There were many strong ciphers that seemed impossible to decipher, but only one has the name "Great Cipher." The Great Cipher stood undecipherable for 200 years. Created by Antoine and Bonaventure Rossignol, it was used by King Louis XIV as a way to keep his secrets hidden, "protect details of his plans, plots, and political schemes." He was impressed by the cipher and the Rossignols' so much he gave the father-son duo offices near his apartments.
What made the Great Cipher so great was the combination of its use of syllables as cipher text in the form of numbers, and the death of both Antoine and Bonaventure. The Great Cipher was secure because it turned basic french syllables into cipher text into numbers, specifically 587 of them. As mentioned before, 200 years went by before it was deciphered. Many people tried their hand at the cipher and ultimately failed, died, or gave up before they could solve it. Along with the death of the Rossignols, there was no one to read the messages. This lead to messages being unreadable for years, thus securing the cipher for years until Etienne Bazeries deciphered the Great Cipher. This still took him a total of 3 years of work of using various techniques. Some of these techniques led to gibberish and complete restarts of his journey. He finally considered the numbers could be syllables, then he found a single word, "les ennemis," from a cluster of numbers that appeared several times. From here he could examine the other parts of cipher texts and decipher them.
The Great Cipher is remembered as one of the most secure ciphers in all of history. The techniques used to decipher it are still used in other deciphering techniques, and it is one of the "forefathers" of today's unsolved ciphers.