Fast Correlation Attacks on Nonlinear Filter Generators

The fast correlation attack based on iterative probabilistic decoding is applied to nonlinear filter generators in order to investigate the effect of multiple linear transforms of the same linear recurring sequence being correlated to the keystream sequence. Systematic experimental results on random balanced as well as on some special filter functions show that the attack is successful if the number of parity-checks used is sufficiently large given the correlation coefficient of the best affine approximation to the filter function. In addition, the attack is shown to be more successful when applied to independent correlation noise present in memoryless combiners with distinct input shift registers.