According to your comments the question is: how to implement some form of score board cheat prevention in an offline game with an online score board. Obviously, if the game is played offline, the least of your concerns is that players modify the game elements to throw off the balance, use that to win and post an unwarranted high-score. Your main concern would be that they simply send a couple of packets that say "Hey Mr. Server I got a really great score" and the server will reply "OK, I'm just going to stupidly accept this ... because you claim this to be true, it must be so" and done. They have any score they want with their name on it. What the server should say is "Pics or it didn't happen!".
In order to have some minimal control over the correctness of the scores I would suggest you make the game's behavior deterministic (in this sense):
- Any randomness needs to be controlled by a seed (so it's reproducible).
- Any floating point computation needs to be removed or replaced by a deterministic alternative.
Now record the players input during the game and once the game is over send the recording if the score is high enough to warrant being checked. Play the recording on the server in a checker and see if the official game constraints are broken. This does not prevent them from creating something like a "TAS" (tool assisted speedrun) with some hacks on the machine. For additional measures you may want to have form of control over the seed of the game like seed
A is reasonable if the game was started at time
If you want to prevents players from cheating offline: that's impossible. If it were possible then people would have found ways to prevent users from cracking games in the first place.
If you want to add some measures to make it more difficult to cheat offline and then publish that as an achievement online then there might be some more things you can do but this really deserves it's own question.
BTW, encrypting the data and then decryption it during run-time is just a form of obfuscation and isn't considered a real measure of protection.