Vigenère Ciphers

The Vigenère Ciphers
The Vigenère Ciphers is a simple polyalphabetic cipher based on the tableau in Figure below, in which each row of the tableau is shifted one letter to the left from the row above.



Encipherment and Decipherment

The cipher works by taking a keyword, in this case “BUCKNELL”, and uses it to encode a plaintext, “HAS A NICE CAMPUS”. First, the keyword is repeated on top of the plaintext:



For each letter in the plaintext, the corresponding ciphertext letter is found by selecting the letter in the column determined by the keyword-letter and the row determined by the plaintext There are two methods to decrypt the Vigen`ere cipher. The first simply does the encoding process backwards: Repeat the keyword above the ciphertext, and select the ciphertext letter out of the column determined by the keyword. The row that the ciphertext letter appears in corresponds to the plaintext letter that was enciphered. This is rather time consuming as one must search the column for the appropriate value.

The second method is to ‘encode’ again using the inverse of the keyword, which is easier for a computer to do. This is done by taking each keyword letter’s numerical value (A = 0,B = 1, etc.), subtracting it from 26, and then moding by 26. Once the key is modified as discussed above, the ciphertext is deciphered by running the encipherment algorithm with the inverted key.