Substitution Cipher - The One-time Pad

The One-time Pad

One type of substitution cipher, the one-time pad, is quite special. It was invented near the end of WWI by Gilbert Vernam and Joseph Mauborgne in the US. It was mathematically proven unbreakable by Claude Shannon, probably during WWII; his work was first published in the late 1940s. In its most common implementation, the one-time pad can be called a substitution cipher only from an unusual perspective; typically, the plaintext letter is combined (not substituted) in some manner (e.g., XOR) with the key material character at that position.

The one-time pad is, in most cases, impractical as it requires that the key material be as long as the plaintext, actually random, used once and only once, and kept entirely secret from all except the sender and intended receiver. When these conditions are violated, even marginally, the one-time pad is no longer unbreakable. Soviet one-time pad messages sent from the US for a brief time during WWII used non-random key material. US cryptanalysts, beginning in the late 40s, were able to, entirely or partially, break a few thousand messages out of several hundred thousand. (See VENONA)

In a mechanical implementation, rather like the ROCKEX equipment, the one-time pad was used for messages sent on the Moscow-Washington hot line established after the Cuban missile crisis.