The Great Cipher of Louis XIV proved to be one of the most challenging ciphers ever recorded. This was brought together by the father son duo of Antoine and Bonaventure Rossignol. They immersed themselves in studying and perfecting methods of cryptography from the past and eventually determined how to create a form of encryption stronger than anyone had seen before.
Previously, cryptographers focused on discovering the identities of individual letters. When Bazeries first attempted to decipher the text he used these methods mainly believing that it was a homophonic cipher after discovering that there were 587 different numbers being used. This was another big advantage to the Rossignol cipher, instead of using a normal 26 number system they used double and triple digit numbers. Believing to be homophonic, Bazeries attempted to align each number with a letter. Of course this proved to be futile as the cipher was not homophonic. One method that had never been used previously was assigning numbers with syllables which is exactly what the Rossignol's did. The code may have remained unsolved had Brazeries not deciphered the text "les ennemis". The use of traps and numbers representing "delete" also delayed the solving of the cipher.
One big form of security for the cipher that can often be forgotten is that the Rossignols died without letting anyone know how to solve their cipher. It remained a complete mystery, along with the Man in the Iron Mask, until Brazeries. Their greatest achievement was going past the typical ciphertext of representing one letter for a number and instead using numbers to represent syllables.