Mary, Queen of Scots’ secret letters decoded 436 years after her death

Over 50 letters written in a secret code by Mary, Queen of Scots more than 400 years ago were deciphered by an international team of codebreakers.
Over 50 letters written in a secret code by Mary, Queen of Scots more than 400 years ago were deciphered by an international team of codebreakers. Copyright AFP - François Clouet, Mary, Queen of Scots via Canva
By Giulia CarbonaroAFP
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The letters shed new light on Mary's captivity.


Over four hundred years after Mary, Queen of Scots was beheaded for allegedly plotting to assassinate Queen Elizabeth I, some of the mysterious correspondence she exchanged while imprisoned, written in her own personal code, is no longer a secret.

It took a team of three international codebreakers ten years to crack the cypher in which some 57 letters written by the Catholic queen and kept at the Bibliothèque Nationale de France in Paris were written.

The trio – a French computer scientist and cryptographer, George Lasry; a German pianist and music professor, Norbert Biermann; and Japanese physicist Satoshi Tomokiyo – used computer algorithms, linguistic analysis and manual codebreaking techniques to reveal the meaning of the mysterious letters.

Lasry said in a statement that cracking the code was like “solving a very large crossword puzzle” with most of the effort “spent on transcribing the ciphered letters (150,000 symbols in total), and interpreting them.” What they gave us is a total of 50,000 words shedding light on Mary’s life in captivity. 

“Enough to fill a book”, Lasry said.

The trio discovered that the text was written in French, and not in Italian, as experts had long believed. They were also able to determine that the letters were likely written by Mary between 1578 and 1584 – six years out of the Catholic queen’s total of 19 years spent imprisoned by her cousin Elizabeth – and almost all of them were sent to the French ambassador to England, Michel de Castelnau.

HANDOUT/AFP or licensors
After being deciphered, the code had to be translated from French.HANDOUT/AFP or licensors

Mary was not a newcomer when it came to using codes and other subterfuges to ‘encrypt’ her mails.

After she was imprisoned by the Protestant Elizabeth – who saw her as an obvious threat since Catholics favoured Mary as the rightful heir to the throne – she knew her correspondence was closely scrutinised by her enemies, and relied on several techniques to avoid being spied upon.

She’s known to have used a hyper-complicated folding method known as the spiral lock – a series of folds, slits and tucks to make her missive tamper proof – which experts still struggle to understand how it worked. And then, there were the coded letters that nobody, until now, could decipher.

But now we know what she wrote that Elizabeth and her allies could not read.

In the letters to de Castelnau, the trio of codebreakers has revealed that Mary complained about her captivity and her poor health. She lamented that negotiations for her release weren’t done in good faith by Elizabeth. She badmouthed Elizabeth’s allies and her captors. 

According to the documents, she also tried to bribe the queen’s officials – apparently to no avail as she was eventually executed in 1587.

The letters somehow snagged to the French ambassador were likely then shared by de Castelnau with Mary’s agents in France.

Lasry and his colleagues believe there could be more to their discovery.

“In our paper, we only provide an initial interpretation and summaries of the letters. A deeper analysis by historians could result in a better understanding of Mary’s years in captivity,” Lasry said. “It would also be great, potentially, to work with historians to produce an edited book of her letters deciphered, annotated, and translated.”

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