main()
{
    String guard = "someencryptedtextofwhichyouknowthedecryptresult";
    String key = GetGey();
    Buffer<byte>buf(guard.GetCount());
    memcpy(buf, ~guard, guard.GetCount());
    RC4 rc4(key);
    rc4.Crypt(buf, buf, guard.GetCount());
    if(strncmp(buf, "yourknowndecryptedtext", guard.GetCount())
    {
        PromptOK("Invalid license key!!!");
        exit(1);
    }

    // here rest of your app

}