# Implementing Dark chess without a third party

Hello everyone,

I have recently been working on a theoretical game that requires some advanced cryptography. I have made multiple forum posts about it. They all looked something like this:

TL;DR: How can one implement Dark chess without relying on a third party?

Zero-knowledge tools make it possible to play some games without having to rely on and trust a third party. A common example is battleship. Both player A and player B keep the arrangements of their ships private throughout the game. When player A fires a shot on player Bâ€™s grid, player B replies with a zero-knowledge proof telling player A whether it was a hit or a miss. When it is player Bâ€™s turn to fire, it simply picks a coordinate on player Aâ€™s grid that it hasnâ€™t picked before. When it is their turn, each player knows what their options are â€“ they do not need to know anything about the other playerâ€™s private game state (in this case, the arrangement of their ships on the grid) to know every valid move.

This contrasts with Dark chess, where â€śa player does not see the entire board â€“ only their own pieces and the squares that they can legally move to.â€ť For a player to know which squares their pieces can move to, they frequently need to know where their opponentâ€™s pieces are, which is private information! So, players should 1) keep the positions of their pieces private, 2) zero-knowledge prove the validity of their moves, 3) somehow reveal the positions of the pieces that can be captured by the opponentâ€™s pieces. The catch is that the opponentâ€™s pieces are hidden, which makes #3 nontrivial.

If there is a trusted third party, the players can just share the positions of their pieces with it and rely on it for the management of the board. The challenge is to implement Dark chess without relying on a third party. It seems to me that zero-knowledge proofs would not be enough and that we would need homomorphic encryption in some form. When player A makes a move, they can send 1) a zero-knowledge proof proving that their move is valid, 2) a homomorphically-encrypted copy of the positions of their pieces. Player B can pass #2 and the positions of their own pieces to a function, which can somehow tell only the positions of the pieces that player B is supposed to see. Now that player B has updated its board after player Aâ€™s latest move, they send a homomorphically-encrypted copy of the positions of their own pieces to player A. Player A passes this data to the same function that player B used and updates their own board.

Do we currently have the math/software tools that can implement this? Does anyone have any ideas?

Some people suggested me to check out private set intersection (PSI), shared secret keys, and key switching.

Because chess has so many rules and intricacies, I decided to use a simpler game while trying to solve the cryptography part. Consider battleship. A similar problem in battleship would be to keep the coordinates of the shots private while still learning the correct outcome (hit/miss).

My questions to this forum:

• A protocol that I came up with for battleship is the following: Player A fires a shot. It is apparently possible for PSI to reveal the intersection to only one party. For the PSI, player Aâ€™s set will contain its shot coordinate, while player Bâ€™s set will contain the squares that are occupied by player Bâ€™s ships. If they intersect, player A learns that the outcome is a hit, otherwise itâ€™s a miss. They will also send each other zero-knowledge proofs to ensure that the elements in the sets are what they are supposed to be. Would a protocol like this work? How can this game be implemented using shared keys?
• I read that private set intersection can be implemented using homomorphic encryption. Can Zamaâ€™s Concrete be used to implement the games that I described?

Thank you very much!

Hello @YigitKilicoglu

Sounds like a very nice use case. It seems that what you describe and need is Multi-Party Computation. MPC via FHE is currently an active research topic on our side.

I will ask the team if someone has an idea in the meantime for a PSI solution with concrete.

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Following this enthusiastically.

hello @YigitKilicoglu

I donâ€™t have much to add on the topic at the moment but maybe on fhe.org discord server you could
find people that could help or share insights.
worth trying!

Hi @alex , thanks for your support. I reached out to the fhe.org Discord server 2 months ago actually, but didnâ€™t get any replies that suggested a solution.

However, I did find a PSI solution in the meantime that is explained here. Iâ€™m currently participating in Minaâ€™s zkApps Builders Program and started implementing the protocol. The code can be found in the same repository that contains the protocol description.

My next step will be to find a general HE-based solution for all role-based applications. That is, the framework will let parties exchange homomorphically-encrypted data, possibly with zero-knowledge proofs. ZheroTag (the current name of my game) will later be migrated to this framework.

Thanks again for your support and I am looking forward to solving this next puzzle.

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