Listener Email: Vigenère Cipher
26 Oct. 2019

Listener Email: Vigenère Cipher


Lynn wrote in to share some information about Vigenère Ciphers that were mentioned in the Supergirl season five episode titled “Blurred Lines”!

Hello Super Ladies,

Sorry I’m a little late, but I wanted to share with you a bit about encryption. In a nutshell: The Supergirl writers used real information security (infosec) terms in —  shall I say — a rather, um, “creative” way? (Translation: they were like Humpty Dumpty in that they made the words mean what they wanted them to mean.)

A Vigenère (pronounced roughly as “vision air,” with the vowel of the second syllable of “vision” being eliminated as much as one’s dialect will permit.), or polyalphabetic, substitution cipher, operates at the character level. In a monoalphabetic substitution cipher, you replace each plaintext letter with a different letter. If you substitute “g” for “a” in one part of the message, then every “a” in the plaintext message will be replaced with a “g” in the ciphertext.

Such a cipher is trivial to crack based on known language patterns. For example, if “htn” appears several times in an English message, and if the letter “n” shows up extremely frequently in the message, chances are very good that the ciphertext “htn” represents the plaintext “the.” Similarly, one letter words are probably going to be either “a” or “I.”

Polyalphabetic substitution ciphers overcome this weakness by cycling through different mappings between plaintext and ciphertext. So, for example, an “a” in plaintext may be replaced with a “d” the first time it appears in the message, “y” the second time, and “s” the third time. The cycle will then begin again; the fourth appearance of “a” will be replaced by “d,” the fifth with a “y,” and so on.

Clearly, ciphertext based on such a cipher would not look like a regular English message, much such as one discussing horses. (It would still be relatively easy for a computer to crack, but it would take more time to do so.)

I think it far more likely that instead of using any sort of encryption, Lex was putting his messages in code. (Codes substitute at the word, phrase, or sentence level based on meaning; ciphers works at the bit or character level.) So, for example, “horse” might have meant, “Why, Lena? Why won’t you worship me the way I deserve?” 😉

I hope I presented the information clearly. If you have any follow-up questions, I would be happy to answer them if I can.

Incidentally, I teach an introductory infosec course at a local community college.

Stay Super,

Lynn

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