feat: prevent zero-cost delay grief

Author: jaybuidl

contracts/deploy/01-foreign-chain.ts

@@ -58,6 +58,7 @@ const deployForeignGateway: DeployFunction = async (hre: HardhatRuntimeEnvironme
   }
   const { claimDeposit, challengeDuration, homeChainId } = paramsByChainId[chainId];
   const challengeDeposit = claimDeposit;
+  const bridgeAlpha = 5000;
   const homeChainIdAsBytes32 = hexZeroPad(homeChainId, 32);
 
   const homeGatewayAddress = getContractAddress(deployer, nonce);
@@ -72,6 +73,7 @@ const deployForeignGateway: DeployFunction = async (hre: HardhatRuntimeEnvironme
       claimDeposit,
       challengeDeposit,
       challengeDuration,
+      bridgeAlpha,
     ],
     log: true,
   });

contracts/src/bridge/FastBridgeReceiverOnEthereum.sol

@@ -46,9 +46,11 @@ contract FastBridgeReceiverOnEthereum is SafeBridgeReceiverOnEthereum, IFastBrid
     // *             Storage               * //
     // ************************************* //
 
+    uint256 public constant ONE_BASIS_POINT = 1e4; // One basis point, for scaling.
     uint256 public override claimDeposit;
     uint256 public override challengeDeposit;
     uint256 public override challengeDuration;
+    uint256 public override alpha; // Basis point of claim or challenge deposit that are lost when dishonest.
     mapping(uint256 => Ticket) public tickets; // The tickets by ticketID.
 
     // ************************************* //
@@ -64,11 +66,13 @@ contract FastBridgeReceiverOnEthereum is SafeBridgeReceiverOnEthereum, IFastBrid
         address _inbox,
         uint256 _claimDeposit,
         uint256 _challengeDeposit,
-        uint256 _challengeDuration
+        uint256 _challengeDuration,
+        uint256 _alpha
     ) SafeBridgeReceiverOnEthereum(_governor, _safeBridgeSender, _inbox) {
         claimDeposit = _claimDeposit;
         challengeDeposit = _challengeDeposit;
         challengeDuration = _challengeDuration;
+        alpha = _alpha;
     }
 
     // ************************************* //
@@ -154,7 +158,7 @@ contract FastBridgeReceiverOnEthereum is SafeBridgeReceiverOnEthereum, IFastBrid
         require(ticket.claim.bridger != address(0), "Claim does not exist");
         require(ticket.claim.verified == true, "Claim not verified: deposit forfeited");
 
-        uint256 amount = ticket.claim.claimDeposit + ticket.challenge.challengeDeposit;
+        uint256 amount = ticket.claim.claimDeposit + (ticket.challenge.challengeDeposit * alpha) / ONE_BASIS_POINT;
         ticket.claim.claimDeposit = 0;
         ticket.challenge.challengeDeposit = 0;
         payable(ticket.claim.bridger).send(amount); // Use of send to prevent reverting fallback. User is responsibility for accepting ETH.
@@ -167,7 +171,7 @@ contract FastBridgeReceiverOnEthereum is SafeBridgeReceiverOnEthereum, IFastBrid
         require(ticket.challenge.challenger != address(0), "Challenge does not exist");
         require(ticket.claim.verified == false, "Claim verified: deposit forfeited");
 
-        uint256 amount = ticket.claim.claimDeposit + ticket.challenge.challengeDeposit;
+        uint256 amount = ticket.challenge.challengeDeposit + (ticket.claim.claimDeposit * alpha) / ONE_BASIS_POINT;
         ticket.claim.claimDeposit = 0;
         ticket.challenge.challengeDeposit = 0;
         payable(ticket.challenge.challenger).send(amount); // Use of send to prevent reverting fallback. User is responsibility for accepting ETH.
@@ -203,6 +207,10 @@ contract FastBridgeReceiverOnEthereum is SafeBridgeReceiverOnEthereum, IFastBrid
         challengeDuration = _challengeDuration;
     }
 
+    function changeAlpha(uint256 _alpha) external onlyByGovernor {
+        alpha = _alpha;
+    }
+
     // ************************ //
     // *       Internal       * //
     // ************************ //

contracts/src/bridge/interfaces/IFastBridgeReceiver.sol

@@ -28,4 +28,6 @@ interface IFastBridgeReceiver {
     function challengeDeposit() external view returns (uint256 amount);
 
     function challengeDuration() external view returns (uint256 amount);
+
+    function alpha() external view returns (uint256 amount);
 }