An English Spy King Louis XIV's Mistress King Louis XIV's Twin A Disloyal Military Officer 
Answer: A Disloyal Military Officer
For centuries people have been intrigued by the story of The Man in the Iron Mask. Imprisoned by King Louis XIV in the late 17th century, a mysterious prisoner at a prison located in Savoy, France caused quite a stir in his day (and has remained a topic of discussion and debate ever since). The prisoner’s identity was kept secret at the time of his imprisonment, and he was infrequently seen on the battlements of the prison wearing an iron mask that concealed his face. His identity was never revealed and, to this day, remains unconfirmed.
Although we cannot say with absolute certainty who The Man in the Iron Mask was, we do have very strong evidence in favor of the man being a disloyal military officer. In the 19th century, French cryptanalyst Etienne Bazeries undertook the task of decoding Louis XIV’s “Great Cipher,” a cryptographic code the king used that was composed of around 600 unique numbers. Despite the number far exceeding the number of letters in the French alphabet, Bazeries was able to almost completely crack the cipher by decoding it against letter pairings (of which there are 676 possible pairs in the French alphabet).
In doing so he opened up a trove of information about the King and the country at the time, including a letter written from the king to the ministry of war discussing the fate of one French military officer Vivien de Bulondes. Bulondes had been in charge of a skirmish on the Italian border but, upon hearing his position was about to be compromised, fled (leaving his men and supplies behind). King Louis was furious over the event and ordered Bulondes imprisoned at the prison in Savoy where he would be kept under lock and key, guarded, and permitted only to walk the battlement during the day with something.
That something remains unknown because the number pair indicating what exactly Bulondes was permitted to walk the battlements with is described by a number pairing that occurs only once in the entire body of the king’s correspondence and, with no comparative instance available, will likely never be decoded. Given that The Man in the Iron Mask appeared at the time of Bulonde’s cowardly desertion, however, Bazeries cryptographic work provides us with the strongest evidence to date regarding the identity of the iron-clad man.
Image courtesy of the United States Library of Congress.