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Vigenere Cipher

Vigenere Cipher

A Vigenere Cipher is an extended Caesar Cipher where a message is encrypted using various Caesar shifted alphabets. A key is used to determine how many shifts each letter receives. It adds an additional layer of complexity that relies on the shared key instead of a predetermined shift length.

Example

We'll use the following table can be used to encode a message: Vigenere Square

Encryption

Plaintext: SUPERSECRET
KEY: CODE

  1. CODE gets padded to the length of SUPERSECRET so the key becomes CODECODECOD.
  2. For each letter in SUPERSECRET we use the table to get the Alphabet to use, in this instance row C and column S.
  3. The ciphertext's first letter then becomes U.
  4. We eventually get UISITGHGTSW.

Decryption

  1. Go to the row of the key, in this case C
  2. Find the letter of the cipher text in this row, in this case U
  3. The column is the first letter of the decrypted ciphertext, so we get S
  4. After repeating this process we get back to SUPERSECRET

Cryptanalysis

The key part of breaking a Vigenere Cipher is (not a pun) the key itself. Because it repeats, it's vulnerable to brute forcing the rotation by figuring out what the length of the key is. After, frequency analysis or key elimination is used to reverse the secret. We're not going to cover it here, but check out the footnotes for more!2

Online cipher solvers automatically use these steps!

Info

For more information on how to determine the key length, check out this video on the Kasiski Examination.

  1. https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher#Cryptanalysis ↩

  2. https://www.youtube.com/watch?v=LaWp_Kq0cKs ↩