HMAC_OPAD
opad[i] = (i < klen ? k[i] : 0) ^ HMAC_OPAD;
memset( k_opad, HMAC_OPAD, sizeof k_opad);
hkey->hk_opad[i] ^= HMAC_OPAD;
opad[i] = (key8[i] ^ HMAC_OPAD);
opad[i] = HMAC_OPAD;