You know how people say you should layer up to keep the cold out in the winter? That’s exactly what the Rossignols did with their cipher to protect it from the prying eyes of enemy spies.
Because they had been successfully working on decrypting ciphers for so long, the Rossignols were able to identify the weak points in the ciphers they intercepted. Their hard work, coupled with this insight led to the production of The Great Cipher, used by King Louis XIV to encrypt his most secret messages. Generations upon generations of cryptanalysts were stumped by The Great Cipher, and it remained an enigma for almost two centuries.
To create such a secure cipher, the Rossignols added several layers of complexity to it. The first layer appears in the fact that they used numbers instead of letters, which although, in essence, is the same as using a regular alphabet, is still counterintuitive.
The second layer is that there are thousands of numbers. Specifically, five hundred and eighty-seven different ones, which means that each letter in the alphabet is not assigned to exactly one number in the cipher text. In fact, each number is not even assigned to a letter, or a combination of letters, but is instead assigned to a syllabic value. This brings us to our next layer of complexity.
The syllabic values were often misleading, because they were not only assigned to syllables but also to single letters, meaning there was no clear pattern that cryptanalysts could identify, and follow to determine the rest of the cipher alphabet.
To add a final layer of complexity, the Rossignols laid traps within the cipher text. They used operational numbers that represented neither syllables nor single letters, but instead had a functional role. For example, some numbers deleted the previous number, thereby removing additional syllables in words.