The History and Mathematics of Codes and Code Breaking

The Great Headache

The Great Cipher used by King Louis XIV was an extremely strong cipher for several reasons. First, the cipher text included 587 different numbers. This magnitude of possible decipherments was the first line of defense. Additionally, because multiple cipher types had been created by the time the Great Cipher was implemented, there were many more possibilities for the encryption than just the simple monoalphabetic cipher. However, the Rossignols chose not to use any previously created ciphers, vouching instead to devise their own. This creativity further complicated the matter of decipherment. Because the Great Cipher substituted numbers for syllables, any letter-based frequency analysis was useless. This major difference was probably the biggest reason why the cipher stayed unbroken for so long. Once discovering the substitution for syllables, the hard work of decipherment was still far from completion. The next stumbling block came in the form of inconsistency. Some of the numbers stood not for syllables, but for individual letters. Finally, the Rossignols laid “traps”, as Singh refers to them, within the cipher itself. One such example is a number which deletes the previous number. Combining all the individual parts of the Great Cipher results in a code which is devilishly difficult to decipher. Considering all the intricacies of the Great Cipher, it is little wonder that it remained a mystery for two centuries.


The Great Decipherment


The Complexity of the Great Cipher

1 Comment

  1. Derek

    Good points, particularly that there were still plenty of challenges in decrypting the message even after Bazeries got the idea about syllables.

    One tip for future blog posts: You could use bullet points for a post structured like this one. Long paragraphs are harder to read on-screen than they are in-print, which is why much Web writing involves shorter paragraphs and visual aids like bullet points.

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