Optimizing the Placement of Tap Positions

Although there are many different approaches used in cryptanalysis of nonlinear filter generators, the selection of tap positions in connection to guess and determine cryptanalysis has not received enough attention yet. In a recent article [18], it was shown that the so- called filter state guessing attack (FSGA) introduced in [15], which applies to LFSR based schemes that use (vectorial) Boolean filtering functions, performs much better if the placement of tap positions is taken into account. In this article, for a given LFSR of length L, we analyze the problem of selecting n (where n << L) tap positions of the driving LFSR (used as binary inputs to a filtering function) optimally so that the complexity of FSGA like attacks is maximized. An algorithm which provides a suboptimal solution to this problem is developed and it can be used for real- life applications when the choice of tap positions is to be made.