, the team fed the transcription into an AI-assisted software program that attempts to unlock a code’s key. However, for that program to work, they still needed to know in what language the original code had been written. Given where they’d first found the ciphers, Lasry figured Italian was a good guess. However, the thousands of key variations generated by the software program failed to turn up a single identifiable word.

“We just kept getting garbage,” says Lasry. “So, then we tried Spanish. Next Latin. Still no luck.”

Finally, they decided to use French. Lasry estimates the software program then ran hundreds of thousands of variations—possibly over a million. Along the way, identifiable words written in a feminine grammar began to appear: terms like fils, the French word for son, and ma liberté.

“The only reason to write about your liberty is if you are not free,” says Lasry. “So we began to piece together that we were dealing with a mother who was being held captive.”

Each time the team found another identifiable French word, they would lock it into the software program, generating a better possible key—a process known in cryptology as hill climbing. Eventually, the program stumbled upon the name Walsingham, Elizabeth’s spymaster.

“That’s when we began to speculate that these letters might have been written by Mary,” says Lasry. “But when we took them to historians, they told us there was no way–that we were wasting our time.”

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Unraveling Mary’s enigma

The trio kept at it for several months. However even their best computer-generated key was still only able to unlock about a third of the ciphers’ contents. That’s when they realized Mary wasn’t just encoding individual letters of the alphabet; she had also devised symbols for discrete names and words, as well as parts of words. The codebreakers began working manually, trying to fill in the blanks based on existing patterns and linguistic context, which Lasry likens to completing the world’s biggest and most byzantine Sunday crossword puzzle.

“It was incredibly time consuming,” says Lasry. “We all have day jobs, so it was a year of dedicating our nights and weekends to the project.”

Once they were fully cracked, Lasry brought the ciphers to John Guy, a history fellow at the University of Cambridge’s Clare College and a world-renowned expert on Mary, Queen of Scots. Using existing ciphers of Mary’s, Guy was able to authenticate these 57 new ones.

“It’s a stunning piece of research,” says Guy. “They mark the most important new find on Mary Stuart in over a hundred years.”

Guy and other historians have only just begun to dig into the voluminous contents of these letters, which were all written in Middle French and are marked by a complex diction and syntax.

However, they’ve already found much that is of great historic significance. Many of the letters detail Mary’s shrewd skills at political subterfuge, from her attempts at bribing Elizabeth’s councilors to orchestrating a marriage between Elizabeth and Mary’s brother-in-law, France’s Duke of Anjou. They also include her efforts to reclaim the Scottish throne and her involvement in the 1583 Throckmorton Plot to overthrow Elizabeth.

Such details will shed important new light not only on Mary’s legacy, but also the strength of the Catholic league and historic tensions between European nations, says Ball State’s Jennifer DeSilva.

A scholarly article written by Lasry and his team detailing the ciphers and their solutions appears in a special issue of the academic journal Cryptologia, published on February 8, the anniversary of Mary’s execution.

“These letters are very long, complex, and not easy to understand,” says Lasry. “I think they’ll keep historians busy for decades.”

Kathryn Miles is the author of five books including, most recently Trailed: One Woman’s Quest to Solve the Shenandoah Murders. She teaches at Colby College and Eastern Oregon University.

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