There is a guy who can determine if a diamond is genuine or fake by just looking at it. His name is Richard.
One day, he is summoned by the leader of a gang of thieves. It turns out that thieves have stolen a lot of diamonds from a jewelery store. However, the problem is, some of the diamonds are fake. If the thieves try to cash out a fake diamond, they will be caught.
Naturally, none of the thieves want to go to a jewelery store and borrow a jeweler’s loupe, because this will cause them to be discovered.
The leader, Nick, wants to learn which diamonds are fake ones. Richard agrees to tell them, but only if they agree to share 20%.
Both parties do not trust each other. Richard does not want to tell thieves which ones are real, because then the thieves would take them and run away. The thieves think Richard might lie to them and cause them to go to jail.
Thus, they set some rules:
- Richard has to prove that he can distinguish any diamond by only looking at them.
- Richard should not tell if any of the diamonds are fake or real.
- Thieves know that out of 500 diamonds, only 100 of them are genuine. Therefore, they will let Richard take any 20 and leave. If he takes some fake ones, then he won’t be able to cash them out.
How can Richard prove that he is able to do what he says, without showing them any real diamond among all the fake ones?
Note that all the diamonds look, weigh and feel identical.
Richard takes one fake diamond and one genuine diamond from the stack, and hands them to the leader.