Add CustomDigestItem type

contracts/src/Verification.sol

@@ -76,6 +76,9 @@ library Verification {
     uint256 public constant DIGEST_ITEM_PRERUNTIME = 6;
     uint256 public constant DIGEST_ITEM_RUNTIME_ENVIRONMENT_UPDATED = 8;
 
+    /// @dev Prefix of enum variants of DigestItem(0 is reserved for snowbridge)
+    bytes1 public constant DIGEST_ITEM_PREFIX = 0;
+
     /// @dev Verify the message commitment by applying several proofs
     ///
     /// 1. First check that the commitment is included in the digest items of the parachain header
@@ -133,8 +136,17 @@ library Verification {
     {
         for (uint256 i = 0; i < header.digestItems.length; i++) {
             DigestItem memory item = header.digestItems[i];
-            if (item.kind == DIGEST_ITEM_OTHER && item.data.length == 32 && commitment == bytes32(item.data)) {
-                return true;
+            if (item.kind == DIGEST_ITEM_OTHER && item.data.length == 33 && item.data[0] == DIGEST_ITEM_PREFIX) {
+                bytes memory commitment_in_header = new bytes(32);
+                for (uint256 j = 1; j < 33;) {
+                    commitment_in_header[j - 1] = item.data[j];
+                    unchecked {
+                        ++j;
+                    }
+                }
+                if (commitment == bytes32(commitment_in_header)) {
+                    return true;
+                }
             }
         }
         return false;

contracts/test/Verification.t.sol

@@ -88,7 +88,7 @@ contract VerificationTest is Test {
         digestItems[2] = Verification.DigestItem({
             kind: 0,
             consensusEngineID: 0x00000000,
-            data: hex"b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c"
+            data: hex"00b5bb9d8014a0f9b1d61e21e796d78dccdf1352f23cd32812f4850b878ae4944c"
         });
         digestItems[3] = Verification.DigestItem({
             kind: 5,

parachain/pallets/outbound-queue/src/lib.rs

@@ -90,7 +90,7 @@ mod mock;
 #[cfg(test)]
 mod test;
 
-use bridge_hub_common::AggregateMessageOrigin;
+use bridge_hub_common::{AggregateMessageOrigin, CustomDigestItem};
 use codec::Decode;
 use frame_support::{
 	storage::StorageStreamIter,
@@ -104,7 +104,10 @@ use snowbridge_core::{
 use snowbridge_outbound_queue_merkle_tree::merkle_root;
 pub use snowbridge_outbound_queue_merkle_tree::MerkleProof;
 use sp_core::{H256, U256};
-use sp_runtime::traits::{CheckedDiv, Hash};
+use sp_runtime::{
+	traits::{CheckedDiv, Hash},
+	DigestItem,
+};
 use sp_std::prelude::*;
 pub use types::{CommittedMessage, FeeConfigRecord, ProcessMessageOriginOf};
 pub use weights::WeightInfo;
@@ -277,10 +280,10 @@ pub mod pallet {
 			// Create merkle root of messages
 			let root = merkle_root::<<T as Config>::Hashing, _>(MessageLeaves::<T>::stream_iter());
 
+			let digest_item: DigestItem = CustomDigestItem::Snowbridge(root).into();
+
 			// Insert merkle root into the header digest
-			<frame_system::Pallet<T>>::deposit_log(sp_runtime::DigestItem::Other(
-				root.to_fixed_bytes().into(),
-			));
+			<frame_system::Pallet<T>>::deposit_log(digest_item);
 
 			Self::deposit_event(Event::MessagesCommitted { root, count });
 		}

parachain/pallets/outbound-queue/src/test.rs

@@ -207,3 +207,15 @@ fn convert_local_currency() {
 		assert_eq!(fee, fee2);
 	});
 }
+
+#[test]
+fn encode_digest_item() {
+	new_tester().execute_with(|| {
+		let digest_item: DigestItem = CustomDigestItem::Snowbridge(H256::default()).into();
+		let enum_prefix = match digest_item {
+			DigestItem::Other(data) => data[0],
+			_ => u8::MAX,
+		};
+		assert_eq!(enum_prefix, 0);
+	});
+}

relayer/relays/parachain/digest_item.go

@@ -7,12 +7,16 @@ import (
 func ExtractCommitmentFromDigest(digest types.Digest) (*types.H256, error) {
 	for _, digestItem := range digest {
 		if digestItem.IsOther {
-			var commitment types.H256
-			err := types.DecodeFromBytes(digestItem.AsOther, &commitment)
-			if err != nil {
-				return nil, err
+			digestItemRawBytes := digestItem.AsOther
+			// Prefix 0 reserved for snowbridge
+			if digestItemRawBytes[0] == 0 {
+				var commitment types.H256
+				err := types.DecodeFromBytes(digestItemRawBytes[1:], &commitment)
+				if err != nil {
+					return nil, err
+				}
+				return &commitment, nil
 			}
-			return &commitment, nil
 		}
 	}
 	return nil, nil