We frequently talk about cybersecurity, but perhaps not so much about cryptography. Cryptography is one method used to protect sensitive information, while cybersecurity refers to keeping data secure.

This is one of the greatest cryptography stories ever told.

A Catholic queen, an alleged plot to kill her cousin, and a 445-year-old code that revealed her secrets.

The Catholic queen was Mary, Queen of Scots. Her cousin was Elizabeth 1, Queen of England, and the cipher, cracked by three cryptanalysts in Tokyo, Berlin, and Tel Aviv, revealed the secrets of a series of letters written by Mary to France’s ambassador in London, Michel de Castelnau Mauvissière, between 1578 and 1584. At the heart of this story is cryptography, a system used to encrypt and decrypt messages that cannot be deciphered by unauthorized users.

The letters, written on handmade paper, contained a series of strange, unpunctuated characters, written with a quill using iron-gall ink, the standard ink formulation used in Europe for the 1400 years between the 5th and 19th centuries. The writing reads like something created by an ancient civilization, according to the Financial Times, which reported on the letters’ discovery and deciphering.

Cryptography – the cipher’s journey from the 16th century to the present day   

The letters were discovered in the online archive of the French National Library, the  Bibliothèque Nationale de France, by Satoshi Tomokiyo, a Japanese cryptographer. Mysteriously described as “dépêches chiffrées” or enciphered messages, the letters had no details of author or recipient.

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The only thing that was known about them was that they had been kept in a volume of the library for hundreds of years, bound in red goatskin, part of the collection of the family of a French noble. They only truly came to light when the Bibliothèque Nationale started digitising manuscripts in the 2010s. Had digitization not taken place, the letters would have remained largely forgotten and certainly their cipher uncracked.

The language of ciphers: substitutions, homophones, and diacritics

The historical background behind Mary’s need to write her letters in code is described on Tomokiyo’s cryptography website, Cryptiana, and in an academic paper that explains the cracking process in detail.

The basic principle of ciphers has remained the same for centuries, with each letter of the alphabet being substituted with a symbol. The simplest form is the monoalphabetic substitution cipher where each letter is represented by a unique symbol.

As early as the 10th century, Arabic scholars developed effective methods for codebreaking based on frequency analysis, which relied on the fact that some letters of the alphabet, say ‘e’ or ‘t’ in English, are much more frequent than other letters of the alphabet and the symbols representing them are likely to be the most frequent in ciphertexts and the easiest to identify.

To thwart codebreaking based on frequency analysis, ‘homophonic ciphers’ were introduced, in which, for the most common letters, such as ‘e’, there was a choice of several symbols—i.e., ‘homophones—which can be used to encipher that letter. Because common words or syllables were also likely to reveal recurring patterns useful to codebreakers, a ‘nomenclature’—a list of symbols for frequently used words, names, or syllables—was also included in ciphers.

A key distinguishing feature of Mary’s ciphers is ‘diacritics’, which are glyphs attached to symbols, like accents on a letter. Such glyphs attribute a standalone word or part of a word to each symbol. They throw the decipherer off because they look like they could be recurring letters.

Spymasters, moles – and betrayal

Where there are ciphers, there are spies, and where there are spies, there is betrayal. What led to Mary’s downfall and, ultimately, her execution, were intercepted letters that showed her involvement in the Babington Plot, a scheme to kill Elizabeth I.

Several deciphered letters, from 1583 to 1584, were leaked to Francis Walsingham, Queen Elizabeth I’s secretary and spymaster, by a mole in Castelnau’s embassy.

The painstaking effort to decipher the letters by cryptanalysts—Tokyo-based Tomokiyo, Norbert Biermann in Berlin, and George Lasry in Tel Aviv—make this story a compelling one that arguably rivals the World War II Enigma code-cracking saga. The Hollywood incarnation of the latter, ‘The Imitation Game’, won an Oscar for adapted screenplay. Perhaps someone should write one for this story. Preferably one that doesn’t need deciphering.