The 1600's were a strange time in the history of cryptography. Monoalphabetic ciphers had run their course, with cryptanalysts having the resources and know-how to crack any monoalphabetic cipher quickly. On the other hand, the newly developed polyalphabetic cipher, a cipher that uses multiple cipher alphabets, was effective but too tedious to be embraced by codemakers. People needed a method of encryption that was unbreakable and also not so difficult to use. That's when the Rossingols, cryptanalysts employed by the French government, developed the Great Cipher of Louis XIV. It is important to wonder why this particular cipher, which is just an enhanced monoalphabetic cipher, took 200 years to decipher.
The first reason why the Great cipher took so long to decipher was its complexity compared to ciphers cryptanalysis had seen in the past. The Great Cipher was a monoalphabetic cipher, meaning each symbol in the “cipher alphabet” mapped to one and only one thing in the plaintext alphabet. Still, it was extremely different from all other monoalphabetic ciphers. First, it used numbers. The use of numbers to map to letters had been a relatively new development in cryptography, and cryptanalysts still didn’t know the best method to decipher numeric codes. More importantly, however, the Great Cipher included 576 numbers: many more letters than there are in the alphabet. This great of a mismatch between the quantity of symbols and the quantity of letters had never been seen before, so there was initially a huge gap between the experience of the cryptallaists and the complexity of this cipher.
The second reason the Great Cipher took so long to decipher was the technologies available at the time. This was a cipher of 576 numbers. If it were 26 numbers, it would be somewhat obvious that each number matches a letter. However, with 576 numbers, each number could mean anything. So, many possibilities needed to be tested out. However, due to the fact that everything had to be written out, testing a possibility was extremely tedious, time consuming, and daunting. Bazeries, the man who eventually deciphered the Great Cipher 200 years later, spent months testing if it was a homophone and then spent months testing whether the numbers represented pairs of words. In short, no one wanted to do the herculean task of testing failing decriptions over and over again with pen and paper, so it took a long time before someone took up the task.
Lastly, the Great Cipher took so long to decipher because of the creativity and effectiveness of the cipher itself. Each cipher in the past had been based off of letters, but the Rossingols based their cipher off of syllables, matching each number to a syllable in the english language. This system made it just as easy to send messages and decode them with a key, but made the cipher extremely difficult to crack, given that there are so many potential syllables. The creativity put into this cipher showed, as no one thought to look at the syllables for 200 years. The Great Cipher took so long to crack because it was everything a great cipher is: complex, daunting, and way before its time.