We present a new model for finding a suitable differential characteristics with applying intelligent techniques. We describe the model for Serpent, and find suitable differential characteristics with it through an approach which we call forward-backward technique. Our description may be generalized to find suitable differential characteristics for any block cipher. The model represents the problem of finding the best differential characteristic for a block cipher algorithm as the problem of finding the shortest path in a weighted directed graph. Then, we applied Ant-colony technique for finding the shortest path in the directed graph. In this way, we reached two advantages. Firstly, by applying this method, one can obtain a suitable result without examining the whole search space. Secondly, intelligent techniques such as Ant-colony technique may reduce dependency of cryptanalysis to cryptanalyser.

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