Compiler: Perry Wang
Compiler: Perry Wang
From a community perspective, we are very excited about the off-chain protocol (off-chain protocol) as a way to scale the blockchain network, because this will move most transactions from the layer 1 blockchain to the off-chain system, This avoids network fees and latency issues on the L1 blockchain.
I would like to use this article to discuss aboutoff-chain protocolsecondary title
What is a "bridge"?
Before answering this question, we should talk about the often overlooked but important aspect of evaluating bridges.Fund securityEssential building blocks:
In short, bridges are used whenever an L1 blockchain like Ethereum is connected to any other system. All bridges have similar actions:
deposit assets. Users can deposit assets to the bridge, and the bridge will give proofs representing this asset in other systems;
update account balance. The bridge will be notified about the new account balance, which can be used to facilitate the withdrawal process.
Asset withdrawal. Users can withdraw assets from the bridge by virtue of their balances in other systems, and issued tokens burned in other systems.
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Most cryptocurrency exchanges (probably all) offer a single organizational form of bridging services
If we only consider bridges and nothing else, we can sayA centralized cryptocurrency exchange is an off-chain protocol. Users can lock funds into the services of the centralized exchange, bypass network fees and delays when making transactions, and eventually withdraw their funds back to the L1 blockchain.
In addition to this single-organization bridge, there are two other bridges that rely on a set of custodians:
Multiple tissue bridges. A fixed number of independent individuals (K out of N representatives) hold locked funds.
The bridge of the encrypted economy. A dynamic number of individuals keep locked assets, and the specific amount is determined by their asset weight.
An important insight is that all three of the above bridges on the L1 blockchainUnable to verifyCustodianCustodiandetermine whetherall withdrawalssecondary title
Sidechains and bridges are independent of any blockchain network
In the above discussion, we mainly considered the adoption of custodial service bridges such as cryptocurrency exchanges. An increasingly popular use case for bridges is toOne blockchain connects to another blockchainimage description
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There are also several instances of bridging:
WBTC: A single organizational bridge that connects BTC assets to the Ethereum network.
Liquid Network or RSK: A multi-organizational bridge, multi-party consortium with Hardware Security Modules (HSMs) to lock/unlock funds in BTC assets to another blockchain.
Polygon Bridge: A crypto-economic bridge in which 2/3 + 1 of the locked assets in the bridge regularly reach an agreement on the account balances of all users in Polygon, and users can use this agreement to withdraw funds on Ethereum (in fact, Polygon Ultimately it will be governed by small multi-signature contracts, but this example focuses on long-term goals).
Rainbow Bridge: A cryptoeconomic bridge whose bridge contract is a light client that can verify the progress of other blockchains. It does not check the validity of the other blockchain, and the safety of funds ultimately depends on the continued development of the other blockchain (secured through cryptoeconomics).
Crucially, each bridge has its ownsecurity model, and is not bound to any blockchain network.
We can take a simple caseWBTC To elaborate further:BitGo TrustHosting the funds locked in the Bitcoin blockchain, they are responsible for issuing the same amount of WBTC on Ethereum. Smart contracts on Ethereum track account balances for all transfers of WBTC. BitGo is the trustworthy party, managing account balances recorded in smart contracts.
There are several aspects to consider in the case of WBTC:
single custodian. WBTC's bridge relies on a single custodian to guarantee its integrity. They could theoretically issue more WBTC on Ethereum than locked in Bitcoin, and they could decide not to honor the withdrawal of WBTC into BTC.
Standalone security model. Ethereum has its own security model that is independent of the Bitcoin blockchain. The bridge again has its own security model that is independent of the two aforementioned blockchain networks.
Ethereum is a sidechain. Compared to the Bitcoin blockchain, transactions have been moved off-chain, from the Bitcoin blockchain to Ethereum.
What the three bridges mentioned above have in common is that they do not check the side chain informationhonesty,And if the custodian (or sidechain) goes offline, there is no self-enforcingemergency plansecondary title
What does the L2 protocol have to do with this article?
L2 scalability aims to move transaction throughput from an L1 blockchain to another off-chain system, requiring bridges to hold assets issued on other systems.
However, unlike all the other bridge types explored in this article, the L2 protocol strives to secure funds with the same security mechanisms as the L2 blockchain, and it cannot rely on a set of custodians (or other off-chain systems) to secure funds.
It requires a new type of bridge:
L2 bridge. The L1 blockchain has custody of the funds, and the bridge must trust that the message integrity of the L2 protocol will not be compromised. In the worst case, the bridge will automatically increase the liveness of the L2 protocol until all funds can be withdrawn.
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There are several companies focused on building L2 bridges, and naturally new blockchain networks will also be built
This isL2 protocolWhat makes it so exciting is that it took years for the aforementioned teams to come up with a solution. The first competitors using L2 protocols in the market are mainly focused on how to implement a secure L2 bridge (and not necessarily deploy other blockchain networks).
This is a good opportunity to further explore technical issues and definitions. We make it clear: it is imperative to ensure that bridges do not break the L2 protocol, and that breaches of L2 data integrity can be broken down into four issues:
Data availability. How can the bridge be confident that all data of another blockchain network is publicly available so that users can independently recompute the L2 database?
Integrity of state changes. How do we convince the bridge that all state transitions of the L2 network are well-formed and valid?
Integrity of withdrawals. If the L2 network is compromised, how does the bridge guarantee that all honest users' funds can be withdrawn?
Protocol activity. How does the bridge guarantee that transactions can still be executed when the L2 protocol stops or goes offline?
Of course, the above problems must be solved, and at the same time, the bridge contract has significantly less computing resources than the off-chain system, so the bridge irreversibly re-executes all transactions in real time. Otherwise, it's not a scalable solution.
Solving the above problems allows us to fall intorabbit hole dilemma. Specifically includeOn-Chain Challenge, Fraud Proof, Validity Proof, Publish transaction data to the L1 blockchain (rollups) and the world on the chain.
Although our article does not highlight the various solutions, we emphasize that all solutions are not the same. Some of the L2 protocols to be deployed will not be able to meet the above security goals. They cannot be called L2 protocols due to the lack of L2 bridges.
As in the case explored in this article, there are four types of bridges that allow funds to be locked in a blockchain while the same asset is represented in another off-chain system (and possibly another blockchain).
Managed bridge. The first three types of bridges mentioned above focus on which groupCustodians control locked funds. The role of the custodian is to verify that the off-chain system is correct before allowing any assets to be withdrawn from the bridge. The assumption is that the integrity of the off-chain system is a client-side issue, and that the custodian has sufficient computing resources to handle it. While there are efforts to ease the role of custodians and introduce cryptoeconomic incentives to encourage custodians to follow the protocol, the bridging protocolinability to fully restrain the custodian. There have been several instances of bridges losing user funds (e.g. Mt. Gox) because the integrity of such bridges ultimately depends on trust in people.
L2 bridgeCustodianCustodianrole, they are responsible for custody of funds and check the status of off-chain systemsintegrity. At the heart of the matter is that such a bridging system must be confident that the off-chain system cannot be compromised, and it lacks the means to independently check each transaction.computing resources(otherwise it's not a scalable solution). Aside from the high level of technical challenges it entails, it doesn't come for free. There is an ongoing financial cost to convincing the L1 blockchain that the off-chain system is indeed well-structured and that its integrity has not been compromised in the slightest. Ultimately, however, it will be the bridge that will keep custody of the funds, not the off-chain system operator.
Overall, the jury is still out on whether users really care about L2 bridges and whether we should extend Ethereum's security model to off-chain systems.
As with all things in life, I doubt all four types of bridgeswill continue to exist, as they are both integral to promoting among users.
My only request is this: you (the user) take a careful look at the type of bridge your favorite protocol uses. Important: Get a better understanding of how your money stays safe from the bad guys.
