Louis XIV’s code was one that was not deciphered for over 200 years. It used 587 number codes which represented letters and syllables, among other things. The cypher was created by Antoine and Bonaventure Rossignol as a means to keep military, economic, and other personal information hidden.
The cypher however did not use a standard substitution alphabet, as could be discovered by the fact that there were 587 number codes. Commandant Etienne Bakeries, who was an expert in military code breaking, devoted himself to trying to break Louis XIV’s cypher in the 1890’s. He first discovered that the codes did not represent a homophone alphabet, which would use multiple symbols for the same letter to remove the use for frequency analysis. Next he discovered that the cypher did not use a digraph, or each symbol representing a pair of letters. Bakeries’ first major breakthrough came in recognizing a cluster of numbers that was repeated often. He found this to mean “les ennemis”, with the numbers representing syllables in the French language. Little by little he was able to begin solving for more letters, followed by some words, which lead to eventually solving the entire code.
It may seem that the cypher in fact failed, but one has to remember, a cypher’s effectiveness doesn’t come from if it can be solved, but actually on if it can’t be solved in time to be able to use the knowledge against the author. 200 years later, the cypher had done its job and was already obsolete.