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Abstract. In this paper, we improve the impossible differential attack on 20-round LBlock given in the design paper of the LBlock cipher. Using relations between the round keys we attack on 21-round and 22-round LBlock with a complexity of 2 69.5 and 2 79.28 encryptions respectively. We use the same 14-round impossible differential characteristic observed by the designers to attack on 21 rounds and another 14-round impossible differential characteristic to attack on 22 rounds of LBlock.

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