1. A common key block encryption apparatus for computing 8-bit output data U −1 ∘f r from 8-bit input data U∘g r , where U is arbitrary 8-bit data, in a composite field GF((. .. (2 β1 ). .. ) βγ ) that satisfies βγ=2 by using 4-bit c r , 8-bit g r , 4-bit d r and 8-bit f r as fixed mask values selected with a random number r, and by using 8-bit R and R′ as random mask values, comprising: a masked U 16 ⊚U calculation circuit configured to calculate a 4-bit value U 17 ∘c r from the input data U∘g r with a U 16 ⊚U calculation for XORing with a random mask value, and to calculate 8-bit U 16 ∘R′ and R′; a 4-bit-to-4-bit transformation table circuit that is selected with a random number r, and configured to calculate 4-bit U −17 ∘d r from the U 17 ∘c r output from the masked U 16 ⊚U calculation circuit; and a masked U −17 ⊚U 16 calculation circuit, to which U −17 ∘d r output from the 4-bit-to-4-bit transformation table circuit, U 16 ∘R′output from the masked U 16 ⊚U calculation circuit, and R′ are input, and which is configured to calculate the output data U −1 ∘f r .