The fact that the Rossignols' Great Cipher remained invincible to decryption for over 200 years can be linked to both the complexity of the cipher and its novelty. The 587 different numbers used in the cipher creates thousands of possibilities; with hundreds of substitutions, any combination of multiple letters can be represented by a variety of numbers, and multiple letters or combinations of letters can have more than one number assigned to them. In Simon Singh’s The Code Book, he says that Bazeries spent months testing theories, only to find that they were incorrect (56). Immense time and effort were required to test simple possible theories, and traps were laid by the Rossignol to derail decryption efforts.

Another important factor in the Great Cipher was its ability to render frequency analysis obsolete. The cryptanalysts’ most useful tool was useless against this cipher. In order to decrypt the cipher, cryptanalysts needed to develop a completely new method, not just adapt an old one. In addition, the use of the cipher slowly faded after the death of the Rossignols, so no new messages could be created and examined. The urgency to decrypt the cipher also lessened after the cipher was no longer being used; the value of the messages became purely historical and held no political, military, or strategic value. The industrialization of cryptanalysis occurred after the Great Cipher and focused on monoalphabetic ciphers and messages in circulations, so the Great Cipher remained relevant to historians, but not to those with power and resources.