Abstract: To analyze impossible differential cryptanalysis on the block cipher CLEFIA,the results are presented based on two 9-round impossible differential role．It uses the output and input differences of S-boxes to recover round keys,which apply one impossible differential attack to 11-round reduced CLEFIA．Improved result on 14round CLEFFIA-256is given with the number of chosen plaintexts being reduced to 2104.23and the time complexity reduced to 2221.5．At the same time,exploiting the key relations,using Early -Abort technique and S-boxes differential technique,impossible differential attack is proposed on 12and 13-round CLEFIA-128based on two impossible differential roles respectively．

Key words: Block cipher,CLEFIA,Impossible differential attack,S-box,Early-abort technique