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

6 despite the possible confusion these operations are

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Unformatted text preview: symmetric algorithms, that tag won’t be used here. Encryption using public key K is denoted by: EK(M) = C Even though the public key and private key are different, decryption with the corresponding private key is denoted by: DK(C) = M Sometimes, messages will be encrypted with the private key and decrypted with the public key; this is used in digital signatures (see Section 2.6). Despite the possible confusion, these operations are denoted by, respectively: EK(M) = C DK(C) = M Cryptanalysis The whole point of cryptography is to keep the plaintext (or the key, or both) secret from eavesdroppers (also called adversaries, attackers, interceptors, interlopers, intruders, opponents, or simply the enemy). Eavesdroppers are assumed to have complete access to the communications between the sender and receiver. Cryptanalysis is the science of recovering the plaintext of a message without access to the key. Successful cryptanalysis may recover the plaintext or the key. It also may find weaknesses in a cryptosystem that eventually lead to the previous results. (The loss of a key through noncryptanalytic means is called a compromise.) An attempted cryptanalysis is called an attack. A fundamental assumption in cryptanalysis, first enunciated by the Dutchman A. Kerckhoffs in the nineteenth century, is that the secrecy must reside entirely in the key [794]. Kerckhoffs assumes that the cryptanalyst has complete details of the cryptographic algorithm and implementation. (Of course, one would assume that the CIA does not make a habit of telling Mossad about its cryptographic algorithms, but Mossad probably finds out anyway.) While real-world cryptanalysts don’t always have such detailed information, it’s a good assumption to make. If others can’t break an algorithm, even with knowledge of how it works, then they certainly won’t be able to break it without that knowledge. There are four general types of cryptanalytic attacks. Of course, each of them assumes that the cryptanalyst has complete knowledge of the encrypti...
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