Biclique Cryptanalysis of Block Ciphers LBlock and TWINE80 with Practical Data Complexity

In the biclique attack, a shorter biclique usually results in less data complexity, but at the expense of more computational complexity. The early abort technique can be used in partial matching part of the biclique attack in order to slightly reduce the computations. In this paper, we make use of this technique, but instead of slight improvement in the computational complexity, we keep the amount of this complexity the same and reduce the data complexity enormously by a shorter biclique. With this approach, we analysed fullround of LBlock, and also LBlock with modified key schedule (which was designed to resist biclique attack) both with data complexity 2^12, while the data complexity of the best biclique attack on the former was 2^52 and for the latter there is no attack on the fullround cipher, so far. Then we proposed a new key schedule that is more resistant against biclique cryptanalysis, though the low diffusion of the cipher makes it vulnerable to this attack regardless of the strength of the key schedule. Also using this method, we analyzed TWINE80 with 2^12 data complexity. The lowest data complexity for the prior attack on the TWINE80 was 2^60. In all the attacks presented in this paper, the computational complexities are slightly improved in comparison to the existing attacks.