Deciphering England’s Royal “Lost Letters” from Late 1500s

This old cipher story never really seems to get old. A team deciphering letters in a French library has just announced their discovery of the “lost letters” of England’s monarchy.

In “Under the Molehill – an Elizabethan Spy Story”, John Bossy writes that a secret correspondence with [a Queen’s] associates and allies, prior to its compromise in mid-1583, was “kept so secure that none of it has survived, and we don’t know what was in it.” We have found over 55 letters fully in cipher in the Bibliothèque nationale de France, which, after we broke the code and deciphered the letters, unexpectedly turned out to be letters from Mary Stuart, addressed mostly to Michel de Castelnau Mauvissière, the French ambassador to England. Written between 1578 and 1584, those newly deciphered letters are most likely part of the aforementioned secret correspondence considered to have been lost, and they constitute a voluminous body of new primary material on Mary Stuart – about 50,000 words in total, shedding new light on some of her years of captivity in England.

I put “lost letters” in quotes because to be fair they weren’t supposed to be found and deciphered. Semantics, I know.

The fun new part to the old story of solving antique substitution ciphers is how researchers put a graphical user interface tool into the mix to speed things up.

Due to a large amount of material (more than 150,000 symbols in total), and since automated transcription such as off-the-shelf OCR software was not applicable,Footnote44 we utilized a special GUI (graphical user interface) tool developed by the CrypTool 2 project. After transcribing some documents, we performed an initial computer analysis and decipherment, applying the GUI tool codebreaking function described in Appendix A, identifying the original plaintext language, which turned out to be French, and recovering fragments of plaintexts. We then recovered the homophones – the symbols representing single letters of the alphabet, also identifying special symbols (e.g., a symbol to duplicate the last symbol), and the structure of the cipher. After that, we could identify the symbols for common prefixes, suffixes, prepositions, and words. Based on the partial decipherment of several documents, we were able to attribute the letters to Mary, Queen of Scots, addressed to Castelnau, the French ambassador. By reviewing the text of previously-known letters between Mary and Castelnau, we found several documents matching our decipherments, enabling us to determine or validate the meaning of other symbols. Finally, we identified symbols representing names, places, and the twelve months of the year and completed the transcription and decipherment of all the documents.

It’s a great read that focuses a lot on integrity controls and attacks relative to confidentially.

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