Impossible differential attacks on FOX

Some 4-round impossible differentials were found by means of "meet-in-the-middle".By using impossible differential cryptanalysis methods and "time-memory" balanced technique,the results of previous attacks on FOX cipher were improved.The experiment results show that For FOX64,5-round attack needs 239 chosen plaintexts and 268 encryptions,6-round attack needs 256 chosen plaintexts and 2133 encryptions,7-round needs 256 and 2213.For FOX128,5-round attack needs 272 chosen plaintexts and 2134 encryptions.Therefore,7-round FOX64/256 and 5-round FOX128/192/256 are not immune to impossible differential attack.