Vernam Cipher

The Vernam-cipher (One-time Pad (OTP)) is a crypto algorithm where plaintext is combined with a random key. The basic idea is simple. You have a pad of paper with randomly chosen key letters, the same size as your message. You add one key letter to each plaintext letter, and never repeat the key letters. Though sounds simple, it is the only existing mathematically unbreakable encryption, and thus it provides very long-term message secrecy. However, practical implementation of this simple concept beyond a narrow field of top secret communications has proven to be a challenge. OTP requires two copies of the encryption key–one for the sender and one for the receiver—a truly random key equal to the size of the message, used once and then destroyed. Until now, these important considerations limited the OTP commercial use.

The notion of security of the perfect cipher is that the encrypted message provides no information about the original message to a cryptanalyst. The perfect cipher notion was evaluated by Claude Shannon during WWII and proved, mathematically, to be true for the One-time Pad.[1] This is because, given a truly random key that is used only once, a cipher-text can be translated into any plaintext of the same length, and all are equally likely. With different keys, the cipher-text HELLO could decrypt to ABOUT, BREAK, GREEN, or WRJFS. Conventional algorithms, such as 3DES, AES, RSA, and others, can tell you which key is correct because only one key can produce a reasonable plaintext. Thus, OTP is immune even to brute-force attacks where the adversary has infinite computational power and infinite time.


[1] Bell Labs Technical Journal 1949.



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