Codebreakers chapter 4: On the Origin of a Species

The species in question is polyalphabetic substitution. Though it took some time for its use to take hold, it was a big step up in cryptography. So without any ado, here is a visual to start off:

Visual of a terrible poly alphabetic cipher system

In regular mono-alphabetic substitution, a single cipher symbol will always refer to the same plain text. Homophones add a bit of “fun” to the mix by mapping a plain text symbol to a set of cipher text symbols. Regardless, the relationship between plain text and cipher text is static. This makes the cryptanalyst’s job easy. With some work, they can deduce what cipher symbols are likely to correspond to what plain text symbols through linguistic analysis. They can guess at common words, and if correct, use their findings to decipher other parts of the cipher text and guess at even more words.

In a (decent) poly-alphabetic system, the mapping from plain text to cipher text continuously changes. This is how it goes down:

  • Alice and Bob agree on a key. Let’s say the key they agree on is “CAB”.
  • When Alice wants to send the message “DECAF” to Bob:
    She finds the cipher text equivalent for the letter “D” under the key “C”, which is v. She finds the cipher text equivalent of the letter “E” under the key “A”, which t.
    She finds the cipher text equivalent of the letter “C” under the key “B”, which is d.
    The key repeats. She then goes on to produce the remaining cipher text, yielding the message ‘vtdzy’.

Blaise de Vigenère took this a step further with his concept of the autokey. Instead of agreeing on a single key or set of keys to use for communication, two parties need only agree on a single letter “priming key” — the key to decrypting the first plain text letter. From there, the current plain text letter serves as the key for the next letter. This works great because it avoids the use of short keys that repeat throughout the message (a weakness). The tabula recta (that’s the fancy name of the visual above) that is used to map a key to an alphabet still needs to be kept secret.

Poly alphabetic ciphers did not have significant practical use until after the 1800s-ish. The nomenclator reigned supreme. Part of the reason is that a small mistake when enciphering the message can garble everything else from that point on. Not comforting, especially for a species with the idiom “To err is human”.

I’ll leave you with the last sentence of this chapter — “The authors did not know the real cryptology that was being practiced in locked rooms here and there throughout Europe, by uncommunicative men working stealthily to further the grand designs of state” (page 156)

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