Differential Sieving for 2-Step Matching Meet-in-the-Middle Attack with Application to LBlock

In this paper, we propose a modified approach for the basic meet-in-the-middle attack which we call differential sieving for 2-step matching. This technique improves the scope of the basic meet in the middle attack by providing means to extend the matching point for an extra round through differential matching and hence the overall number of the attacked rounds is extended. Our approach starts by first reducing the candidate matching space through differential matching, then the remaining candidates are further filtered by examining non shared key bits for partial state matching. This 2-step matching reduces the total matching probability and accordingly the number of remaining candidate keys that need to be retested is minimized. We apply our technique to the light weight block cipher LBlock and present a two known plaintexts attack on the fifteen round reduced cipher. Moreover, we combine our technique with short restricted bicliques and present a chosen plaintext attack on Lblock reduced to eighteen rounds.