Applied Cryptography: Protocols, Algorithms and Source Code in C, 20th Anniversary Edition

CHAPTER 1

Foundations

1.1 TERMINOLOGY

Sender and Receiver

Suppose a sender wants to send a message to a receiver. Moreover, this sender wants to send the message securely: She wants to make sure an eavesdropper cannot read the message.

Messages and Encryption

A message is plaintext (sometimes called cleartext). The process of disguising a message in such a way as to hide its substance is encryption. An encrypted message is ciphertext. The process of turning ciphertext back into plaintext is decryption. This is all shown in Figure 1.1.

(If you want to follow the ISO 7498-2 standard, use the terms “encipher” and “decipher.” It seems that some cultures find the terms “encrypt” and “decrypt” offensive, as they refer to dead bodies.)

The art and science of keeping messages secure is cryptography, and it is practiced by cryptographers. Cryptanalysts are practitioners of cryptanalysis, the art and science of breaking ciphertext; that is, seeing through the disguise. The branch of mathematics encompassing both cryptography and cryptanalysis is cryptology and its practitioners are cryptologists. Modern cryptologists are generally trained in theoretical mathematics—they have to be.

Figure 1.1 Encryption and Decryption.

Plaintext is denoted by M, for message, or P, for plaintext. It can be a stream of bits, a text file, a bitmap, a stream of digitized voice, a digital video image . . . whatever. ...