The Great Cipher used by Louis XIV encoded syllables and single letters using 587 different numbers and remained unbroken for 200 years. One factor contributing to the strength of this cipher could be that during the time period in which the cipher was in use, the most well known ciphers included the monoalphabetic substitution cipher, the polyalphabetic substitution cipher, and the homophonic substitution cipher. People had commonly being encoding their message one letter at a time. Consequently, many trying to break The Great Cipher may have not considered syllables, consisting of a varying series of letters, were being encoded as one number. To further increase the strength the cipher, some single letters did correspond to a single number which would further confuse the cryptanalyst as to which numbers represented single letters and which represented syllables. To add to the confusion there were numbers that were traps which did not represent a syllable or a letter. Trap numbers deleted the previous number in the ciphertext. This cipher incorporated three layers of complexity which could attribute to why it remain unbroken for two centuries.
Furthermore, Louis XIV’s plaintext was in French meaning that enemies who potentially intercepted the messages would be unlikely to known common French syllables unless they were literate in French. It was probably more likely for a French-speaking people to loyal to his or her king than to Spain or other enemy countries during Louis XIV’s reign.
Lastly, after some time passed after Louis XIV’s reign the group of people interested in decoding his secret messages shifted from enemies to historians. Historians acknowledged the value of decoding the king’s secret messages to gain insight regarding the 17th century however, the urgency was nowhere near that of the enemies. Enemies need to decode his messages of his political scheming and planned attacks within days for the information to be of any benefit to them. A century later, the details of the dead king’s plans did not need to be deciphered within days considering they were events of the past. The lack urgency may have also contributed to the long lasting unbreakability of the cipher.