SEMICONDUCTOR DEVICE AND ENCRYPTION PROCESSING METHOD

PROBLEM TO BE SOLVED: To provide an effective method for protecting against the side-channel attack using the sign of Xp-Xq.SOLUTION: A semiconductor device (3) decrypts a ciphertext X obtained by RSA encryption, by using an algorithm that uses the Chinese remainder theorem. When assuming Dp=dmod(p-1), Dq=dmod(q-1), Xp=Xmodp, and Xq=Xmodq for secret keys d, p, and q, the semiconductor device (3) is provided with a first register (RE1) that stores Xp-Xq and a second register (RE2) that stores Xp-Xq+p. An computing unit (10) obtains a plaintext Y by using a value stored in the first register (RE1) when the sign of Xp-Xq is positive; and obtains a plaintext Y by using a value stored in the second register (RE2) when the sign of Xp-Xq is negative.