int load_privkey(std::string privkey)
{
    FILE *fp;

    fp = fopen(privkey.c_str(), "r");
    if (!fp) {
        return -1;
    }

    privatekey = PEM_read_ECPrivateKey(fp, NULL, NULL, NULL);
    if (!privatekey) {
        ERR_print_errors_fp(stderr);
        return -1;
    }

    // validate the key
    EC_KEY_check_key(privatekey);

    evp_sign_key = EVP_PKEY_new();

    int ret;

    ret = EVP_PKEY_assign_EC_KEY(evp_sign_key, privatekey);
    if (ret != 1) {
        ERR_print_errors_fp(stderr);
        return -1;
    }

    fclose(fp);

    std::cout << "privkey load ok" << std::endl;

    return 0;
}