The substitution tables (S-boxes) s_box and inv_s_box are used by the expanded key

schedule function key_expansion and the en- and decrypting functions cipher and

inv_cipher to directly substitute a byte (element of GF(28)) by another byte of the

same finite field.

In any speed optimized real-world application the substitution tables would definitely be

hard coded a priori in a constant but in the scope of this educational

paper it seems to be interesting, how the S-boxes can be generated.-The substitution tables (S-boxes) s_box and inv_s_box are used by the expanded key

schedule function key_expansion and the en- and decrypting functions cipher and

inv_cipher to directly substitute a byte (element of GF(28)) by another byte of the

same finite field.

In any speed optimized real-world application the substitution tables would definitely be

hard coded a priori in a constant　 but in the scope of this educational

paper it seems to be interesting, how the S-boxes can be generated.