Vernam Cipher (One Time Pad Vigenere)

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 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.

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