ecccdh_shared_secret_size
ATTRIBUTE_WARN_UNUSED_RET int ecccdh_shared_secret_size(const ec_params *params, u8 *size);
ret = ecccdh_shared_secret_size(our_priv_key->params, &expected_shared_secret_len); EG(ret, err);
ret = ecccdh_shared_secret_size(¶ms, &shared_secret_len); EG(ret, err);
ret = ecccdh_shared_secret_size(¶ms, &sharedsecretsize);