int pkey_ctrl_string(EVP_PKEY_CTX *ctx, char *value)
{
    int rv;
    char *stmp, *vtmp = NULL;
    stmp = OPENSSL_strdup(value);
    if (!stmp)
        return -1;
    vtmp = strchr(stmp, ':');
    if (vtmp) {
        *vtmp = 0;
        vtmp++;
    }
    rv = EVP_PKEY_CTX_ctrl_str(ctx, stmp, vtmp);
    OPENSSL_free(stmp);
    return rv;
}