HMAC_OPAD_VAL
hmac_key[i] = key[i] ^ HMAC_OPAD_VAL;
hmac_key[i] = 0 ^ HMAC_OPAD_VAL;
s->hmac.opad[i] ^= HMAC_OPAD_VAL;
hmac_init_pad(axf, key, klen, auth_ctx, HMAC_OPAD_VAL);