Attacking the Combination Generator

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Executive Summary

The combination generator is, together with the filter generator, one of the simplest and most analyzed construction of stream ciphers. It uses as an internal state many Linear Feedback Shift Registers (LFSRs). The authors will write m for their total size in bits. These registers are filtered using a n-variable balanced Boolean function f (from Fn2 into F2) to produce the keystream (zt)t??0. The inputs of this function are taken from some bits in the LFSRs internal states. They will write xt for the n-bit vector corresponding to the inputs of f at time t. Notice that they will always write such vector of bits in bold.

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