In cryptography, the one-time pad (OTP) is an encryption algorithm where the plaintext is combined with a random key or “pad” that is as long as the plaintext and used only once. A modular addition is used to combine the plaintext with the pad. (For binary data, the operation XOR amounts to the same thing.)

It was invented in 1917 and patented a couple of years later. If the key is truly random, never reused, and kept secret, the one-time pad provides perfect secrecy. It has also been proven that any cipher with perfect secrecy must use keys with the same requirements as OTP keys. The key normally consists of a random stream of numbers, each of which indicates the number of places in the alphabet (or number stream, if the plaintext message is in numerical form) which the corresponding letter or number in the plaintext message should be shifted. For messages in the Latin alphabet, for example, the key will consist of a random string of numbers between 0 and 25; for binary messages the key will consist of a random string of 0s and 1s; and so on.

The “pad” part of the name comes from early implementations where the key material was distributed as a pad of paper, so the top sheet could be easily torn off and destroyed after use. For easy concealment, the pad was sometimes reduced to such a small size that a powerful magnifying glass was required to use it. Photos accessible on the Internet show captured KGB pads that fit in the palm of one’s hand, or in a walnut shell. To increase security, one-time-pads were sometimes printed onto sheets of highly flammable nitrocellulose.

The one-time pad is derived from the Vernam cipher, named after Gilbert Vernam, one of its inventors. Vernam’s system was a cipher that combined a message with a key read from a paper tape loop. In its original form, Vernam’s system was not unbreakable because the key could be reused. One-time use came a little later when Joseph Mauborgne recognized that if the key tape was totally random, cryptanalytic difficulty would be increased.

There is some term ambiguity due to the fact that some authors use the term “Vernam cipher” synonymously for the “one-time-pad”, while others refer to any additive stream cipher as a “Vernam cipher”, including those based on a cryptographically secure pseudorandom number generator (CSPRNG).

 

Excerpt from a one-time pad

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