Unformatted text preview: then compute the output of the first filter generator. If the output of LFSR0 is 0, then compute the output of the second filter generator. The final output is the first output XOR the second. 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 © 19962000 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)
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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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 Gollmann Cascade
The Gollmann cascade (see Figure 16.16), described in [636,309], is a strengthened version of a stopandgo generator. It consists of a series of LFSRs, with the clock of each controlled by the previous LFSR. If the output of LFSR1 is 1 at time t  1, then LFSR2 clocks. If the output of LFSR2 is 1 at time t  1, then LFSR3 clocks, and so on. The output of the final LFSR is the output of the generator. If all the LFSRs have the same length, n, the linear complexity of a system with k LFSRs is n(2n  1)k1 Figure 16.15 Multispeed innerproduct generator. Cascades are a cool idea: They are conceptually very simple and they can be used to generate sequences with huge periods, huge linear complexities, and good statistical properties. They are vulnerable to an attack called lockin [640]. This is a technique by which a cryptanalyst reconstructs the input to the last shift register in the cascade, then proceeds to break the cascade register by register. This is a serious problem in some situations and weakens the effective key length of the algorithm, but precautions can be...
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