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Unformatted text preview: y cryptosystem that achieves perfect secrecy. Perfect secrecy aside, the ciphertext unavoidably yields some information about the corresponding plaintext. A good cryptographic algorithm keeps this information to a minimum; a good cryptanalyst exploits this information to determine the plaintext. Cryptanalysts use the natural redundancy of language to reduce the number of possible plaintexts. The more redundant the language, the easier it is to cryptanalyze. This is the reason that many real-world cryptographic implementations use a compression program to reduce the size of the text before encrypting it. Compression reduces the redundancy of a message as well as the work required to encrypt and decrypt. The entropy of a cryptosystem is a measure of the size of the keyspace, K. It is approximated by the base two logarithm of the number of keys: H(K) = log2 K A cryptosystem with a 64-bit key has an entropy of 64 bits; a cryptosystem with a 56-bit key has an entropy of 56 bits. In general, the greater the entropy, the harder it is to break a cryptosystem. Previous Table of Contents Next Products | Contact Us | About Us | Privacy | Ad Info | Home Use of this site is subject to certain Terms & Conditions, Copyright © 1996-2000 EarthWeb Inc. All rights reserved. Reproduction whole or in part in any form or medium without express written permission of EarthWeb is prohibited. Read EarthWeb's privacy statement. To access the contents, click the chapter and section titles. Applied Cryptography, Second Edition: Protocols, Algorthms, and Source Code in C (cloth)
Brief Full Advanced Search Search Tips (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier ISBN: 0471128457 Publication Date: 01/01/96 Search this book:
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----------- Unicity Distance
For a message of length n, the number of different keys that will decipher a ciphertext message to some intelligible plaintext in the same language as the original plaintext (such as an Engl...
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