I am making a game which i will be open sourcing. Its a simple arcade like game but requires a network connection because it is meant to be played with other people.

The thing i am worrying about is how would i be sure that the client is the one that i put out for the end user to play with? Kind of a like of sv_pure for Team Fortress 2.

I was thinking of different ways to combat this such as the server requesting the client's version or even it's md5 hash but people with simple java knowledge could just force a method to always return what the server wants.