You can play mental chess just fine, there are no unknown variables. But what about poker?
“Mental poker” is the act of playing a game without a trusted third party, even though one is traditionally needed. The most common example, playing poker without a dealer. Of course you can’t trust the other players, so how do you make sure a fair deck is used and no bad shuffles take place? How do you guarantee someone isn’t lying about the cards they have?
Because it’s a trust problem, we can’t solve it with human things. No, we need to bring the power of maths into this. Specifically: cryptography. Through clever use of encryption, the different parties can verify the deck, each other’s shuffles, and both face-up and face-down cards on their validity. Everyone can have all the data available, but only when all parties agree to it will parts of that data actually become readable.
I’m not going to bore you with the implementation details, I’ve yet to dive into it deeper than the surface of the protocol anyway, but I can tell you it’s really cool. Also not very hard to turn the poker protocol into something more generically useful, so you can use it for other card games as well.
Would be fun to implement a mental poker protocol sometime. Urbit seems like a good fit, of course.