The History and Mathematics of Codes and Code Breaking

The Complexity of the Great Cipher

Antoine and Bonaventure Rossignol created the Great Cipher of Louis XIV and made it so complex that it took over 200 years to decipher. Generation after generation attempted to crack the cipher, yet no progress was made. The Rossignol’s both died, which terminated the ciphers use, as well as cutting off any potential collaboration with the creators and knowledge of exact details that could have been useful to the hundreds of codebreakers that tried to uncover the mystery. It was not until Commandant Etienne Bazeries came along and spent three years of his life working on deciphering letters of Louis XIV that the code was finally solved. Bazeries knew that it was not a substitution cipher, as there were 587 different numbers instead of the usual 26 different numbers. To Bazeries’ dismay, the cipher was also not a homophonic cipher, which was a possibility he entertained for months.

His final attempt proved to be worth it after all. The main factor that made this cipher so secure was the fact that each number represented a whole syllable, not a pair of letters. Bazeries finally got on a roll, guessing the remaining letters of an unfinished word, which enabled him to recognize other syllables. Another major deceiving factor in the Great Cipher was the traps that the Rossignols inserted; some numbers occasionally deleted previous numbers instead of standing for another syllable. The combination of the traps, the vague numbers, and the inability to collaborate with the Rossignols created an extremely secure and virtually unbreakable cipher.




The Great Headache


The Great Cipher Eludes Great Minds

1 Comment

  1. Derek

    Yes, Bazeries did "get on a roll" with a (lucky?) insight. I'm guessing you've had the same experience decrypting messages on our problem sets. It's a good feeling to see letters start making sense!

    A note about writing: Your post involves a fair amount of summary. There's nothing really wrong with that, since it's helpful for readers who haven't read the material in Singh. But it also doesn't help answer the question. Deciding on the right balance of summary and analysis is often a tricky part of writing.

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