Abstract: This paper studied and analyzed the ability of the block cipher algorithm ESF resisting the impossible diffe-rence,a 8-round impossible differential routz was used and the related results were given.On the basis of the 8-round impossible differential route,according to the relationship of the round keys,by changing the original order of round number extension and key guessing,the paper attacked 11-round ESF,improving the result of the 11-round ESF impossible differential.Computing result shows that the attack of 11-round ESF needs O(2⁵³) chosen plaintext operations and O(2³²) encrypting computations.At the same time,it also shows that the 11-round ESF is not immune to the impossible difference.

Key words: Block cipher,Impossible differential,ESF,Round key