Elementary Information Security, 3rd Edition

▶ 8.2 The Reused Key Stream Problem

Stream ciphers using the xor operation were first put into operation on teletype networks in the 1920s. Most messages were copied and transmitted from punched paper tape. The encryption key was also punched on tape. To encrypt the data, they installed a very simple circuit that computed the xor of the data and key values at each punch on the tape.

Gilbert Vernam, the developer, realized that an attacker could crack the encryption if they used the same key tape to transmit more than one message. Cryptosystem designers seem to learn this lesson all over again every generation or two.

Soviet spies used one-time pads in the 1940s. (See Section 7.3.) In 1998, Microsoft used the RC4 cipher to protect data transmitted ...