3 amateur codebreakers set out to decrypt old letters. They uncovered royal history

We can now learn more about one of the most controversial monarchs in European history, thanks to a trio of amateur cryptologists who unknowingly stumbled upon a treasure trove of long-lost letters by Mary, Queen of Scots and worked painstakingly to crack their code.

George Lasry of Israel, Norbert Biermann of Germany and Satoshi Tomokiyo of Japan are part of an international community of codebreakers and scholars working on historical ciphers (aka secret codes). Though they all have other day jobs: Lasry is a computer scientist, Biermann is a pianist and musical coach and Tomokiyo is an astrophysicist who works for a patent firm.

They joined forces on this particular project, which began as part of a larger effort to decipher archival documents and ended up uncovering more than 55 letters that Mary Stuart wrote during her years in captivity in England. The codebreakers published an article detailing their methods and results in the journal Cryptologia on Wednesday, the 436th anniversary of the Scottish queen's execution in 1587.

"Due to the sheer amount of deciphered material, about 50,000 words in total and enough to fill a book, we have only provided preliminary summaries of the letters, as well as the full reproduction of a few of them, hoping to provide enough incentive to historians with the relevant expertise to engage in in-depth analysis of their contents, to extract insights that would enrich our perspective on Mary's captivity during the years 1578-1584," they wrote.

A brief history lesson: Mary was first in line to succeed Queen Elizabeth I of England (a Protestant) and considered by Catholics to be the legitimate sovereign. But when Mary fled from Scotland to England in 1568 after a tumultuous series of events (including the murder of her husband and her forced abdication in favor of her 1-year-old son), her cousin Elizabeth I took her presence as a threat and imprisoned her.

Mary spent 19 years in captivity — during which time she corresponded in code with her associates and supporters — before she was convicted of treason and beheaded at age 44 for her alleged role in a plot to have Elizabeth I murdered.

The newly deciphered letters, which cover a wide range of personal and political topics, were written between 1578 and 1584 and mostly addressed to Michel de Castelnau Mauvissière, the French ambassador to England. They suggest that a secure line of communication between the two opened earlier than historians had previously known.

John Guy, a British historian and author — whose 2004 biography of Mary won the prestigious Whitbread Award and formed the basis of the popular 2018 film — said the discoveries "mark the most important new find on Mary Stuart, Queen of Scots, for 100 years."

"The letters show definitively that Mary, during the years of her captivity in England ... closely observed and actively involved herself in political affairs in Scotland, England and France, and was in regular contact, either directly, or indirectly through de Castelnau, with many of the leading political figures at Elizabeth I's court," he said in a statement provided to NPR, adding that they prove Mary was a "shrewd and attentive analyst of international affairs."

"They will occupy historians of Britain and Europe and students of the French language and early modern ciphering techniques for many years to come."

How they did it

The codebreakers found the letters in the online archives of the Bibliothèque Nationale de France, but had no idea what they would turn out to be — in part because the collections were described as related to Italian affairs and dating back to the 1520s.

That particular collection in the French National Library was the "last place you would expect" to find Mary's letters, Lasry told NPR over email. He says his team doesn't know how they ended up there, but is glad they did.

"Historical decipherment is like archeology," he wrote. "You never know what you may find, you need to keep digging, and if you do, with a little bit of luck, you will find (historical) treasure."

The three decided to work together on deciphering the letters — an effort sponsored by the DECRYPT Project at European universities — because of the complexity of the code and amount of material: about 200 distinct graphical symbols and 150,000 in total, Lasry explains.

They employed a mix of methods including computerized codebreaking and linguistic as well as contextual analysis. The actual code breaking took one to two months, but Lasry says the main challenge was transcribing the vast amount of material so that it could be processed by computer algorithms. All told, the project took a year of their free time.

"After the code was cracked, we needed to decipher all the letters one by one, and edit the transcriptions, also a time-consuming process, with 50,000 decrypted words in total," he adds.

The team realized relatively quickly that the letters had nothing to do with Italy and were in fact written in French. Other clues alerted them to their author's identity: participles and adjectives in the feminine form, mentions of captivity and a son, and the name "Walsingham" (Elizabeth's secretary and spymaster).

After deciphering additional letters and finding a copy of the text of several letters in Walsingham's papers in the British Library, the researchers said they could definitely prove that the letters were written by Mary to Castelnau.

They kept searching for similar letters and ultimately found more than 55 with the same cipher. Roughly 50 of those had never been published or known to historians, they say.

"Mary Queen of Scots is a tragic, very famous figure from the 16th Century," Lasry says. "Every piece of information to be found about her would generate a substantial history. [Fifty] letters at once is a huge thing."

What they found

Historians already knew that the exiled queen and French ambassador were in touch, but their communication channels were so secure that they didn't know what exactly that involved before they were compromised in mid-1583.

"With our new decipherments, we provide evidence that such a secret channel was already in place as early as May 1578," the codebreakers wrote. "Also, while some details were already known, our new decipherments provide further insights into how this channel was operated, and on the people involved."

The article briefly summarizes the contents of the dozens of letters and reprints several of them in full.

Some of the recurring topics include: Mary's efforts to keep her communications secure, her thoughts on the queen, her associates and alleged plots against them, negotiations about her potential release and return to the Scottish throne, complaints about her conditions in captivity and efforts to ensure her servants and allies were financially rewarded.

"From time to time, she suggests enticing various people with financial rewards so that they would switch sides, or soften their attitude toward her," the article reads. "She also asks for Castelnau's assistance in recruiting new spies and couriers, while sometimes she warns him – rightly – that some people working for her might be Walsingham's agents."

The article also details the pains Mary and her associates took to keep up their secret correspondence, from writing in cipher and using aliases to delivering letters through indirect channels and trusted messengers in attempts to avoid interception. They were not always successful.

Enciphering letters was common for monarchs, nobles and diplomats at the time, Mary included, according to Lasry.

"At the age of 9, she was taught how to write in cipher," he wrote. "Throughout her life, she made extensive use of ciphers, especially while she was captive."

Mary famously used an intricate folding technique known as a spiral lock to safeguard the letter she wrote on the eve of her execution — a method so elaborate that researchers weren't able to crack it until 2021.

What happens next

The codebreakers say deciphering the letters is just the first step in learning more about Mary's life, captivity and connections. For one, they say there is "evidence that some enciphered letters known to have existed are still missing."

They only had access to online archives, and say inspecting the physical documents could yield even more information — like filling in gaps caused by low-quality scans and allowing for examination of things like paper and handwriting. Plus, they'd like to know more about how all of these documents ended up in such unrelated collections.

And there's still much to learn from the newly discovered letters themselves.

The codebreakers have been focusing mostly on what they could learn about Mary's communications channels, but say experts could use them to dive deeper into specific events and people. They could also compare the writing style and subjects of the enciphered letters to the regular ones that Mary sent through official channels.

The study authors hope that scholars will get in touch, both to gain access to the materials and ideally collaborate on "an annotated edition" of all the new letters.

Lasry says the next step of the process will involve helping historians work with the existing documents — and looking for new ones, too.

"There is so much material, that will keep them busy for quite a while," he writes. "And we will continue to break new ciphers, which is our passion — there will always be ciphers to break."