LBlock is a new lightweight Block cipher which was proposed by Wu and Zhang at ACNS2011. The block size of LBlock is 64 bits and its key size is 80 bits. It was reported that the cipher is very suitable for many resource constrained environments (for instance RFID and sensor networking) which usually have weak computation ability, strict power constraints, or a small storage space and so on. In this paper, the security of LBlock cipher against a meet-in-the-middle attack is examined. An 11-round distinguisher is constructed by using the structure of LBlock encryption algorithm. Moreover, it is shown that 19-round LBlock is breakable with a data complexity of 238 chosen plaintexts and a time complexity of 265 19-round LBlock encryptions. This is the first known evaluation of the LBlock cipher against a meet-in-the - middle attack.
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