Patent ID: 8090957

Claim:
An information security apparatus for processing an input value securely and reliably by operating a modular exponentiation that raises the input value to a power of an integer over a predetermined modulus, the modular exponentiation including repeated multiplications performed on each digit of the integer so as to obtain an exponential value, the information security apparatus comprising: a modulus setting unit operable to set a first value for the modulus; an integer setting unit operable to randomly select an integer k which is greater than a number of valid bits of the modulus; a judgment unit operable to judge whether a value of a digit of the integer is 0 or not; a Montgomery transform unit operable to perform a Montgomery transform on a numeral value of 1 using the integer k in accordance with the first value for the modulus so as to calculate a Montgomery transform value; and a multiplication unit operable, when the value of the digit is judged to be 0, to operate a modular multiplication using the Montgomery transform value calculated by the Montgomery transform unit.