rnd + 2); S = sm4_subword(B); rk6 = ROUND_KEY(rk2, S); B = x1 ^ x2 ^ x3 ^ x4 ^ x5 ^ rk2; S = sm4_subword(B); x5= sm4_round(x1, S); B = rk4 ^ ck(4 * rnd + 2); S = sm4_subword(B); x4