The one-time pad, which is an unbreakable cryptosystem, was described by Frank Miller in 1882 as a means of encrypting telegrams. It was rediscovered by Gilbert Vernam and Joseph Mauborgne around 1918. In terms of security, it is the best possible system, but implementation makes it unsuitable for most applications.

In this chapter, we introduce the one-time pad and show why a given key should not be used more than once. We then introduce the important concepts of perfect secrecy and ciphertext indistinguishability, topics that have become prominent in cryptography in recent years.