Vigenère Cipher

The Vigenère cipher, for a significant period in history, was regarded as such a secure cryptosystem that it was regularly used for protecting sensitive political and military information and referred to as the ‘indecipherable cipher’. The Vigenère cipher is of interest to us because it illustrates the use of positional dependency to defeat single letter frequency analysis.

Encryption using the Vigenère cipher

The Vigenère cipher is fairly straightforward to understand. The key of the Vigenère cipher consists of a string of letters that form a keyword. Associating the letters A through Z with the numbers 0 through 25, respectively, the encryption process proceeds as follows:

1. Write out the keyword repeatedly underneath the plaintext until every plaintext letter has a keyword letter beneath it.
2. Encrypt each plaintext letter using a Caesar cipher with a key that is the number associated with the keyword letter written beneath it.

The figure below provides an example of the Vigenère cipher with the keyword DIG in which the plaintext appears in the top row and the ciphertext appears in the bottom row: