Basic fraud proofs and slashing

[adlrocha]

Users in HC should have a way to submit fraud proofs to notify consensus faults in a subnet. These proofs are committed in the parent to trigger the slashing of miners' collateral. The most straightforward way in which we can build a fraud proof is:

• If the consensus fault is in epoch n, and the latest checkpoint committed to the parent is in checkpoint n-i, a user could send to the parent SCA the full state from n-i to n (where the attack is).
• The parent SCA would replay the state and check if any of the consensus rules have been violated for the blocks from n-i to n to determine if there was a consensus fault and miners should be slashed. This can be generalized not for the latest checkpoint but for any n and i in the subnets history.

(In the future we should explore the use of Lurk and ZKProofs for fraud proofs)