To aim to provide an information security device capable of reducing a period necessary for performing a power operation used for secret communication or authentication. The information security device performs secret communication or authentication by calculating an exponentiation X̂d based on target data X and a secret value d using the window method. In the process of calculating the exponentiation X̂d, immediately after square of a random value R acquired for multiplication is repeatedly performed a predetermined number of times, for example 256 times, a result of square of the random value R is cancelled using a cancellation value S (=R̂(−2̂256)). This makes it unnecessary to perform cancellation processing that has been conventionally performed.