Cheating protection for highscore list

I'm making a small game and would like it to upload/download highscores to/from a central database. I'm using C++ and Qt, so the game will be multi-platform. I would also like to give people the source code so that they can compile the games themselves on platform I don't have access to.

But given the source code, people can easily figure out how to upload fake highscores. Is there any way to make this impossible (or very hard) even with the source code available?

Make it so ungodly complicated that no one can replicate it. Encrypt things so they cannot change them effectively.

What about all the problems of dealing with say changing
sendHiscoreToServer(score);
to
sendHiscoreToServer(score*1000);

Or changing
lives--;
to
// lives--; // infinite lives hack

If they've got the code, you're sunk.

I think you would need some kind of continual communication with the server which allows you to monitor game play, say points scored in time taken, to try and gauge whether a genuine game has been played. But even then, once people figure out the protocol, writing another program which just mimics a long plausible game will be pretty easy.

This is the classic mistake of applying a technological solution to a social problem. Politicians do it all the time, so you're in good company.

I used to try do the sort of thing you're doing, then I came to the following realization:

If someone cares enough about my game to hack and rebuild the code just to cheat the high-score list, I should be honored. The vast majority of freeware games get downloaded, played once, and then thrown away. If someone wants to cheat, I must not only have made a good game, but made a game interesting enough that someone WANTED to cheat at it!

In short, if you're giving out the code (which I highly recommend), don't bother with anti-cheating safeguards. You're ultimately wasting your time trying to prevent something that likely won't happen.

Well, you could have 2 separate programs; the game and one to send the score. Don't release the source to the latter. Each time you send a score, send a long a hash of the game binary, the game version, OS and the score (encrypted). Server side, you can check the hash against a DB of versions (and configurations and OS). That leaves you with one problem; locating the right binary to hash.

[edit]
really 2 problems. You need to build up the DB of hashes as well of course.
[/edit]

Originally Posted by AverageSoftware

If someone cares enough about my game to hack and rebuild the code just to cheat the high-score list, I should be honored. The vast majority of freeware games get downloaded, played once, and then thrown away. If someone wants to cheat, I must not only have made a good game, but made a game interesting enough that someone WANTED to cheat at it!

You're giving way too much credit to cheaters. People steal, kill, and cheat all the time in life for stuff that has little to no gain whatsoever. Games are no exception when it comes to cheating. There is no reason to cheat at a game where you compete vs other people. If it makes you feel honored that people want to cheat at your game..... enjoy the feeling. I know as a player, I'm fed up with it, and it makes me want to avoid decent games if cheating can't be controlled.

Originally Posted by AverageSoftware

In short, if you're giving out the code (which I highly recommend), don't bother with anti-cheating safeguards. You're ultimately wasting your time trying to prevent something that likely won't happen.

As Salem said, you can do it if you have a server/client model. Open source or not, it's possible.

Have it send the recorded game with exact timings and rng state, and have the server do a full check for correctness. Depends on how computationally intensive your game is :-). But then have some gametime() function encode information into low significance bits of the time values, and have the server check for that too. Maybe they'll miss that. You'll catch a few, and then after, say, a week or two to wait for their 'real' cheating playernames (their first might be a fake name for testing), you can put them up on the board as listed cheaters.

Another option would be to use encryption in your communication with the high score server. Release the source code, minus the encryption key. If somebody ports it, and you trust them sufficiently, send them the key so that they can make an official binary for that platform.

It requires some manual intervention, but is probably less effort than attempting to have the server verify proper gamestate with the client (saves you bandwidth, not to mention the fact that some games can still be cheated).

You could divide the thing into official and unofficial builds. Unofficial builds are normal source builds. They don't include the ability to upload scores.
Official builds are available only as binaries, and they include an additional server communication module, which employs all the anti-cheat measures enjoyed by closed-source software.

hmm... but if you trust those people sufficiently to give them your secret key, then why not just release source code to selected maintainers?

That's basically the idea, except you allow the general public to play with the source code if they wish, minus high scores.