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

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Unformatted text preview: y will rise out of all the random alternatives. So, the basic differential attack on n-round DES will recover the 48-bit subkey used in round n, and the remaining 8 key bits are obtained by brute-force guessing. There are still considerable problems. First, there is a negligible chance of success until you reach some threshold. That is, until you accumulate sufficient data you can’t tell the correct subkey from all the noise. And the attack isn’t practical: You have to use counters to assign different probabilities to 248 possible subkeys, and too much data is required to make this work. At this point, Biham and Shamir tweaked their attack. Instead of a using a 15-round characteristic on 16-round DES, they used a 13-round characteristic and some tricks to get the last few rounds. A shorter characteristic with a higher probability worked better. And they used some clever mathematics to obtain 56-bit key candidates which could be tested immediately, eliminating the need for counters. This attack succeeds as soon as a right pair is found; this avoids the threshold and gives a linear success probability. If you have 1000 times fewer pairs, then you have 1000 times smaller chance of success. This sounds terrible, but it is a lot better than the threshold. There is always some chance of immediate success. 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! Previ...
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