Question

Encryption has a remarkably long and varied history. Spies have been using it to convey secret messages ever since there were secret messages to convey. One powerful method of encryption uses random digits. Two documents are prepared with the same random sequence of numbers. The spy is sent out with one and the spy master retains the other. The digits are used as follows. Suppose that the word to be encrypted is SPY and the random digits are 352. Then S becomes V (three letters after S), P becomes U (five letters after P), and Y becomes A (two letters after Y, restarting at A after Z). The spy would encrypt a message and then destroy the document used to encrypt it. This is an early example of

Answer 1

One-time pad. The one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a one-time pre-shared key the same size as, or longer than, the message being sent. In this technique, a plaintext is paired with a random secret key (also referred to as a one-time pad). Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using modular addition. If the key is truly random, at least as long as the plaintext, never reused in whole or in part, and kept completely secret, then the resulting ciphertext will be impossible to decrypt or break. It has also been proven that any cipher with the perfect secrecy property must use keys with effectively the same requirements as OTP keys. Digital versions of one-time pad ciphers have been used by nations for some critical diplomatic and military communication, but the problems of secure key distribution have made them impractical for most applications.