State recovery of RC4 and Spritz Revisited

We provide an improved complexity analysis of backtracking-based state recovery attacks on RC4 and Spritz. Comparing new estimates with known results on Spritz, our analysis shows a signi cantly lower complexity estimate for simple state recovery attack as well as special state recovery attack. We validated the estimates by performing experiments for selected feasible parameters. We also propose a pre x check optimization for simple state recovery attack on Spritz. We believe that the simple state recovery attack with this optimization and so-called “change order” optimization inspired by Knudsen et al. attack on RC4 constitutes currently the best state recovery attack on Spritz (when no special state is observed).