In the initialization step the S-boxes (S_BOX and INV_S_BOX) and the polynomial matrices (POLY_MAT and INV_POLY_MAT) are created and an example cipher key is expanded into the round key schedule (W).aes part4-5-In the initialization step the S-boxes (S_BOX and INV_S_BOX) and the polynomial matrices (POLY_MAT and INV_POLY_MAT) are created and an example cipher key is expanded into　the round key schedule (W).aes part4-5