Compare 23 cross-chain Bridges to understand the status quo of L2 bridge

原文标题：《 The Current State of Layer 2 Bridges 》

Andreas Freund, L2 Standard Working Group, Enterprise Ethereum Alliance (EEA) Community Project

Source: entethalliance.org

The Kyle DeFi Way

We live in a multi-chain world, with billions of dollars of asset value locked up in more than 100 chains. And the owners of these blockchain assets are behaving just like their assets in traditional finance: they are looking for arbitrage opportunities to make money. However, unlike the traditional financial world, where assets in one country can be used in arbitrage activities in another without transferring the assets through trusted intermediaries, the same approach won't work for blockchain for a long time for three reasons:

Blockchains can't talk to each other,

Because of the trust-free nature of public blockchains, arbitrage on a particular blockchain requires that all relevant assets exist on that blockchain,

And there are no trusted intermediaries in traditional finance between untrusted blockchains.

To address capital inefficiencies on the blockchain, and make money in the process, enterprising individuals have created blockchain Bridges to address these three challenges and begin to connect the blockchain ecosystem together - yes, you can now trade bitcoin on Ethereum. Of course, cross-chain Bridges can also be used for other types of functions; However, the main function is to improve capital efficiency.

What is a blockchain bridge?

At a high level, a blockchain bridge connects two blockchains, facilitating secure and verifiable communication between these blockchains through the transfer of information and/or assets.

This provides many opportunities, for example

The transfer of assets across chains, new decentralized applications (dApps) and platforms that allow users to access the advantages of various blockchains -- thereby empowering them, and developers from different blockchain ecosystems to collaborate and build new solutions.

There are two basic types of Bridges:

1. Trust Bridge

Rely on a central entity or system for operations. Trust assumptions about the custody of funds and the safety of Bridges. Users mainly rely on the reputation of the bridge operator. Users need to relinquish control of their crypto assets.

2. Don't trust the bridge

Operate with decentralized systems, such as smart contracts with embedded algorithms. The security of the bridge is the same as that of the underlying blockchain. Enabling users to control their money through smart contracts.

Among the two sets of trust assumptions, we can distinguish the different, common types of cross-chain bridge design:

Locking, casting, and destroying Token Bridges: Instant finality is guaranteed, as casting assets on the target blockchain can occur as needed without the possibility of transaction failure. The user receives a synthetic asset on the target blockchain, often called an encapsulated asset, rather than a native asset. Liquidity network with local pools of assets with uniform liquidity: Individual pools of assets on one blockchain are connected to other pools of assets on other blockchains, sharing access to each other's liquidity. This approach does not achieve instant, guaranteed finality because transactions may fail if there is a lack of liquidity in the shared pool.

However, all designs, and under any assumption of trust, must solve two difficult problems that blockchain Bridges face.

Bridging Trilemma by Stargate's Ryan Zarick

The bridge protocol may have only two of the following three attributes:

Immediacy Guaranteed finality: Guaranteed receipt of assets on the target blockchain immediately after execution of transactions on the source blockchain and finalization of transactions on the target blockchain.

Unified Liquidity: A single pool of liquidity for all assets between the source and target blockchain. Local asset: An asset that receives the target blockchain asset, rather than one that represents the bridge of the original asset on the source blockchain.

The interoperability Trilemma proposed by Arjun Bhuptani of Connext

An interoperability protocol may have only two of the following three attributes:

No trust: Same security guarantees as the underlying blockchain, no new trust assumptions.

Scalability: The ability to connect different blockchains.

Versatility: Allows for arbitrary data messaging

Aside from the trilemma, which can be solved by clever design, the biggest challenge for blockchain Bridges is security, as evidenced by the many hacks that took place in 2021 and 2022; Whether it's Wormhole, Ronin, Harmony or Nomad events. Fundamentally, the Bridges between blockchains are only as secure as the least secure blockchains used in the Bridges (chains) of assets. However, the latter issue is not a problem for Bridges between Tier 2 (L2) platforms anchored on the same Tier 1 (L1) blockchain, as they share the same security guarantees from the shared L1 blockchain.

Why are cross-chain Bridges important for L2?

So far, we haven't specifically discussed L2 platforms that aim to extend L1 blockchains while inherits L1 security guarantees, because L2 is strictly a specific type of bridge: a local bridge. However, there are some features of the L2 platform when creating Bridges between L2, such as optimistic rollups vs. zk-rollups vs. Validium rollups vs. Volition rollups. These differences make them special because there are differences in trust assumptions and finality between L2 and L1 and between different L2s.

Bridges between L2 are important for the same reasons as L1: L2 assets are seeking the capital efficiency of other L2s, as well as portability and other functions.

As mentioned earlier, differences in local trust assumptions on L2 platforms can be overcome if the bridged L2 is anchored to the same L1. And the bridge requires no additional assumptions of trust. However, differences in L2 transaction finality over anchoring L1 make it challenging to bridge assets between L2 in a way that minimizes trust.

Types of blockchain Bridges in L2: An Overview

Digging into L2 Bridges, we found that L2-L2 Bridges should ideally meet the following criteria:

Clients must be abstracted from each L2 protocol they connect to through the abstraction layer -- the loosely coupled paradigm.

The client must be able to verify that the data returned from the abstraction layer is valid, ideally without changing the trust model to the one used by the target L2 protocol.

Interface L2 does not require structure/protocol changes.

A third party must be able to independently build an interface to the target L2 protocol -- ideally a standardized interface.

As things stand, one will find that most L2 Bridges treat L2 as just another blockchain. Note that the proof of fraud used in Optimistic rollups and the proof of validity used in the zk-rollups solution replace the block header and Merkle proof used in the "normal" L1 to L1 bridge.

Current L2 bridge pattern

Below we summarize the current and very diverse landscape of L2 Bridge, including name, brief summary and bridge design type:

1.Hope Exchange

描述：Rollup-rollup 通用 Token 桥。它允许用户几乎立即将 Token 从一种 rollup 发送到另一种 rollup，而无需等待 rollup 的挑战期。

https://hop.exchange/whitepaper.pdf

Type of design: Fluid Network (uses an AMM)

2.Stargate

Description:

Composable native asset Bridges and DApps built on LayerZero. DeFi users can exchange native assets across the chain on Stargate in a single transaction. Applications compose Stargate to create native cross-chain transactions at the application level. These cross-chain exchanges are supported by the community-owned Stargate Consolidated Liquidity Pool.

Design type: Mobility network

3.Synapse Protocol

Description:

一个 Token 桥，利用链和流动性池之间的验证者来执行跨链和同链交换。

设计类型：混合设计（ Token 桥/流动性网络）

4.Across

Description:

A cross-chain Optimistic bridge that uses actors called Repeaters to fulfill user transmission requests on the target chain. The repeater is then compensated by providing proof of its behavior to the Optimsitic Prophecy machine on Ethereum. The architecture utilizes a single liquidity pool on Ethereum and separate deposit/repayment pools on the target chain, which are rebalanced using canonical Bridges.

Design type: Mobility network

5.Beamer

Description:

使用户能够将 Token 从一个 rollup 移动到另一个 rollup。用户通过在源 rollup 上提供 Token 来请求传输。流动性提供者然后填写请求并直接将 Token 发送给目标 rollup 上的用户。该协议的核心重点是尽可能方便最终用户使用。这是通过分离两个不同的关注点来实现的：向最终用户提供的服务，以及流动性提供者回收资金。请求一到达，就乐观地提供服务。源 rollup 的退款由其自身的机制保证，并与实际服务分离。

6.Biconomy Hyphen

Description:

多链中继网络利用基于智能合约的钱包，供用户与流动性提供者交互，在不同（Optimistic）L2 网络之间转移 Token 。

Design type: Mobility network

7. Bungee

Description:

The bridge is built on top of the Socket infrastructure and SDK, with the Socket Mobility Layer (SLL) as its main component. The SLL pooled liquidity from multiple Bridges and DEX, and also allowed P2P settlement. This is different from a liquidity pool network in that this single meta-bridge allows funds to be dynamically selected and routed through the best bridge based on user preferences (such as cost, delay, or security).

Design type: Liquidity pool aggregator

8.Celer cBridge

Description:

一个去中心化的非托管资产桥，支持跨越 30 多个区块链和 L2 rollup 的 110 多种 Token 。它建立在 Celer 链间消息框架之上，而 Celer 链间消息框架建立在 Celer State Guardian Network (SGN) 之上。SGN 是建立在 Tendermint 上的权益证明 (PoS) 区块链，充当不同区块链之间的消息路由器。

Design type: Mobility network

9.Connext

Description:

Scheduling and processing messages related to sending money across chains. Custody funds used to regulate assets, fast liquidity and stable conversion. The Connext contract uses a diamond pattern, so it contains a set of Facets that act as a logical boundary for the function group. Facets shares the contract storage and can be upgraded separately.

设计类型：混合设计（ Token 桥/流动性网络）

10.Elk Finance

Description:

Use ElkNet with the following features:

用于价值转移的跨链实用 Token （$ELK）与传统桥相比安全可靠的传输在 Elk 支持的所有区块链之间通过 ElkNet 在几秒钟内进行跨链价值转移桥接即服务 (BaaS) 为开发人员提供基础设施，以利用 ElkNet 实现自定义桥接解决方案所有连接的区块链之间的跨链交换为我们的流动性提供者提供无偿损失保护 (ILP) 具有独特能力和特性的不可替代 Token （Moose NFT）

设计类型：混合设计（ Token 桥/流动性网络）

11.LI.FI

Description:

Bridges and DEX aggregators, which route any asset on any chain to a desired asset on a desired chain, provided at the API/ contract level via the SDK, or as embeddable widgets in DApps

Design type: Liquidity pool aggregator

12.LayerSwap

Description:

以低廉的费用将 Token 从中心化交易账户直接桥接到第 2 层（L2）网络（Optimistic 和 zk-rollups）。

Type of design: Fluid Network (uses an AMM)

13.Meson

Description:

一个使用哈希时间锁定合约 (HTLC) 的原子互换应用程序，使用用户之间的安全通信与流动性提供者中继网络相结合，用于支持的 Token 。

Design type: Mobility network

14.O3 Swap

Description:

O3 Swap and Bridge cross-chain mechanism aggregate multiple cross-chain liquidity pools, allowing simple one-time confirmation transactions with planned gas stations to solve the demand for gas charges on each chain.

Design type: Liquidity pool aggregator

15.Orbiter

Description:

A decentralized cross rollup bridge for transporting Ethereum's native assets. The system has two roles: Sender and Maker. The "Maker" must deposit an excess margin on Orbiter's contract before it is eligible to be a cross-rollup service provider for the "Sender". In the usual process, the "Sender" sends the asset to the "Maker" on the Source Network, and the "Maker" sends the asset back to the "Sender" on the Destination Network.

Design type: Mobility network

16.Poly Network

Description:

Allows users to transfer assets between different blockchains using Lock-Mint exchanges. It uses a Poly Network chain to validate and coordinate messaging between Repeaters on the supported chain. Each chain has a set of Relayers, while the Poly Network chain has a set of Keepers for signing cross-chain messages. Chains integrated with Poly Bridge need to support light client validation, as validation of cross-chain messages involves verifying bulk and transactions through Merkle proofs. Some smart contracts used by the bridge infrastructure were not validated on Etherscan.

设计类型： Token 桥

17.Voyager (Router Protocol)

Description:

The router protocol uses a pathfinding algorithm to find the best path, using a router network similar to Cosmos IBC to move assets from the source chain to the target chain.

Design type: Mobility network

18.Umbria Network

Description:

Umbria has three main protocols that work together:

Cross-chain asset bridge; Support for transferring assets between other incompatible blockchain and cryptocurrency networks.

A pledge pool where users can earn interest on their crypto assets by providing liquidity to the bridge. UMBR's liquidity provider earns 60% of all fees incurred by the bridge.

Decentralized trading platform (DEX); Automated liquidity protocols are supported by a constant product formula, deployed using smart contracts, and managed entirely on-chain.

The two protocols work together to provide for the migration of assets between cryptocurrency networks.

Type of design: Fluid Network (uses an AMM)

19. Via Protocol

Description:

This protocol is an aggregator of chains, DEX, and Bridges to optimize asset transport paths. This allows for asset bridging in three ways:

Multiple transactions on different blockchains

Make a transaction through a decentralized bridge that integrates DEX

A transaction over a semi-centralized bridge triggers a second transaction on the target chain

设计类型：混合设计（ Token 桥/流动性网络）

20.Multichain

Description:

Multichain 是经过外部验证的桥梁。它使用运行 SMPC（安全多方计算）协议的节点网络。它通过 Token 桥和流动性网络支持数十条区块链和数千种 Token 。

设计类型：混合设计（ Token 桥/流动性网络）

21.Orbit Bridge

Description:

Orbit Bridge 是 Orbit Chain 项目的一部分。它是一个跨链桥，允许用户在支持的区块链之间转移 Token 。 Token 存放在源链上，「表示 Token 」在目标链上铸造。存入的 Token 没有精确锁定，Orbit Farm 可以在 DeFi 协议中使用。应计利息不会直接传递给 Token 存款人。桥合约实施和 Farm 合约源代码未在 Etherscan 上验证。

设计类型： Token 桥

22.Portal (Wormhole)

Description:

The Portal Token Bridge is built on top of Wormhole, a messaging protocol that utilizes a dedicated network of nodes to perform cross-chain communication.

设计类型： Token 桥

23.Satellite (Axelar)

Description:

Satellite 是由 Axelar 网络提供支持的 Token 桥

Design type: Mobility network

The L2Beat project maintains a list of blockchain Bridges associated with L2, along with their total value locking (TVL), along with a description and brief risk assessment (if any).

L2 bridge risk profile

Finally, when users use L2 Bridges, virtually any bridge needs to be careful, and the following risks need to be assessed for a given bridge:

Loss of funds

Predictors, Repeaters, or verifiers collude to submit fraudulent proofs (e.g., block hashes, block headers, Merkle proofs, fraud proofs, validity proofs) and/or relay unmitigated fraudulent transmissions

The authenticator/repeater private key is leaked

验证者恶意铸造新 Token

False claims not contested in a timely manner (Optimistic Information Agreement)

The target blockchain reorganization occurs after Optimistic's prognosticator/repeater controversy time has passed (Optimistic messaging protocol).

Unverified contract source code involved in or used in the protocol contains malicious code or functionality that can be misused by the contract owner/administrator

Token 桥所有者行为不端，或发起影响用户资金的时间敏感的紧急行动，并且没有与用户群进行适当的沟通

Protocol Contract suspension (if function exists)

Protocol The contract received a malicious code update

Frozen funds

The repeater/liquidity provider does not take action on user transactions (messages)

Protocol Contract suspension (if function exists)

Protocol The contract received a malicious code update

桥上目标 Token 流动性不足

Audit user

A predictor or repeater on a target or target L2 or both cannot facilitate transmission of (a message).

Protocol Contract suspension (if function exists)

While this list is not exhaustive, it provides a good overview of the risks associated with the current use of Bridges.

New developments using Proof of Zero Knowledge (ZKP) technology are underway that aim to mitigate some of these risk factors and address both bridge challenges. In particular, the use of ZKP allows for the following bridge design features:

No trust required and secure, as the correctness of the block headers on the source and target blockchains can be proved by zk-SNARKs, which can be verified on EVM-compatible blockchains. Therefore, no external trust assumption is required, assuming that the source and target blockchains and the light client protocols used are secure, and that we have 1-of-N honest nodes in the relay network.

No licensing and decentralization is required because anyone can join the bridge's relay network, and no PoS style or similar validation scheme is required to be scalable because applications can retrieve ZKP validation block headers and perform application-specific validation and functional efficiency, This is because the new optimized proof scheme has short proof generation and fast proof verification time

Although still early, these types of developments are expected to accelerate the maturation and security of the bridge ecosystem.

generalize

We can summarize the above discussion and overview of L2 Bridges as follows:

L2 Bridges are an important glue in the L2 ecosystem, further facilitating L2 interoperability and efficient use of assets and applications across the ecosystem.

L2 Bridges used on L2 anchored to the same L1, such as the Ethereum mainnet, are more secure than Bridges between L1-assuming the source code is secure, which is often a big assumption.

As with all distributed system architectures, important trade-offs need to be made, as expressed by two hypothetical trilemma -- the blockchain bridge trilemma and the interoperability trilemma.

L2 Bridges have very different trust assumptions, for example, trusted versus untrusted Bridges, and very different design options, for example, lock-cast-destroy versus fluidity networks.

The L2 Bridges ecosystem is still in its infancy and is in a state of constant change.

Users are advised to perform due diligence to assess which L2 Bridges provide the best risk-reward profile to meet their needs.

New developments are being made using the latest ZKP technologies that effectively solve the two bridge trilemma and contribute to improving the overall safety of the bridge.

Many thanks to Tas Dienes (Ethereum Foundation), Daniel Goldman (Offchain Labs) and Bartek Kiepuszewski (L2Beat) for carefully reading the manuscript and making valuable content suggestions.

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