The Present State of Layer 2 Bridges


By Dr. Andreas Freund (Co-Chair) on behalf of the EEA Neighborhood Initiatives L2 Requirements Working Group

We stay in a multi-chain world, with billions of USD in asset worth locked in 100+ chains. And the house owners of these blockchain belongings behave identical to they might with belongings in conventional finance: they’re in search of arbitrage alternatives to generate income. Nevertheless, in distinction to the world of conventional finance the place belongings in a single nation may be utilized in arbitrage performs out of the country with out shifting belongings by utilizing trusted intermediaries, the identical method didn’t work for blockchains for a very long time for 3 causes:

  1. blockchains can’t speak to 1 one other,
  2. arbitrage performs on a selected blockchain require that every one concerned belongings are current on that blockchain due to the trustless nature of public blockchains, 
  3. and there was no equal to the trusted middleman as in conventional finance between trustless blockchains.

To unravel the issue of capital inefficiency on blockchains, and generate income within the course of, enterprising people created blockchain bridges that addressed these three challenges and began to hyperlink the blockchain ecosystem collectively – sure, now you can commerce bitcoin on Ethereum. In fact, bridges can be utilized for different sorts of performance too; nonetheless, the first operate is to enhance capital effectivity. 

At a excessive stage, a blockchain bridge connects two blockchains facilitating safe and verifiable communication between these blockchains by way of the switch of data and/or belongings.

This permits for a mess of alternatives equivalent to

  • cross-chain switch of belongings,
  • new decentralized functions (dApps), and platforms that enable customers to entry the strengths of varied blockchains – thus enhancing their capabilities,
  • and builders from totally different blockchain ecosystems can collaborate and construct new options.

There are two fundamental sorts of bridges:

Trusted Bridges Trustless Bridges
Rely on a central entity or system for his or her operations. Function utilizing decentralized techniques equivalent to sensible contracts with embedded algorithms.
Belief assumptions regarding funding custody and bridge safety. Customers largely depend on the bridge operator’s popularity. The safety of the bridge is similar as that of the underlying blockchain.
Customers want to surrender management of their crypto belongings. Allow customers to stay in charge of their funds by way of sensible contracts.

Inside each units of belief assumptions one can distinguish totally different, widespread sorts of bridge designs:

  • Lock, mint, and burn token bridges: Immediate assured finality as minting belongings on the vacation spot blockchain can happen at any time when required with out the opportunity of a failed transaction. Customers obtain an artificial, typically known as a wrapped asset, on the vacation spot blockchain, not the native asset.
  • Liquidity networks with swimming pools of native belongings with unified liquidity: A single asset pool on one blockchain is linked with further asset swimming pools on different blockchains with shared entry to 1 one other’s liquidity. This method doesn’t allow on the spot, assured finality since transactions can fail if there’s a lack of liquidity within the shared swimming pools.

Nevertheless, all designs, and underneath no matter belief assumptions, have to deal with two trilemmas that blockchain bridges face.

Bridging Trilemma as posited by Ryan Zarick, Stargate

Bridging protocols could solely have two of the three properties under:

  • Immediate Assured Finality: Assure to obtain belongings on the goal blockchain instantly after transaction execution on the supply blockchain and transaction finality on the goal blockchain.
  • Unified Liquidity: Single liquidity pool for all belongings between supply and goal blockchains.
  • Native Belongings: Obtain goal blockchain belongings as a substitute of belongings minted by the bridge representing the unique asset on the supply blockchain.

Interoperability Trilemma as posited by Arjun Bhuptani, Connext

Interoperability protocols could solely have two of the three properties under:

  • Trustlessness: Identical safety assurances because the underlying blockchain with out new belief assumptions.
  • Extensibility: Capability to attach totally different blockchains.
  • Generalizability: Permits for arbitrary information messaging

Moreover the trilemmas which may be addressed with intelligent design, the largest problem for blockchain bridges is safety as the various hacks in 2021 and 2022 have demonstrated; be it the Wormhole, Ronin, Concord, or Nomad incidents. And basically, a bridge between blockchains is just as safe because the least safe blockchain used within the (chain of) bridge(s) for an asset. Nevertheless, this latter subject shouldn’t be an issue for bridges between Layer 2 platforms which can be anchored on the identical Layer 1 (L1) blockchain as they share the identical safety ensures from their shared L1 blockchain. 

Till this level, we’ve not particularly talked about L2 platforms which can be designed to scale L1 blockchains whereas inheriting the L1 safety ensures, since L2s are strictly talking a selected sort of bridge: a local bridge. There are, nonetheless, a number of idiosyncrasies of L2 platforms when making a bridge between L2s e.g. optimistic rollups vs. zk-rollups vs Validium rollups vs Volition rollups. These variations make them particular due to the distinction in belief assumptions and finality for L2s versus L1s and between totally different L2s.

The explanation why bridges between L2s are necessary is similar as for L1s: L2 belongings are in search of capital effectivity on different L2s, in addition to portability and different functionalities.

The distinction in native belief assumptions on L2 platforms may be overcome if bridged L2s are, as remarked already, anchored on the identical L1. And that the bridge doesn’t require further belief assumptions. Nevertheless, variations in L2 transaction finality on the anchor L1 make it difficult to bridge belongings between L2s in a trust-minimized method. 

Digging a bit deeper into L2 bridges, we see that L2-to-L2 bridges ideally ought to fulfill the next standards:

  • Purchasers have to be abstracted away from every L2 protocol they interface with by way of an abstraction layer – loose-coupling paradigm.
  • Purchasers should be capable to confirm that the info returned from the abstraction layer is legitimate, ideally with out altering the belief mannequin past the one utilized by the focused L2 protocol.
  • No structural/protocol adjustments are required from the interfacing L2 protocol.
  • Third events should be capable to independently construct an interface to a focused L2 protocol – ideally a standardized interface.

When wanting on the present panorama, one sees most L2 bridges are treating L2s identical to one other blockchain. Observe that fraud proofs as utilized in Optimistic rollups, and validity proofs as utilized in zk-rollups options, take the place of block headers and Merkle proofs as utilized in “regular” L1-to-L1 bridges.

Under we summarize the present and really various panorama of L2 bridges with a reputation, temporary abstract, and bridge design sort:

Bridge Identify Description Kind of Design
Hop Trade Rollup-to-rollup common token bridge. It permits customers to ship tokens from one rollup to a different virtually instantly with out having to attend for the rollup’s problem interval. https://hop.alternate/whitepaper.pdf Liquidity Community (utilizing an Automated Market Maker)
Stargate Composable native asset bridge, and dApp constructed on LayerZero. DeFi customers can swap native belongings cross-chain on Stargate inside a single transaction. Functions compose Stargate to create native cross-chain transactions on the utility stage. These cross-chain swaps are supported by the community-owned Stargate unified liquidity swimming pools. Liquidity Community
Synapse Protocol A token bridge leveraging a validator between chains and liquidity swimming pools to carry out cross-chain and same-chain swaps. Hybrid (Token Bridge/Liquidity Community)
Throughout A cross-chain optimistic bridge that makes use of actors known as Relayers to satisfy consumer switch requests on the vacation spot chain. Relayers are later reimbursed by offering proof of their motion to an Optimistic Oracle on Ethereum. The structure leverages a single liquidity pool on Ethereum and separate deposit/reimburse swimming pools on vacation spot chains which can be rebalanced utilizing canonical bridges. Liquidity Community
Beamer Permits customers to maneuver tokens from one rollup to a different. The consumer requests a switch by offering tokens on the supply rollup. Liquidity suppliers then fill the request and immediately ship tokens to the consumer on the goal rollup. The core focus of the protocol is to be as simple to make use of as doable for the top consumer. That is achieved by separating two totally different considerations: the service offered to the top consumer, and the reclaiming of funds by the liquidity supplier. The service is offered optimistically as quickly because the request arrives. Being refunded on the supply roll-up is secured by its personal mechanism and decoupled from the precise service. Liquidity Community
Biconomy Hyphen Multi-chain relayer community using sensible contract-based wallets for customers to work together with liquidity suppliers to switch tokens between totally different (optimistic) L2 networks. Liquidity Community
Bungee The bridge is constructed on the Socket infrastructure and SDK with the Socket Liquidity Layer (SLL) as its fundamental part. The SLL aggregates liquidity throughout a number of bridges and DEXs and in addition permits for P2P settlements. That is totally different from a Liquidity Pool community since this single meta-bridge permits dynamical choice and routing of funds through the most effective bridge as per a consumer’s preferences equivalent to price, latency, or safety.  Liquidity Pool Aggregator
Celer cBridge A decentralized and non-custodial asset bridge that helps 110+ tokens throughout 30+ blockchains and layer-2 rollups. It’s constructed on high of the Celer Inter-chain Message Framework which is constructed on the Celer State Guardian Community (SGN). The SGN is a Proof-of-Stake (PoS) blockchain constructed on Tendermint that serves because the message router between totally different blockchains. Liquidity Community
Connext Dispatches and handles messages associated to sending funds throughout chains. Custodies funds for canonical belongings, quick liquidity, and steady swaps. The Connext contract makes use of the Diamond sample so it includes a set of Sides that act as logical boundaries for teams of features. Sides share contract storage and may be upgraded individually. Hybrid (Token Bridge/Liquidity Community)
Elk Finance Makes use of ElkNet with options equivalent to 

  • Cross-chain utility token for worth switch ($ELK)
  • Protected and safe transfers in comparison with conventional bridges
  • Cross-chain worth switch through ElkNet in seconds between all of the blockchains Elk helps
  • Bridging-as-a-Service (BaaS) to offer infrastructure for builders to leverage the ElkNet for customized bridging options
  • Cross-chain swaps between all linked blockchains
  • Impermanent Loss Safety (ILP) for our liquidity suppliers
  • Non-Fungible Tokens (Moose NFTs) with distinctive talents and options
Hybrid (Token Bridge/Liquidity Community)
LI.FI A bridge and DEX aggregator that routes any asset on any chain to the specified asset on the specified chain made accessible on the API/Contract stage by way of an SDK, or as an embeddable widget in a dApp  Liquidity Pool Aggregator
LayerSwap Bridge for tokens from a centralized alternate account on to a Layer 2 community (each optimistic and zk- rollups) with low charges. Liquidity Community (utilizing an Automated Market Maker)
Meson An atomic swap utility utilizing Hash Time Lock Contracts (HTLC) utilizing safe communications between customers mixed with a liquidity supplier relayer community for the supported tokens. Liquidity Community
O3 Swap The O3 cross-chain mechanisms of Swap and Bridge aggregating a number of liquidity swimming pools throughout chains enable for easy one-time affirmation transactions with a deliberate Fuel Station fixing the gasoline price demand on every chain. Liquidity Pool Aggregator
Orbiter A decentralized cross-rollup bridge for transferring Ethereum-native belongings. The system has two roles: Sender and Maker. The ‘Maker’ is required to deposit extra margin to Orbiter’s contract earlier than they’ll qualify to be a cross-rollup service supplier to the ‘Sender’. Within the regular course of, the ‘Sender’ sends belongings to the ‘Maker’ on the ‘Supply Community’, and the ‘Maker’ sends them again to the ‘Sender’ on the ‘Vacation spot Community’. Liquidity Community
Poly Community Permits customers to switch belongings between totally different blockchains utilizing a Lock-Mint swap. It makes use of a PolyNetwork chain to confirm and coordinate message passing between Relayers on supported chains. Every chain has a set of Relayers, whereas the PolyNetwork chain has a set of Keepers that signal cross-chain messages. Chains built-in with Poly Bridge have to help gentle consumer verification since validation of cross-chain messages contains verifying block headers and transactions through Merkle proofs. A few of the sensible contracts utilized by the bridge infrastructure aren’t verified on Etherscan. Token Bridge
Voyager (Router Protocol) The Router Protocol makes use of a pathfinder algorithm to seek out essentially the most optimum route to maneuver belongings from the supply chain to the vacation spot chain using the Router community which has similarities to Cosmos’ IBC. Liquidity Community
Umbria Community Umbria has three main protocols working collectively:

  • A Cross-chain Asset Bridge; enabling the switch of belongings between in any other case incompatible blockchains and cryptocurrency networks.
  • A Staking Pool, the place customers can earn curiosity on their crypto-assets by offering liquidity to the bridge. Liquidity suppliers of UMBR earn 60% of all charges generated by the bridge.
  • A Decentralised Trade (DEX); and automatic liquidity protocol powered by a continuing product formulation, deployed utilizing sensible contracts, ruled totally on-chain.

Each protocols work in tandem to offer asset migration between cryptocurrency networks

Liquidity Community (utilizing an Automated Market Maker)
Through Protocol The protocol is an aggregator of chains, DEXs, and bridges to optimize asset switch routes. This permits asset bridging in 3 ways:

  • Make a number of transactions on totally different blockchains
  • Make one transaction by way of decentralized bridges which have built-in DEXs
  • Make one transaction by way of semi-centralized bridges, which is able to set off a second transaction on the goal chain
Hybrid (Token Bridge/Liquidity Community)
Multichain Multichain is an externally validated bridge. It makes use of a community of nodes working SMPC (Safe Multi-Celebration Computation) protocol. It helps dozens of blockchains and hundreds of tokens with each Token Bridge and Liquidity Community. Hybrid (Token Bridge/Liquidity Community
Orbit Bridge Orbit Bridge is a part of the Orbit Chain mission. It’s a cross-chain bridge that permits customers to switch tokens between supported blockchains. Tokens are deposited on the supply chain and “illustration tokens” are minted on the vacation spot chain. Deposited tokens aren’t exactly locked and can be utilized in DeFi protocols by Orbit Farm. Accrued curiosity shouldn’t be handed on to token depositors. Bridge contract implementation and farm contract supply code aren’t verified on Etherscan. Token Bridge
Portal (Wormhole) Portal Token Bridge is constructed on high of Wormhole, which is a message-passing protocol that leverages a specialised community of nodes to carry out cross-chain communication. Token Bridge
Satellite tv for pc (Axelar) Satellite tv for pc is a token bridge powered by the Axelar community Liquidity Community

The L2Beat mission maintains an lively listing of bridges related to L2s with the Complete-Worth-Locked (TVL) within the bridge in addition to an outline and temporary danger evaluation, if accessible.

L2 Bridges Danger Profiles

Lastly, when customers make the most of L2 Bridges, actually, any bridge, care must be taken, and the next dangers should be evaluated for a given bridge:

Lack of Funds

  • Oracles, relayers, or validators collude to submit fraudulent proofs (e,g, block hash, block header, Merkle proof, Fraud proof, Validity proof) and/or relay fraudulent transfers that aren’t mitigated
  • Validator/Relayer non-public keys are compromised 
  • Validators maliciously mint new tokens
  • False claims aren’t disputed in time (optimistic messaging protocols)
  • A vacation spot blockchain reorganization happens after optimistic oracle/relayer dispute time passes (optimistic messaging protocols).
  • Supply code of unverified contracts concerned in or utilized by a protocol accommodates malicious code or performance that may be abused by a contract proprietor/administrator
  • Token Bridge house owners behave maliciously, or provoke time-sensitive emergency actions that impression consumer funds, and don’t correctly talk to the consumer base
  • Protocol contract(s) paused (if performance exists)
  • Protocol contract(s) obtain a malicious code replace

Freezing of Funds

  • Relayers/Liquidity Suppliers don’t act on consumer transactions (messages)
  • Protocol contract(s) paused (if performance exists)
  • Protocol contract(s) obtain a malicious code replace
  • Inadequate liquidity within the goal token on the bridge

Censoring Customers

  • Oracles or relayers on both vacation spot or goal L2s or each fail to facilitate a switch (message)
  • Protocol contract(s) paused (if performance exists)

Whereas this listing shouldn’t be exhaustive, it offers a great overview of the present dangers related in utilizing bridges.

There are new developments underway utilizing zero-knowledge-proof (zkp) applied sciences designed to mitigate a number of the above danger elements and tackle the 2 bridge trilemmas. Specifically, using zkps permits for the next bridge design traits:

  • Trustless and Safe as a result of the correctness of block headers on the supply and goal blockchains may be confirmed by zk-SNARKs that are verifiable on EVM-compatible blockchains. Therefore, no exterior belief assumptions are required, assuming the supply and goal blockchains and the utilized light-client protocols are safe and we’ve 1-of-N trustworthy nodes within the relay community.
  • Permissionless and Decentralized as a result of anybody can be part of the bridges’ relay community, and PoS-style or related validation schemes aren’t wanted 
  • Extensible as a result of functions can retrieve zkp-verified block headers, and execute application-specific verification and performance 
  • Environment friendly due to new, optimized proof schemes with brief proof era and quick proof verification occasions

Albeit early, these kind of developments promise to speed up the maturation and safety of the bridge ecosystem.

We are able to summarize the above dialogue and overview of L2 Bridges as follows:

  • L2 Bridges are an necessary glue of the L2 ecosystem to additional L2 interoperability and environment friendly use of belongings and functions throughout the ecosystem. 
  • L2 bridges used on L2s anchored on the identical L1, equivalent to Ethereum Mainnet, are safer than bridges between L1s – assuming the supply code is secure, which is commonly a giant if.
  • As with all distributed system architectures, there are vital tradeoffs to be made, as expressed within the two posited Trilemmas – Bridging Trilemma and Interoperability Trilemma.
  • L2 Bridges have very totally different belief assumptions, e.g., trusted vs. trustless bridges, and really totally different design decisions, e.g., lock-mint-burn vs. liquidity networks.  
  • The L2 Bridges ecosystem continues to be nascent and in a state of flux.
  • Customers are suggested to do their due diligence to evaluate which L2 bridges supply the most effective risk-reward profile for his or her wants.
  • There are new developments underway utilizing current zkp-technologies which can be successfully addressing the 2 bridge trilemmas, and assist to extend the safety of bridges total.

Whereas nonetheless early within the journey in direction of a standardized L2 interoperability framework, these are necessary developments, and should be taken critically as any a kind of tasks may turn into “THE” bridge framework – it’s not but VHS vs Betamax, however we’re getting there.

The L2 WG want to gratefully acknowledge Tas Dienes (Enterprise Basis), Daniel Goldman (Offchain Labs), Bartek Kiepuszewski (L2Beat) for a cautious studying of the manuscript and invaluable content material solutions.


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