The Vigenère encryption algorithm (Menu Crypt/Decrypt \ Symmetric (classic))

Under the Vigenère encryption algorithm, a key that is several characters long is used. The Caesar encryption algorithm may be viewed as a special case of Vigenère encryption with a key of length 1, and conversely the Vigenère encryption algorithm can be thought of as a multiple use of a modified Caesar encryption operation.

Characters that are not to be encrypted are of course ignored during encryption and decryption and do not have the effect of continuing the key on the next key letter either.

As under the Caesar encryption algorithm, only letters from the alphabetic are encrypted (the alphabet is selected in the Text Options dialog). Characters which are not part of the alphabet are normally left out of the encrypted text, so that after decryption these characters have to be inserted from the textual context. This applies to both punctuation characters and special characters (especially spaces between two words). The menu selection Options \ Text Options is used to specify whether punctuation and special characters should be retained during encryption and decryption.

The key for the Vigenère encryption algorithm is entered in the Key entry dialog.

Example:

In the 3-letter long key " KEY",

Comment:

In the literature, the Vigenère encryption algorithm is often described in terms of the Vigenère tableau rather than as a multiple use of the Caesar encryption algorithm, since this tableau was used as a manual tool for encryption and decryption earlier.

In the classical Caesar procedure, the letter "A" means a shift of 1 character position, the letter "B" a shift of 2. In the Vigenère system on the other hand the shift is always one character less, i.e. no character for "A", one character for "B", two characters for "C", .... 25 characters for "Z".

Under the Vigenère method, the plaintext and the encrypted text are identical when the key is "A" (and not "Z", as with the Caesar encryption algorithm).

The Vigenère encryption algorithm can be broken relatively easily with a ciphertext-only attack (see Automatic analysis, Vigenère encryption algorithm). An example of this will be found in the Examples chapter.