On the truncated path search for the maximum differential characteristic probability on a generalized Feistel-type block cipher

The upper bound of DCP_max is a security index of a cipher against differential cryptanalysis. It is usually evaluated by a truncated-path search with Viterbi algorithm. The search often gives an imaginary path as the resultant optimum one in generalized Feistel-type cipher, which cannot be connected as a real path. We discuss the phenomenon on example base, and propose a new search method to avoid such imaginary paths. Our method is based on carrying over a constraint equation in state transition, and can give the tighter upper bound of DCP_max than conventional one.