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

No matter how much work the cryptanalyst invests he

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Unformatted text preview: onditions, 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 ----------- LFSR/FCSR Summation/Parity Cascade The theory is that addition with carry destroys the algebraic properties of LFSRs, and that XOR destroys the algebraic properties of FCSRs. This generator combines those ideas, as used in the LFSR/FCSR Summation Generator and the LFSR/FCSR Parity Generator just listed, with the Gollmann cascade. The generator is a series of arrays of registers, with the clock of each array controlled by the output of the previous array. Figure 17.6 is one stage of this generator. The first array of LFSRs is clocked and the results are combined using addition with carry. If the output of this combining function is 1, then the next array (of FCSRs) is clocked and the output of those FCSRs is combined with the output of the previous combining function using XOR. If the output of the first combining function is 0, then the array of FCSRs is not clocked and the output is simply added to the carry from the previous round. If the output of this second combining function is 1, then the third array of LFSRs is clocked, and so on. This generator uses a lot of registers: n*m, where n is the number of stages and m is the number of registers per stage. I recommend n = 10 and m = 5. Alternating Stop-and-Go Generators These generators are stop-and-go generators with FCSRs instead of some LFSRs. Additionally, the XOR operation can be replaced with an addition with carry (see Figure 17.7). — FCSR Stop-and-Go Generator. Register-1, Register-2, and Register-3 are FCSRs. The combining...
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