The Great Cipher used by Louis XIV remained unbroken for 200 years.  What were the factors that led to such a secure cipher?

The father-son team of Antoine and Bonaventure Rossignol invented The Great Cipher while working closely with Louis XIV as his cryptanalysts. Initially, they were mainly code breakers, but their skill gave them the idea to create a much stronger way to encrypt messages. This idea turned into the Great Cipher. This cipher was very useful for the French and no enemy cryptanalysts were able to crack it. Unfortunately, the Rossignols's death also meant that the Great Cipher's secrets were lost and any archives encoded using it could no longer be read. Although this was inconvenient for the French, the real struggle would be experienced by future generations of code breakers. Eventually letters encrypted by the Great Cipher were passed on to Étienne Bazeries who worked tirelessly to decipher the letters. The high security of the cipher made it nearly impossible to decode.

The first factor that led to this secure cipher was the amount of characters included. 587 different characters immediately made it clear that the it was not a substitution cipher and later, Étienne also discovered it was not a homophonic cipher (a cipher that replaces letters with a proportional number of symbols to how often that letter is used). Later he would also try to decipher it as a digraph (one number represents a pair of letters), but this also was not correct. The grunt work that decoding the Great Cipher must have required is astonishing because the text says each idea could take Étienne multiple months to prove wrong. Eventually, Étienne was struck with the idea that each number represented a whole syllable. After tirelessly working on this idea, he was able to decode 124-22-125-46-345 as meaning "les ennemis". This crucial breakthrough led to Étienne's eventual success despite variations in the cipher and traps laid by the Rossignols. This elaborate cipher truly deserved its name as "The Great Cipher".