Biclique Cryptanalysis of LBlock with Modified Key Schedule

Biclique Cryptanalysis of LBlock with Modified Key Schedule

LBlock is a lightweight block cipher proposed in ACNS 2011 as a solution to the security challenge in extremely constrained environments. Because biclique cryptanalysis had not been invented when this cipher was designed, the designers themselves evaluated the security of this cipher one year later in WISA 2012, where a modified key schedule was also suggested to make this cipher resistant against biclique attack. In this paper, we analyze the full-round of LBlock with this modified key schedule by the biclique attack with data complexity of 􀫛􀫚􀫛 and computational complexity of 􀫛􀫠􀫡.􀫠􀫝. In the biclique attack, a shorter biclique potentially results in less data complexity, but at the expense of more computational complexity. Here, we make use of early abort technique (first employed in impossible differential attack) to keep the amount of computational complexity the same, while reduce the data complexity enormously by a shorter biclique. To the best of our knowledge, it is the first biclique attack on the full-round LBlock with modified key schedule.