(CNN) A trio of codebreakers has found and deciphered a treasure trove of lost letters written by Mary, Queen of Scots.
The 57 secret letters, from Mary Stuart to the French ambassador to England between 1578 and 1584, were written in an elaborate code. The findings come 436 years after Mary's death by execution on February 8, 1587.
Most of the letters were kept in the Bibliothèque Nationale de France in Paris, mainly in a large set of unmarked documents that were also written in cipher — special graphical symbols. The documents were listed as dating from the first half of the 16th century and thought to be related to Italy.
Then, a trio passionate about cracking historical ciphers stumbled upon the documents.
George Lasry, a computer scientist and cryptographer from France; Norbert Biermann, a pianist and music professor from Germany; and Satoshi Tomokiyo a physicist and patents expert from Japan, all worked together to find the truth behind the documents.
The multidisciplinary team has worked together for 10 years to find and understand historical ciphers. Lasry is also a member of the DECRYPT Project, which digitizes, transcribes and identifies the meaning of historical ciphers.
Once the researchers began working through the unique ciphers, they quickly realized the correspondence was written using French, and there was nothing Italian about it.
The team spied verbs and adverbs that used a feminine form, mentions of captivity — and a keyword: Walsingham. Sir Francis Walsingham was Queen Elizabeth I's secretary and spymaster. Together, all signs pointed to the fact that the team may have found letters of Mary Stuart thought lost for centuries.
The results were published Tuesday in the journal Cryptologia.
"Mary, Queen of Scots, has left an extensive corpus of letters held in various archives," Lasry said in a statement. "There was prior evidence, however, that other letters from Mary Stuart were missing from those collections, such as those referenced in other sources but not found elsewhere. The letters we have deciphered are most likely part of this lost secret correspondence."
The newly deciphered material, which is about 50,000 words total, sheds new light on Mary's time spent in captivity in England.
Mary Stuart, a Catholic, was first in line for the succession to the English throne after her Protestant cousin, Queen Elizabeth I. Catholics considered Mary as the rightful, legitimate sovereign. Considering Mary Stuart a threat, Elizabeth I imprisoned her cousin for 19 years, under the custody of the Earl of Shrewsbury in England for the majority of that time. She was executed by decapitation at the age of 44 for her alleged part in a plot to have Elizabeth I murdered.
But Mary wasn't idle in captivity. She maintained regular correspondence with allies and tried to recruit messengers to hide her letters from enemies.
The new letters reveal new details about her communication with Michel de Castelnau, sieur de la Mauvissière, the French ambassador to England. The correspondence may have started as early as 1578. The ambassador forwarded letters from Mary to her agents in France.
The English government was aware of her confidential activities, and in turn, Walsingham spied on Mary during her captivity. He was able to snag some of her letters through a spy inside the French embassy — which is why some of the 57 letters deciphered by the team can also be found in British archives.
In the letters, Mary complained about the conditions of her captivity and her poor health. She lamented that her negotiations with Elizabeth I to be released weren't carried out in good faith. Mary detailed her dislike of Walsingham as well as Robert Dudley, Earl of Leicester — a favorite of her cousin. Mary also tried to bribe the queen's officials.
The letters also showcase the distress Mary felt when in August 1582 her son, James — the man who would eventually become King James I of England two decades later — was abducted.
Dr. John Guy, a fellow in history at Clare College in Cambridge, England, and author of "Queen Of Scots: The True Life of Mary Stuart," was able to read the study ahead of its release.
"It's a stunning piece of research, and these discoveries will be a literary and historical sensation," Guy said. "They mark the most important new find on Mary Stuart, Queen of Scots, for 100 years."
The letters show that even in captivity, Mary was "a shrewd and attentive analyst of international affairs" who was involved in the political affairs of Scotland, England and France, Guy said.
The research team used complex methods combining computer algorithms, linguistic analysis and manual codebreaking techniques to decipher the letters.
"Breaking the code was not a eureka moment — it took quite a while, each time peeling another layer of the 'onion,'" Lasry said.
Initially, the researchers could only read 30% of the text using the computer algorithm. Then, they manually analyzed the symbols and tested their meanings through trial and error using contextual analysis.
"This is like solving a very large crossword puzzle," Lasry said. "Most of the effort was spent on transcribing the ciphered letters (150,000 symbols in total), and interpreting them — 50,000 words, enough to fill a book."
The ciphers were homophonic, meaning each letter of the alphabet could be encoded using several cipher symbols, according to the researchers. This practice ensured that certain symbols weren't used too frequently. The text also included dedicated symbols to signify common places, words and names.
The team was also able to compare the letters with some documents included in Walsingham's papers in the British Library in London and trace similar ciphers.
"We have cracked more difficult codes, and we have deciphered an occasional letter from a king or queen, but nothing compared to 50 new letters from one of the most famous historical figures," Lasry said.
It's likely that other coded letters from Mary are still missing. In the meantime, the letters provide a wealth of information for researchers to dig into.
"In our paper, we only provide an initial interpretation and summaries of the letters," Lasry said. "A deeper analysis by historians could result in a better understanding of Mary's years in captivity. It would also be great, potentially, to work with historians to produce an edited book of her letters deciphered, annotated, and translated."