Codebreakers chapter 8: The Professor, the Soldier, and the Man on Devil’s Island

The chapter is about French cryptography in the years leading up to World War I. The spotlight was primarily on Jean-Guillaume-Hubert-[three other names here]-Auguste Kerckhoffs. We shall call him Kerckhoffs. His contributions to the field can be summarized in three parts:

  1. Established what security people now consider a given: “Compromise of a the system should not inconvenience the correspondents”. That is, the security of the cipher algorithm should not rest on it being kept a secret
  2. Contributed two techniques to cryptanalysis, superimposition and symmetry of position
    1. Superimposition: A fancy term for a technique similar to the Kasiski method for solving poly-alphabetic ciphers. This method lets you avoid guessing the key length if you have multiple cryptograms that have been encrypted with the same key. You line the cryptograms up such that symbols that have been enciphered with the same key are in the same column. The second step is the same.
    2. Symmetry of position: Recognizes that in a straight up Vigenère cipher, the cipher symbols are at fixed positions away from each other. So if the cryptanalyst figures out the distance between two cipher text symbols in one alphabet, and then figures out one of those letters in a second alphabet, they get the other symbol for free. Apparently this method remains useful even when the cipher text alphabets are jumbled up because there remains a “latent symmetry of position”. I am not sure what this means, but my guess is that a given mixed up alphabet won’t be truly random, so there would still be some leftover symmetry.
  3. Invented?Popularized? the cryptographic slide, now called the St.-Cyr system

In a previous post I made it seem like nomenclature was dead because of the telegraph. It turns out that is not technically true. The old form died but gave birth to a more sophisticated version. The main update is super-encipherment. It goes something like this:

The idea is to encipher the plain text twice. This can be done in various ways; for example you could use a nomenclator for the first pass, then a poly-alphabetic for the second. Just don’t use the example I gave because the double-encipherment doesn’t help in that case. A cryptanalyst can use letter frequency analysis to go from the super-enciphered text directly to the plain text.

Speaking of the plain text, “J’accuse…!” was the name of the article Émile Zola (from Au Bonheur des Dames) wrote denouncing the handling of the Dreyfus affair, which makes a cameo in this scene chapter. The debacle is of cryptological interest because the evidence the case stood on was a cryptogram.

Finally, I’ll leave you with the last line of this chapter. After describing the preparations countries were making for a potential war, Khan ends with these words:

“and finally, in an obscure corner of the Balkans, someone helpfully slew an archduke, and the nations leaped recklessly into the bloody cockpit of war”

Codebreakers, page 265

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