feat: NitroRPC app implementation

.gitmodules

@@ -1,3 +1,9 @@
 [submodule "nitro_rpc/lib/forge-std"]
 	path = nitro_rpc/lib/forge-std
 	url = https://github.com/foundry-rs/forge-std
+[submodule "nitro_rpc/lib/exit-format"]
+	path = nitro_rpc/lib/exit-format
+	url = https://github.com/statechannels/exit-format
+[submodule "nitro_rpc/lib/nitro"]
+	path = nitro_rpc/lib/nitro
+	url = https://github.com/erc7824/nitro

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+    "solidity.packageDefaultDependenciesContractsDirectory": "src",
+    "solidity.packageDefaultDependenciesDirectory": "lib"
+}
+

nitro_rpc/src/NitroRPC.sol

@@ -1,23 +1,82 @@
 // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.17;
 
+
+import {INitroTypes} from "nitro/interfaces/INitroTypes.sol";
+import {NitroUtils} from "nitro/libraries/NitroUtils.sol";
+import {IForceMoveApp} from "nitro/interfaces/IForceMoveApp.sol";
+
 /**
  * @title NitroRPC
  * @dev Solidity implementation of Nitro RPC protocol structures
  */
-interface NitroRPC {
-
+contract NitroRPC is IForceMoveApp {
     struct Payload {
         uint256 requestId;
+        uint256 timestamp;
         string method;
         bytes params;
         bytes result;
-        uint256 timestamp;
     }
 
     struct PayloadSigned {
       Payload rpcMessage;
-      INitroTypes.Signature clientSig
-      INitroTypes.Signature serverSig
+      INitroTypes.Signature clientSig;
+      INitroTypes.Signature serverSig;
+    }
+
+    enum AllocationIndices {
+        Client,
+        Server
+    }
+
+    /**
+     * @notice Encodes application-specific rules for a particular ForceMove-compliant state channel.
+     * @dev Encodes application-specific rules for a particular ForceMove-compliant state channel.
+     * @param fixedPart Fixed part of the state channel.
+     * @param proof Array of recovered variable parts which constitutes a support proof for the candidate.
+     * @param candidate Recovered variable part the proof was supplied for.
+     */
+    function stateIsSupported(
+        INitroTypes.FixedPart calldata fixedPart,
+        INitroTypes.RecoveredVariablePart[] calldata proof,
+        INitroTypes.RecoveredVariablePart calldata candidate
+    ) external pure override returns (bool, string memory) {
+        require(fixedPart.participants.length == uint256(AllocationIndices.Server) + 1, "bad number of participants");
+
+        PayloadSigned memory payloadSigned = abi.decode(candidate.variablePart.appData, (PayloadSigned));
+        requireValidPayload(fixedPart, payloadSigned);
+
+        return (true, "");
+    }
+
+    function requireValidPayload(INitroTypes.FixedPart calldata fixedPart, PayloadSigned memory payloadSigned) internal pure {
+        require(recoverPayloadSigner(payloadSigned.rpcMessage, payloadSigned.clientSig) == fixedPart.participants[uint256(AllocationIndices.Client)], "bad client signature");
+        require(recoverPayloadSigner(payloadSigned.rpcMessage, payloadSigned.serverSig) == fixedPart.participants[uint256(AllocationIndices.Server)], "bad server signature");
+
+        // TODO: verify timestamp and requestId
+    }
+
+    // This pure internal function recovers the signer address from the payload and its signature.
+    function recoverPayloadSigner(Payload memory payload, INitroTypes.Signature memory signature) internal pure returns (address) {
+        // Encode and hash the payload data.
+        // Using abi.encode ensures proper padding and decoding, avoiding potential ambiguities with dynamic types.
+        bytes32 messageHash = keccak256(
+            abi.encode(
+                payload.requestId,
+                payload.timestamp,
+                payload.method,
+                payload.params,
+                payload.result
+            )
+        );
+
+        // Apply the Ethereum Signed Message prefix.
+        bytes32 ethSignedMessageHash = keccak256(
+            abi.encodePacked("\x19Ethereum Signed Message:\n32", messageHash)
+        );
+
+        // Recover the signer address using ecrecover.
+        return ecrecover(ethSignedMessageHash, signature.v, signature.r, signature.s);
     }
 }