1. A common key block encryption apparatus for performing a nonlinear transformation with a multiplication process in a binary field or a composite field, comprising: a computing unit configured to execute a computation process other than the nonlinear transformation by using fixed value masked input data obtained by performing an exclusive OR operation (hereinafter referred to as XOR) with a mask using a fixed value (hereinafter referred to as a fixed mask value); and the multiplication process using the nonlinear transformation is configured with an XOR operation circuit configured to transform all pieces of input data into fixed value masked input data by XORing all the pieces of input data with the fixed mask value, and to transform the input data into random value masked input data by XORing the input data with a mask using a random value (hereinafter referred to as a random mask value), a multiplier configured to execute the multiplication process on the basis of the random value masked input data output from the XOR operation circuit, and a random mask value-to-fixed mask value transformation circuit configured to again trans form the random value masked output data output from the multiplier into fixed value masked output data, and to output the fixed value masked output data.