Editing Substitution cipher (section)

===One-time pad===
{{Main|One-time pad}}
One type of substitution cipher, the [[one-time pad]], is unique. It was invented near the end of World War I by [[Gilbert Vernam]] and [[Joseph Mauborgne]] in the US. It was mathematically proven unbreakable by [[Claude Shannon]], probably during [[World War II]]; 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 Union|Soviet]] one-time pad messages sent from the US for a brief time during World War II 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 project]])

In a mechanical implementation, rather like the [[Rockex]] equipment, the one-time pad was used for messages sent on the [[Moscow]]-[[Washington, D.C.|Washington]] [[Moscow–Washington hotline|''hot line'']] established after the [[Cuban Missile Crisis]].