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Cross Chain Transaction Verification

This section provides guidance on verifying transactions across different blockchain chains, assuming that a side chain has already been deployed and indexed by the main chain.

Sending a Transaction​

Any transaction with the status "Mined" can be verified, provided that the transaction has been indexed.

Verifying the Transaction​

There are two scenarios for verification:

  • Verifying a transaction on the main chain.
  • Verifying a transaction on a side chain.
rpc VerifyTransaction (VerifyTransactionInput) returns (google.protobuf.BoolValue) {
  option (aelf.is_view) = true;
}

message VerifyTransactionInput {
    aelf.Hash transaction_id = 1;
    aelf.MerklePath path = 2;
    int64 parent_chain_height = 3;
    int32 verified_chain_id = 4;
}

The VerifyTransaction method is used for verification and returns whether the transaction was mined and indexed by the destination chain. The method is the same for both scenarios; only the input values differ.

Verifying a Main Chain Transaction​

To verify a main chain transaction on a side chain, use the VerifyTransaction method on the side chain with the following input values:

  • parent_chain_height: The height of the block on the main chain where the transaction was packed.
  • transaction_id: The ID of the transaction to verify.
  • path: The Merkle path obtained from the main chain's API using GetMerklePathByTransactionIdAsync with the transaction ID.
  • verified_chain_id: The chain ID of the main chain.

You can retrieve the Merkle path of a transaction in a block by using the chain's API method GetMerklePathByTransactionIdAsync.

Verifying a Side Chain Transaction​

For verifying a side chain transaction:

  1. Obtain the Merkle path using GetMerklePathByTransactionIdAsync, similar to main chain verification.
  2. Retrieve the CrossChainMerkleProofContext of the transaction from the source chain using GetBoundParentChainHeightAndMerklePathByHeight.
rpc GetBoundParentChainHeightAndMerklePathByHeight (google.protobuf.Int64Value) returns (CrossChainMerkleProofContext) {
    option (aelf.is_view) = true;
}

message CrossChainMerkleProofContext {
    int64 bound_parent_chain_height = 1;
    aelf.MerklePath merkle_path_from_parent_chain = 2;
}

Using the result from the above API, call VerifyTransaction on the target chain with:

  • transaction_id: The ID of the transaction to verify.
  • parent_chain_height: Use the bound_parent_chain_height field from CrossChainMerkleProofContext.
  • path: Concatenate the two Merkle paths in order:
    • The Merkle path of the transaction (obtained from GetMerklePathByTransactionIdAsync).
    • The merkle_path_from_parent_chain field from CrossChainMerkleProofContext.
  • verified_chain_id: The chain ID of the side chain where the transaction was mined.