Patent ID: 8265267

Claim:
An information security device that securely and reliably processes predetermined information by performing a power operation d&X over a group defined by a predetermined set and a basic operation that uses elements of the predetermined set, the power operation d&X being the basic operation repeatedly performed d times using an element X, the information security device comprising: a processor; an acquisition unit operable to acquire a value d; a main operation unit operable to perform, using the processor, the power operation d&X by performing a window operation and a carry operation based on a window method for each of a plurality of windows, the window operation being performed using the element X and a window value of a corresponding one of the windows, the windows resulting from division of the value d, and each of the windows having a predetermined bit; and a processing unit operable to (i) decrypt the predetermined information using a result of the power operation d&X or (ii) add a digital signature to the predetermined information using a result of the power operation d&X, wherein with respect to a first window included in the windows, the main operation unit uses a random value R to perform the window operation, wherein with respect to a second window included in the windows, the main operation unit uses a cancellation value S to perform the window operation, and wherein the cancellation value S cancels a cumulative factor that is a result of the carry operations performed using the random value R with respect to windows from the first window to the second window.