The Cipher That Survived for 200 Years

The Great Cipher of King Louis XIV was an enhanced monoalphabetic substitution cipher that managed to remain unsolved for over two centuries.  It was developed by the father-and-son team of Antoine and Bonaventure Rossignol, two of the best cryptanalysts in France.  King Louis XIV used it to securely encrypt sensitive information regarding his political plans.  The first characteristic of the Great Cipher that made it so strong was that it used 587 different numbers to encode messages rather than just 26 symbols, like a standard monoalphabetic substitution cipher.  This meant that there were multiple possibilities for the significance of each number.  Cryptanalysts initially thought that each number corresponded to a single letter, with several ways to represent each letter.  A cipher like this would be quite effective in that it would be immune to frequency analysis, but the Great Cipher was actually even more complicated.  Rather than a single letter, each number represented a full syllable in the French language.  Since there are so many possible syllables, this method is several times more secure, requiring a cryptanalyst to match up far more than just 26 pairs of meanings.  In addition, the Rossignols made the cipher extra deceiving to potential codebreakers by making some of the numbers delete the previous syllable instead of signifying a unique one.  All of these strong encryption techniques contributed to the longevity of the Great Cipher, and it remained unsolved until expert cryptanalyst Commandant Étienne Bazeries finally broke through 200 years later.