feat(protocol)!: sync state root rather than signal service's storage root

packages/protocol/contracts/L1/TaikoData.sol

@@ -119,7 +119,7 @@ library TaikoData {
     struct Transition {
         bytes32 parentHash;
         bytes32 blockHash;
-        bytes32 signalRoot;
+        bytes32 stateRoot;
         bytes32 graffiti;
     }
 
@@ -128,7 +128,7 @@ library TaikoData {
     struct TransitionState {
         bytes32 key; // slot 1, only written/read for the 1st state transition.
         bytes32 blockHash; // slot 2
-        bytes32 signalRoot; // slot 3
+        bytes32 stateRoot; // slot 3
         address prover; // slot 4
         uint96 validityBond;
         address contester; // slot 5

packages/protocol/contracts/L1/TaikoEvents.sol

@@ -43,15 +43,15 @@ abstract contract TaikoEvents {
     /// @param prover The prover whose transition is used for verifing the
     /// block.
     /// @param blockHash The hash of the verified block.
-    /// @param signalRoot The latest value of the signal service storage.
+    /// @param stateRoot The block's state root.
     /// @param tier The tier ID of the proof.
     /// @param contestations Number of total contestations.
     event BlockVerified(
         uint256 indexed blockId,
         address indexed assignedProver,
         address indexed prover,
         bytes32 blockHash,
-        bytes32 signalRoot,
+        bytes32 stateRoot,
         uint16 tier,
         uint8 contestations
     );

packages/protocol/contracts/L1/TaikoL1.sol

@@ -164,6 +164,9 @@ contract TaikoL1 is
     }
 
     /// @inheritdoc ICrossChainSync
+    /// @notice Important: as this contract doesn't send each block's state root as a signal when
+    /// the block is verified, bridging developers should subscribe to CrossChainSynced events
+    /// to ensure all synced state roots are verifiable using merkle proofs.
     function getSyncedSnippet(uint64 blockId)
         public
         view

packages/protocol/contracts/L1/libs/LibProving.sol

@@ -88,8 +88,8 @@ library LibProving {
     {
         // Make sure parentHash is not zero
         // To contest an existing transition, simply use any non-zero value as
-        // the blockHash and signalRoot.
-        if (tran.parentHash == 0 || tran.blockHash == 0 || tran.signalRoot == 0) {
+        // the blockHash and stateRoot.
+        if (tran.parentHash == 0 || tran.blockHash == 0 || tran.stateRoot == 0) {
             revert L1_INVALID_TRANSITION();
         }
 
@@ -110,7 +110,7 @@ library LibProving {
         }
 
         // Each transition is uniquely identified by the parentHash, with the
-        // blockHash and signalRoot open for later updates as higher-tier proofs
+        // blockHash and stateRoot open for later updates as higher-tier proofs
         // become available. In cases where a transition with the specified
         // parentHash does not exist, the transition ID (tid) will be set to 0.
         (uint32 tid, TaikoData.TransitionState storage ts) =
@@ -183,7 +183,7 @@ library LibProving {
             }
         }
 
-        bool sameTransition = tran.blockHash == ts.blockHash && tran.signalRoot == ts.signalRoot;
+        bool sameTransition = tran.blockHash == ts.blockHash && tran.stateRoot == ts.stateRoot;
 
         if (proof.tier > ts.tier) {
             // Handles the case when an incoming tier is higher than the current transition's tier.
@@ -210,7 +210,7 @@ library LibProving {
 
                 ts.prover = msg.sender;
                 ts.blockHash = tran.blockHash;
-                ts.signalRoot = tran.signalRoot;
+                ts.stateRoot = tran.stateRoot;
 
                 emit TransitionProved({
                     blockId: blk.blockId,
@@ -283,7 +283,7 @@ library LibProving {
             // below.
             ts = state.transitions[slot][tid];
             ts.blockHash = 0;
-            ts.signalRoot = 0;
+            ts.stateRoot = 0;
             ts.validityBond = 0;
             ts.contester = address(0);
             ts.contestBond = 1; // to save gas
@@ -376,7 +376,7 @@ library LibProving {
 
         if (!sameTransition) {
             ts.blockHash = tran.blockHash;
-            ts.signalRoot = tran.signalRoot;
+            ts.stateRoot = tran.stateRoot;
         }
     }
 

packages/protocol/contracts/L1/libs/LibUtils.sol

@@ -77,7 +77,7 @@ library LibUtils {
             remoteBlockId: blockId,
             syncedInBlock: blk.proposedIn,
             blockHash: transition.blockHash,
-            signalRoot: transition.signalRoot
+            stateRoot: transition.stateRoot
         });
     }
 

packages/protocol/contracts/L1/libs/LibVerifying.sol

@@ -33,13 +33,13 @@ library LibVerifying {
         address indexed assignedProver,
         address indexed prover,
         bytes32 blockHash,
-        bytes32 signalRoot,
+        bytes32 stateRoot,
         uint16 tier,
         uint8 contestations
     );
 
     event CrossChainSynced(
-        uint64 indexed syncedInBlock, uint64 indexed blockId, bytes32 blockHash, bytes32 signalRoot
+        uint64 indexed syncedInBlock, uint64 indexed blockId, bytes32 blockHash, bytes32 stateRoot
     );
 
     // Warning: Any errors defined here must also be defined in TaikoErrors.sol.
@@ -78,7 +78,7 @@ library LibVerifying {
             assignedProver: address(0),
             prover: address(0),
             blockHash: genesisBlockHash,
-            signalRoot: 0,
+            stateRoot: 0,
             tier: 0,
             contestations: 0
         });
@@ -135,7 +135,7 @@ library LibVerifying {
         // The `blockHash` variable represents the most recently trusted
         // blockHash on L2.
         bytes32 blockHash = state.transitions[slot][tid].blockHash;
-        bytes32 signalRoot;
+        bytes32 stateRoot;
         uint64 processed;
         address tierProvider;
 
@@ -185,7 +185,7 @@ library LibVerifying {
 
                 // Update variables
                 blockHash = ts.blockHash;
-                signalRoot = ts.signalRoot;
+                stateRoot = ts.stateRoot;
 
                 // We consistently return the liveness bond and the validity
                 // bond to the actual prover of the transition utilized for
@@ -222,7 +222,7 @@ library LibVerifying {
                     assignedProver: blk.assignedProver,
                     prover: ts.prover,
                     blockHash: blockHash,
-                    signalRoot: signalRoot,
+                    stateRoot: stateRoot,
                     tier: ts.tier,
                     contestations: ts.contestations
                 });
@@ -237,12 +237,16 @@ library LibVerifying {
                 // Update protocol level state variables
                 state.slotB.lastVerifiedBlockId = lastVerifiedBlockId;
 
-                // Store the L2's signal root as a signal to the local signal
+                // Store the L2's state root as a signal to the local signal
                 // service to allow for multi-hop bridging.
-                ISignalService(resolver.resolve("signal_service", false)).sendSignal(signalRoot);
+                //
+                // This also means if we verified more than one block, only the last one's stateRoot
+                // is sent as a signal and verifiable with merkle proofs, all other blocks'
+                // stateRoot are not.
+                ISignalService(resolver.resolve("signal_service", false)).sendSignal(stateRoot);
 
                 emit CrossChainSynced(
-                    uint64(block.number), lastVerifiedBlockId, blockHash, signalRoot
+                    uint64(block.number), lastVerifiedBlockId, blockHash, stateRoot
                 );
             }
         }

packages/protocol/contracts/L2/TaikoL2.sol

@@ -100,22 +100,24 @@ contract TaikoL2 is CrossChainOwned, ICrossChainSync {
     /// message verification.
     /// @param l1BlockHash The latest L1 block hash when this block was
     /// proposed.
-    /// @param l1SignalRoot The latest value of the L1 signal root.
+    /// @param l1StateRoot The latest L1 block's state root.
     /// @param l1Height The latest L1 block height when this block was proposed.
     /// @param parentGasUsed The gas used in the parent block.
     function anchor(
         bytes32 l1BlockHash,
-        bytes32 l1SignalRoot,
+        bytes32 l1StateRoot,
         uint64 l1Height,
         uint32 parentGasUsed
     )
         external
         nonReentrant
     {
         if (
-            l1BlockHash == 0 || l1SignalRoot == 0 || l1Height == 0
+            l1BlockHash == 0 || l1StateRoot == 0 || l1Height == 0
                 || (block.number != 1 && parentGasUsed == 0)
-        ) revert L2_INVALID_PARAM();
+        ) {
+            revert L2_INVALID_PARAM();
+        }
 
         if (msg.sender != GOLDEN_TOUCH_ADDRESS) revert L2_INVALID_SENDER();
 
@@ -139,18 +141,19 @@ contract TaikoL2 is CrossChainOwned, ICrossChainSync {
             revert L2_BASEFEE_MISMATCH();
         }
 
-        // Store the L1's signal root as a signal to the local signal service to
+        // Store the L1's state root as a signal to the local signal service to
         // allow for multi-hop bridging.
-        ISignalService(resolve("signal_service", false)).sendSignal(l1SignalRoot);
-        emit CrossChainSynced(uint64(block.number), l1Height, l1BlockHash, l1SignalRoot);
+        ISignalService(resolve("signal_service", false)).sendSignal(l1StateRoot);
+
+        emit CrossChainSynced(uint64(block.number), l1Height, l1BlockHash, l1StateRoot);
 
         // Update state variables
         l2Hashes[parentId] = blockhash(parentId);
         snippets[l1Height] = ICrossChainSync.Snippet({
             remoteBlockId: l1Height,
             syncedInBlock: uint64(block.number),
             blockHash: l1BlockHash,
-            signalRoot: l1SignalRoot
+            stateRoot: l1StateRoot
         });
         publicInputHash = publicInputHashNew;
         latestSyncedL1Height = l1Height;

packages/protocol/contracts/common/ICrossChainSync.sol

@@ -25,18 +25,17 @@ interface ICrossChainSync {
         uint64 remoteBlockId;
         uint64 syncedInBlock;
         bytes32 blockHash;
-        bytes32 signalRoot;
+        bytes32 stateRoot;
     }
 
     /// @dev Emitted when a block has been synced across chains.
     /// @param syncedInBlock The ID of this chain's block where the sync
     /// happened.
-    /// @param blockId The ID of the remote block whose block hash and
-    /// signal root are synced.
+    /// @param blockId The ID of the remote block whose block hash are synced.
     /// @param blockHash The hash of the synced block.
-    /// @param signalRoot The root hash representing cross-chain signals.
+    /// @param stateRoot The block's state root.
     event CrossChainSynced(
-        uint64 indexed syncedInBlock, uint64 indexed blockId, bytes32 blockHash, bytes32 signalRoot
+        uint64 indexed syncedInBlock, uint64 indexed blockId, bytes32 blockHash, bytes32 stateRoot
     );
 
     /// @notice Fetches the hash of a block from the opposite chain.