INFORMATION SECURITY DEVICE, INFORMATION SECURITY METHOD, COMPUTER PROGRAM, COMPUTER-READABLE RECORDING MEDIUM, AND INTEGRATED CIRCUIT

The present invention aims to provide an information security apparatus that counters a simple power analysis attack (SPA) on an information security apparatus such as an RSA cryptosystem. The information security apparatus uses a multiplication with 1 in a Montgomery domain. 1 in the Montgomery domain is determined depending on a modulus and an integer k, which is greater than a number of bits of a modulus p. Therefore, it is hard for attackers who do not know p or k to analyze. Also, even if an analyzer can predict the Hamming weight, it is possible to further improve the safety against the SPA by modifying k or the modulus at random.