Codebreakers have finally deciphered the lost letters of Mary, Queen of Scots

The royal wrote letters in code from prison before being beheaded in 1587.
Ruins of Linlithgow Castle, near Edinburgh, Scotland, the birthplace of Mary, Queen of Scots.
Ruins of Linlithgow Castle, near Edinburgh, Scotland, the birthplace of Mary, Queen of Scots. Deposit Photos

On the 436th anniversary of her execution, a team of international codebreakers has uncovered some of the secret, coded letters written by Mary Stuart (aka Mary, Queen of Scots) while she was imprisoned in England. 

These letters had been considered lost to time and were only discovered after George Lasry (a computer scientist and cryptographer), Norbert Biermann (a pianist and music professor), and Satoshi Tomokiyo (a physicist and patents expert) were able to decipher the sophisticated coded writing system that Mary used in her letters. Their code cracking work was published on February 8 in the journal Cryptologia.

[Related: This ancient language puzzle was impossible to solve—until a PhD student cracked the code.]

In the middle to late 16th century, Mary was the first in line of succession to the English throne after her cousin, Queen Elizabeth I. Catholics considered Mary to be the legitimate sovereign instead of the Protestant Queen Elizabeth, and the queen imprisoned her for 19 years since she was seen as a threat. Mary was beheaded on February 8, 1587 when she was 44 years-old due her involvement in the Babington Plot, an alleged plot to assassinate Queen Elizabeth and install Mary as queen.

Fast-forward to the 21st century, and a team found the cryptic documents in the online archives for encrypted documents at France’s national library, the Bibliothèque nationale de France (BnF). The letters date from 1578 to 1574 and offer insights into her captivity. She communicated with her allies and most of the letters are addressed to a supporter named Michel de Castelnau de Mauvissière, France’s ambassador to England.

“Upon deciphering the letters, I was very, very puzzled and it kind of felt surreal,” says co-author George Lasry, in a statement. “We have broken secret codes from kings and queens previously, and they’re very interesting but with Mary, Queen of Scots it was remarkable as we had so many unpublished letters deciphered and because she is so famous.”

Lasry is part of the multi-disciplinary DECRYPT Project which maps, digitizes, transcribes, and deciphers historical ciphers. 

The team used computerized and manual techniques to decode the letters. Some of the documents were in a set of unmarked documents written in cipher and had the same set of graphical symbols. The letters were also originally cataloged as documents pertaining to Italian matters and dating to the first half of the 16th century.

A cipher between Mary and Castelnau. CREDIT: Lasry, Biermann, and Tomokiyo, 2022.

As the team began to crack the code, they noticed that they were written in French and didn’t discuss Italy at all. Eventually, verbs and adverbs using the feminine form, mentions of captivity and the name Walsingham—referring to Sir Francis Walsingham, Queen Elizabeth I’s secretary and spymaster—arose the suspicion that they might be from Mary Stuart.

[Related: Fake Galileo manuscript suspected to be a 20th-century forgery.]

The team took a graphical user interface (GUI) tool which uses symbols to allow a person to communicate with a computer and developed a code breaking algorithm called hill climbing, which determined that some letters–like E or T–have two alternative symbols to encipher them. This is called a homophonic cipher, which was very common in the 16th century, according to Lasry.

After they recovered the homophones, the team identified symbols that represent single letters in the alphabet, common prefixes and suffixes, and eventually the symbols representing names, places, and the twelve months of the year to work out the structure of the cipher to read what was written in the letters.

The team’s final decryption. CREDIT: Lasry, Biermann, and Tomokiyo, 2022.

They compared the letters with some of Walsingham’s papers that were not written in cipher to help confirm that these belonged to her. 

In order to find other encrypted letters from Mary, online searches and physical inspection of documents is needed, but these letters add 57 letters and about 50,000 words of additional primary source material on the complex historical figure.

“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,” added Lasry. “It would also be great, potentially, to work with historians to produce an edited book of her letters deciphered, annotated, and translated.”