Improvement of Search Algorithm for Integral Distinguisher in Subblock-based Block Cipher

Integral distinguisher is the main factor of integral attack. Conventionally, higher order integral distinguisher is obtained as an extension of first order integral (conventional algorithm). The algorithm was applied to many subblock-based block ciphers, however, the conventional algorithm has some problems. We find other integral distinguisher of two sub block-based block ciphers, TWINE and LBlock, which are different from the conventional evaluations. As a solution, we propose a new algorithm to search for higher order integral distinguisher. The point of a proposal algorithm is exploitation of bijective and injective components of cipher functions. Applying the proposal algorithm to TWINE and LBlock, we confirm the results of the proposal algorithm are consistent with the results which are calculated from computer experiment. The results are the optimal distinguisher and the most advantageous one for the attackers. Our proposal algorithm contributes to development of stronger block ciphers by obtaining such integral distinguisher.