Vernam Cipher (One Time Pad)

The Vernam cipher, also known as one time pad cipher, is an encryption method that relies on the use of a long secret key to make a message unreadable to third parties. It takes its name from its inventor, Gilbert S. Vernam. In order to guarantee maximum security, the key must never be reused, hence its other name: one time pad / disposable mask.

Its original implementation is described in a patent which allowed Baudot code to be encrypted via an XOR operation. However, its cryptographic advantages helped to spread its use and the method was applied to various key ciphers, such as Vigenère.

The Vernam Chiffre uses the Vigenere encryption method, but with an encryption key which must have a number of letters identical to or greater than the number of characters in the plain message.

The Vernam cipher can use the XOR encryption method, but with an encryption key that must have a bit size identical to or greater than the bit size of the plain message.

Decryption depends on the encryption method used, Vernam makes no changes to the usual decryption.

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.