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

For a 64 bit block length this is a very long number

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Unformatted text preview: keystream generators are called periodic. Except for one-time pads, all keystream generators are periodic. The keystream generator must have a long period, one far longer than the number of bits the generator will output between key changes. If the period is less than the plaintext, then different parts of the plaintext will be encrypted the same way—a severe weakness. If a cryptanalyst knows a piece of the plaintext, he can recover a piece of the keystream and use that to recover more of the plaintext. Even if the analyst only has the ciphertext, he can XOR the sections encrypted with the same keystream and get the XOR of plaintext with plaintext. This is just the simple XOR algorithm with a very long key. How long a period is long enough depends on the application. A keystream generator encrypting a continuous T-1 link will encrypt 237 bits per day. The keystream generator’s period must be orders of magnitude larger than that, even if the key is changed daily. If the period is long enough, you might only have to change the key weekly or even monthly. 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 ----------- Synchronous stream ciphers also protect against any insertions and deletions in the ciphertext, because these cause a loss of synchronization and will be immediately detected. They do not,...
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