1. An encrypt/decrypt processing apparatus capable of executing any of a decrypt processing operation that contains a first matrix calculation and an encrypt processing operation that contains a second matrix calculation, comprising: a first calculation unit for performing said second matrix calculation within a third matrix calculation, wherein said second matrix calculation by which the same calculation result as that of said first matrix calculation is obtained by exclusive OR-gating the first matrix calculation result and the second matrix calculation result; a second calculation unit for performing said third matrix calculation; and a logic circuit for performing a logic calculation on a calculation result of said first calculation unit and a calculation result of said second calculation unit, wherein: the calculation result of said first matrix calculation is obtained from said logic circuit; said third matrix calculation is provided with a plurality of columns having the same values as each other; said second calculation unit includes a third calculation unit for performing a matrix calculation on said plurality of columns; the matrix of said second matrix calculation corresponds to ( 2 3 1 1 1 2 3 1 1 1 2 3 3 1 1 1 ) ; the matrix of said matrix calculation as to said columns correspond to (c 8 c 8) and (8 c 8 c); the first calculation unit executes the encrypt processing operation based on the second matrix calculation result to output an encrypted result; and the first calculation unit and the third calculation unit execute the decrypt processing operation based on the second matrix calculation result and the first matrix calculation result to output a decrypted result.