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Cross Chain Verification

Verification is the key feature that enables side chains. Because side chains do not have direct knowledge about other side chains, they need a way to verify information from other chains. Side chains need the ability to verify that a transaction was included in another side chain's block.

Indexing​

  • The role of the main chain node is to index all the side chains' blocks.
    • This way, it knows exactly the current state of all the side chains.
    • Side chains also index main chain blocks, which is how they gain knowledge about the inclusion of transactions in other chains.
  • Indexing is a continuous process:
    • The main chain permanently gathers information from the side chains.
    • The side chains permanently gather information from the main chain.
  • When a side chain wants to verify a transaction from another side chain, it must wait until the correct main chain block has been indexed.

Merkle Tree​

  • A Merkle tree is a basic binary tree structure.
    • For cross-chain in aelf, the leaf value is the hash from transaction data.
    • The node value (which is not a leaf node) is the hash calculated from its children's values up to the tree root.

Merkle Tree

Merkle Root​

  • When a transaction is included in a side chain's block, the block will also include a Merkle root of the transactions in this block.
    • This root is local to this side chain's blockchain and, by itself, of little value to other side chains because they follow a different protocol.
    • Communication between side chains goes through the main chain in the form of a Merkle path.
    • During the indexing process, the main chain calculates the root with the data from side chains, and side chains in turn get the root in future indexing.
    • This root is used for the final check in cross-chain transaction verification.

Merkle Path​

  • A Merkle path is the node collection for one leaf node to calculate to the root.
    • A correct Merkle path is necessary to complete any work related to cross-chain verification.
  • For the transaction tx from chain A:
    • You need the whole Merkle path root for tx to calculate the final root if you want to verify the existence of this transaction on other chains.
    • Verify the root by checking whether it is equal to the one obtained from indexing before.

Merkle Path