Inversion Attack and Branching

The generalized inversion attack on nonlinear filter generators is developed and analyzed by the theory of critical branching processes. Unlike the inversion attack which requires that the filter function be linear in the first or the last input variable, this attack can be applied for any filter function. Both theory and systematic experiments conducted show that its time complexity remains close to 2 , M being the input memory size, while the additional memory space required is relatively small for most the filter functions.