Background
Blockchain is becoming a complex and disorderly value society. The future value circulation and clearing mechanism of the blockchain is the cross-chain mechanism. Cross-chain not only increases the scalability of the blockchain, but also opens up the value circulation channel between the chains. It can also solve the problem of “data silos”, which is caused by the difficulties of transaction between public chains, and forms a blockchain society with coordinated development.
It can be said that the Internet is a network in which information is being freely circulated, and the blockchain cross-chain network is a network with free value circulation.
Technical difficulties in the cross-chain mechanism
First: How to guarantee the atomicity of cross-chain transactions
Second: How to confirm the transactions of another chain
Third: Cross-chain transactions, how to keep the constant amount of the total assets.
Fourth: How to guarantee the network security of the two individual chains
Fifth: How to realize cross-chain interconnection among multiple chains
In order to solve the above technical difficulties, lots of solutions have emerged in the blockchain industry. This paper gives a brief introduction to the typical solutions.
1 Transaction Atomicity
The atomic swap protocol is. HTLC (Hashed Time-Lock Contract) is an implementation of the atomic swap protocol, which guarantees the atomicity of transactions through hash lock and time lock mechanism. In different systems, whether it is a blockchain system or a centralized accounting system, the way to implement hash locks and time locks is different. But the principle is the same, that is, the transaction is effective only if certain hash conditions or time conditions are met.
By using hash time locks alone, you can only implement cross-chain asset swaps instead of cross-chain asset transfers. If you want to achieve the cross-chain transfer of assets, you need to use hash time locks to ensure transactional atomicity, in addition to use other cross-chain technologies to ensure the authenticity of cross-chain transactions, in other words, need to complete transaction confirmation of other chains.
2 Transaction Confirmation
Blockchains are closed, independent and unknown to each other. There is no direct communication channel. How can cross-chain transactions confirm that the transaction of sending chain did happened and is confirmed by it? The answer is “middleman”, through the role of “middleman” to exchange information, the “middleman” may be a node or a group, may be a centralized organization or may be a distributed group.
The existing solutions mainly include the following: notary mode, relay mode, side chain mode.
• Notary Mode:
The “middleman” of the notary mode not only collects data, but also confirms and verifies the transaction. According to the way to verify transaction, it can also be divided into the following types: Centralized Notary schemes, Multi-sig Notary schemes and Distributed signature Notary schemes.
• Relay Mode:
The “middleman” of relay mode only acts as a data collector and forwards the collected data to the target chain. The target chain verifies the transaction based on the collected data. The verification methods vary according to the system structure, such as relying on SPV certification, or relying on the number of signatures of verification nodes and so on. Relay mode is a relatively decentralized and loosely coupled way, it has higher scalability.
There are many cross-chain implementation schemes of relay mode, such as Hub in Cosmos, Relay chain in Polkadot, Relayer in BTC-Relayer and so on, they all act as relays.
• Side Chain Mode:
Side chain mode is a strongly coupled design. The two chains collect each other’s data through the “middleman”, and in some way directly embeds the data into their own blocks or storage space. In cross-chain transactions, transaction verification can be completed directly through the original chain data stored in its system.
3 Total Assets Keep Constant
To ensure that the total amount of assets keeps constant, it is necessary to ensure that the process of asset transfer is accurately accounted for on both chains. That is, it is necessary to guarantee: 1) The atomicity of cross-chain transactions, either happen or not. 2) Cross-chain transactions are real and effective on both chains, and transactions are confirmed by the entire network, and the possibility of fork is small. Therefore, under normal circumstances, as long as the difficulty 1 and the difficulty 2 are solved, the difficulty 3 is solved.
4 Independent Security
There are basic security and independence considerations in different cross-chain implementation schemes, but further research on security measures needs to be strengthened, which is not discussed here.
5 Cross-chain Interconnection
This problem contains two potential problems: First, how the existing blockchain system realize interconnection and interoperability. Second, how to prepare and pave the way for the interconnection of the blockchain to be developed in the future.
Polkadot is a cross-chain solution designed to solve the above problems. It uses a typical cross-chain technology, that is relay technology. And Polkadot is also a typical solution of relay technology.
The next paper will give a detailed introduction to how Polkadot solves the above problems and the technologies it uses
by Lichuan Deng
