Imagine the following situation in a game: A series of random numbers is presented to the player. Each number is shown to the player for a short period of time before it goes to the next one. It is the player's aim to pick a high number. He or she just needs to 'click' in the right moment and then the number is chosen.
The question is about how to implement this scenario in a secure way in a client/server scenario.
That means there is a game client which displays the above mentioned scene and there is a server to which the chosen number (in whatever way) needs to be send. The catch is to get this thing secure so that cheating (e.g. by modifying the client) is not possible.
There's really no way to make this completely secure. Even without modifying your client, all someone needs to do is have another program running in the background reading the monitor and looking for a certain number to appear and then send a mouse click event to your client program. This can be done without modifying your client program at all. Even if you somehow managed to make it secure so that they couldn't run any other programs or services at the same time as your program, they could just point a webcam at the screen hooked to a different computer to do the optical digit recognition and send a mouse click event over USB to the computer running your program.
Fortunately, you may be able to get around the problem. Generally if something is to be a "dice roll" you want a random "luck based" outcome. By allowing them to click at a certain time you are making this a skill based game instead of luck based and therefore not really a dice roll. You could make it so there's a slight "delay" from when they click to when the dice stops rolling, so they see it rolling around, click, and it slows down and lands on a number. This way what number was displayed when they actually click does not determine the outcome, but the next (random) number after that one, which would eliminate the possibility of cheating and make this a luck based roll instead of skill based.