A computer system that makes it difficult to analyze the content of a calculation. A power operation unit (262) performs the following operations using the input data "a" and "b": g a = g a mod n, g b = g b mod n. Next, a multiplication unit (264) performs the following calculation using g a and g b : g ab = g a × g b mod n. Next, a discrete logarithm calculation unit (266) calculates c i mod p i - 1 to satisfy g ab = g ci mod p i (i = 1, 2, 3,...,k). Next, a CRT unit (267) calculates "c" to satisfy c i = c mod p i - 1 (i = 1, 2, 3,...,k) using the Chinese remainder theorem CRT.