ICT: A Cryptanalysis Toolbox for Block Cipher Structure With Secret Components

In this paper, we present a new technique for recovering the secret inner components of block cipher structures. This technique does not simply distinguish a block cipher structure from a random permutation but recovers the secret inner components. In addition, our technique is more general than ad hoc structural cryptanalysis for specific structures. A new tool, the Inequality Constraints Table ( $ICT$ ), is introduced to characterize the constraint relation of the secret inner components. If a complete $ICT$ can be constructed, the secret components will be determined by a recursive algorithm. Based on the fundamental structure, an iterative method is proposed to construct an equivalent structure to simplify the initial guess regarding the secret components. Finally, we apply the new technique to several block cipher structures and obtain the secret component recovery results for the 5-round MISTY structure, 23- and 25- round Skipjack structure. To the best of our knowledge, this is the first time to present the structural cryptanalysis against the 5-round MISTY structure, 23- and 25-round Skipjack structure.