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

Although their definition is simple and natural their

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Unformatted text preview: of the previous round: Li = Ri - 1 Ri = Li - 1 • f(Ri - 1,Ki) Ki is the subkey used in the ith round and f is an arbitrary round function. You’ve seen this concept in DES, Lucifer, FEAL, Khufu, Khafre, LOKI, GOST, CAST, Blowfish, and others. Why is it such a big deal? The function is guaranteed to be reversible. Because XOR is used to combine the left half with the output of the round function, it is necessarily true that Li - 1 • f(Ri - 1,Ki) • f(Ri - 1,Ki) = Li - 1 A cipher that uses this construction is guaranteed to be invertible as long as the inputs to f in each round can be reconstructed. It doesn’t matter what f is; f need not be invertible. We can design f to be as complicated as we please, and we don’t have to implement two different algorithms—one for encryption and another for decryption. The structure of a Feistel network takes care of all this automatically. 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) Go! Keyword 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: Go! Previous Table of Contents Next ----------- Simple Relations DES has the property that if EK(P) = C, then EK’(P’) = C’, where P’, C’, and K’ are the bit-wise complements of P, C, and K. This property reduces the complexity of a brute-force attack by a factor of two. LOKI has complementation properties that reduce the complexity of a brute-force attack by a factor of 256. A simple relation can be defined as...
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