An object of the invention is to allow cipher strength evaluation when available resources such as the complexity and the number of plaintext available for decryption have conditions, and to allow comparison of cipher strength under given conditions. The invention combines the exhaustive search with an algebraic method, sets conditions for resources such as the complexity and the number of plaintext available for decryption beforehand, and utilizes the linear dependency of a decryption equation for use in decryption to optimize a decryption method as the maximum number of available plaintext is secured. Thus, it reduces the complexity and allows efficient search of solutions for the decryption equation.