Handle empty user state for sender/reciever on Rollup.disputeTransitionTransfer

contracts/libs/Transition.sol

@@ -124,15 +124,25 @@ library Transition {
         uint256 fee,
         Types.StateMerkleProof memory proof
     ) internal pure returns (bytes32 newRoot, Types.Result) {
+        // disputer must first justify the inclusion of the stateHash
         require(
             MerkleTree.verify(
                 stateRoot,
-                keccak256(proof.state.encode()),
+                proof.stateHash,
                 senderStateIndex,
                 proof.witness
             ),
             "Transition: Sender does not exist"
         );
+        // check if sender is empty
+        if (proof.stateHash == Types.ZERO_BYTES32)
+            return (bytes32(0), Types.Result.BadFromIndex);
+        // sender is non-empty, the disputer now has to justify the preimage of the state
+        require(
+            proof.stateHash == keccak256(proof.state.encode()),
+            "stateHash mismatch"
+        );
+
         (Types.UserState memory newSender, Types.Result result) =
             validateAndApplySender(tokenID, amount, fee, proof.state);
         if (result != Types.Result.Ok) return (bytes32(0), result);
@@ -151,15 +161,25 @@ library Transition {
         uint256 amount,
         Types.StateMerkleProof memory proof
     ) internal pure returns (bytes32 newRoot, Types.Result) {
+        // disputer must first justify the inclusion of the stateHash
         require(
             MerkleTree.verify(
                 stateRoot,
-                keccak256(proof.state.encode()),
+                proof.stateHash,
                 receiverStateIndex,
                 proof.witness
             ),
             "Transition: receiver does not exist"
         );
+        // check if receiver is empty
+        if (proof.stateHash == Types.ZERO_BYTES32)
+            return (bytes32(0), Types.Result.BadFromIndex);
+        // receiver is non-empty, the disputer now has to justify the preimage of the state
+        require(
+            proof.stateHash == keccak256(proof.state.encode()),
+            "stateHash mismatch"
+        );
+
         (Types.UserState memory newReceiver, Types.Result result) =
             validateAndApplyReceiver(tokenID, amount, proof.state);
         if (result != Types.Result.Ok) return (bytes32(0), result);

contracts/libs/Types.sol

@@ -7,6 +7,9 @@ pragma solidity ^0.6.12;
 library Types {
     // prettier-ignore
     uint256 public constant ADDRESS_MASK = 0x000000000000000000000000ffffffffffffffffffffffffffffffffffffffff;
+    bytes32 public constant ZERO_BYTES32 =
+        0x290decd9548b62a8d60345a988386fc84ba6bc95484008f6362f93160ef3e563;
+
     struct SignatureProof {
         Types.UserState[] states;
         bytes32[][] stateWitnesses;
@@ -225,6 +228,7 @@ library Types {
 
     struct StateMerkleProof {
         UserState state;
+        bytes32 stateHash;
         bytes32[] witness;
     }
 
@@ -239,6 +243,9 @@ library Types {
         bytes32[] witness;
     }
 
+    /**
+     * @notice Results of a validation check on a transaction or commit
+     */
     enum Result {
         Ok,
         InvalidTokenAmount,
@@ -250,6 +257,8 @@ library Types {
         BadWithdrawRoot,
         BadCompression,
         TooManyTx,
-        BadPrecompileCall
+        BadPrecompileCall,
+        BadFromIndex,
+        BadToIndex
     }
 }

contracts/rollup/Rollup.sol

@@ -28,8 +28,11 @@ contract Rollup is BatchManager, EIP712, IEIP712 {
     string public constant DOMAIN_NAME = "Hubble";
     string public constant DOMAIN_VERSION = "1";
 
-    bytes32 public constant ZERO_BYTES32 =
-        0x290decd9548b62a8d60345a988386fc84ba6bc95484008f6362f93160ef3e563;
+    /**
+     * @dev If this is not being externally consumed, it can be removed.
+     */
+    // solhint-disable-next-line var-name-mixedcase
+    bytes32 public immutable ZERO_BYTES32;
 
     // External contracts
     BLSAccountRegistry public immutable accountRegistry;
@@ -69,6 +72,8 @@ contract Rollup is BatchManager, EIP712, IEIP712 {
         )
         EIP712(DOMAIN_NAME, DOMAIN_VERSION)
     {
+        ZERO_BYTES32 = Types.ZERO_BYTES32;
+
         accountRegistry = _accountRegistry;
         transfer = _transfer;
         massMigration = _massMigration;
@@ -80,11 +85,11 @@ contract Rollup is BatchManager, EIP712, IEIP712 {
         );
 
         bytes32 genesisCommitment =
-            keccak256(abi.encode(genesisStateRoot, ZERO_BYTES32));
+            keccak256(abi.encode(genesisStateRoot, Types.ZERO_BYTES32));
 
         // Same effect as `MerkleTree.merklize`
         bytes32 commitmentRoot =
-            keccak256(abi.encode(genesisCommitment, ZERO_BYTES32));
+            keccak256(abi.encode(genesisCommitment, Types.ZERO_BYTES32));
         batches[nextBatchID] = Types.Batch({
             commitmentRoot: commitmentRoot,
             meta: Types.encodeMeta(
@@ -355,13 +360,15 @@ contract Rollup is BatchManager, EIP712, IEIP712 {
                 vacant.witness
             );
         bytes32 depositCommitment =
-            keccak256(abi.encode(newRoot, ZERO_BYTES32));
+            keccak256(abi.encode(newRoot, Types.ZERO_BYTES32));
         // Same effect as `MerkleTree.merklize`
-        bytes32 root = keccak256(abi.encode(depositCommitment, ZERO_BYTES32));
+        bytes32 root =
+            keccak256(abi.encode(depositCommitment, Types.ZERO_BYTES32));
         // AccountRoot doesn't matter for deposit, add dummy value
         submitBatch(root, 1, bytes32(0), Types.Usage.Deposit);
     }
 
+    // TODO Add note about proofs beng dif in this one than C2T and MM?
     function disputeTransitionTransfer(
         uint256 batchID,
         Types.CommitmentInclusionProof memory previous,