Improved inversion attacks on nonlinear filter generators

Improved inversion attacks on nonlinear filter generators are proposed with computational complexity equal to O(2^r-m), where r denotes the length of the shift register, and m denotes the largest gap between cells with taps to the filter function or connection polynomial. It is shown that the previously proposed set of design criteria does not prevent the improved inversion attack and an additional criterion is proposed based on the relationship between the positions of taps to the filter function and positions of taps to the connection polynomial