Vernam Cipher (One Time Pad Vigenere)

The Vernam cipher is a Vigenère cipher, but with an encryption key which must have the same number of letters or even greater than the number of characters in the plain message.

A key as long as the text to be encrypted makes it possible to avoid any Vigenère cryptanalysis attempts and makes the message much more difficult to decipher without knowing the key.

In order to guarantee maximum security, the key must never be reused, hence its other name: disposable mask / one time pad.

The Vernam Chiffre uses the Vigenere encryption method but using a key at least as long as the plain text.

Example: To encrypt DCODE, a key of at least 5 letters is needed (KEYWORD, PASSWORD, etc).

If the key is not long enough, it will be repeated, as in the Vigenere cipher, but this introduces a cryptographic weakness in the message.

Decryption is also identical to Vigenere decryption method.

The Vernam cipher message has an indice of coincidence close to a random text.

The disposable mask or one time pad name is also given to this cipher.

If the key length is as long as the text length AND the key is random, then no cryptanalysis method can crack Vernam.

If the key is not random, then a probable plaintext word attack could make it possible to guess part of the key (and can be deduce the rest from it), or an attack by analysis of frequencies could make it possible to find the language of the message and decrease the possibilities for a brute force attack.

If the key is reused on several messages, then Vigeneres' attacks are usable again.

First example were found at the end of the 19th century and Vernam described it in 1917.