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applied cryptography - protocols, algorithms, and source code in c

# The process of turning ciphertext back into plaintext

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Unformatted text preview: r 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. As far as a computer is concerned, M is simply binary data. (After this chapter, this book concerns itself with binary data and computer cryptography.) The plaintext can be intended for either transmission or storage. In any case, M is the message to be encrypted. Ciphertext is denoted by C. It is also binary data: sometimes the same size as M, sometimes larger. (By combining encryption with compression, C may be smaller than M. However, encryption does not accomplish this.) The encryption function E, operates on M to produce C. Or, in mathematical notation: E(M) = C In the reverse process, the decryption function D operates on C to produce M: D(C) = M Since the whole point of encrypting and then decrypting a message is to recover the original plaintext, the followi...
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