An apparatus includes a data storage to store a window table storing a table value with an index value mapped to the table value, the index value having same number of bits as a window width, the table value being a sum of a basic table value and a non-zero table correction value, the basic table value being obtained by multiplying a point G on an elliptic curve. An arithmetic processor generates the index value by reading from a scalar value at a bit position assigned to each bit of the window with the window being shifted, reads the table value from the window table according to the index value, and performs a doubling operation and an addition operation using the read table value. A corrector performs a correction on arithmetic results with a specific correction value responsive to the table correction value.