Unformatted text preview: it. If it doesn’t, then use it. Since the initial state of a FCSR corresponds to the key of the stream cipher, this means that a FCSRbased generator will have a set of weak keys. Table 17.1 lists all connection integers less than 10,000 for which 2 is a primitive root. These all have maximum period q  1. To turn one of these numbers into a tap sequence, calculate the binary expansion of q + 1. For example, 9949 would translate to taps on bits 1, 2, 3, 4, 6, 7, 9, 10, and 13, because 9950 = 213 + 210 + 29 + 27 + 26 + 24 + 23 + 22 + 21 Table 17.2 lists all the 4tap tap sequences that result in a maximallength FCSR for shift register lengths of 32 bits, 64 bits, and 128 bits. Each of the four values, a, b, c, and d, combine to generate q, a prime for which 2 is primitive. q = 2a + 2b + 2c + 2d  1 Any of these tap sequences can be used to create a FCSR with period q  1. The idea of using FCSRs for cryptography is still very new; it is being pioneered by Andy Klapper and Mark Goresky [844,845,654,843,846]. Just as the analysis of LFSRs is based on the addition of primitive polynomials mod 2, analysis of FCSRs is based on addition over something called the 2adic numbers. The theory is well beyond the scope of this book, but there seems to be a 2adic analog for everything. Just as you can define linear complexity, you can define 2adic complexity. There is even a 2adic analog to the BerlekampMassey algorithm. What this means is that the list of potential stream ciphers has just doubled—at least. Anything you can do with a LFSR you can do with a FCSR. There are further enhancements to this sort of idea, ones that involve multiple carry registers. The analysis of these sequence generators is based on addition over the ramified extensions of the 2adic numbers [845,846]. 17.5 Stream Ciphers Using FCSRs
There aren’t any FCSR stream ciphers in the literature; the theory is still too new. In the interests of getting the ball rolling, I propose some here. I am taking two different tacks: I am proposing FCSR stream ciphers that are identical to previously proposed LFSR generators, and I am proposing stream ciphers that use both FCSRs and LFSRs. The security of the former can probably be analyzed using 2adic numbers; the latter cannot be analyzed using algebraic techniques—they can probably only be analyzed indirectly. In any case, it is important to choose LFSRs and FCSRs whose periods are relatively prime. All this will come later. Right now I know o...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.
 Fall '10
 ALIULGER
 Cryptography

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