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The Present State of Layer 2 Bridges

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The Present State of Layer 2 Bridges

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

We stay in a multi-chain world, with billions of USD in asset worth locked in 100+ chains. And the homeowners of these blockchain property behave identical to they might with property in conventional finance: they’re searching for arbitrage alternatives to become profitable. Nonetheless, in distinction to the world of conventional finance the place property in a single nation will be utilized in arbitrage performs abroad with out transferring property through the use of trusted intermediaries, the identical strategy didn’t work for blockchains for a very long time for 3 causes:

  1. blockchains can’t speak to at least one one other,
  2. arbitrage performs on a selected blockchain require that every one concerned property 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 resolve the issue of capital inefficiency on blockchains, and become profitable 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. After all, bridges can be utilized for different kinds of performance too; nevertheless, 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 the switch of knowledge and/or property.

This permits for a large number of alternatives reminiscent of

  • cross-chain switch of property,
  • new decentralized purposes (dApps), and platforms that permit customers to entry the strengths of assorted blockchains – thus enhancing their capabilities,
  • and builders from completely different blockchain ecosystems can collaborate and construct new options.

There are two fundamental kinds of bridges:

Trusted Bridges Trustless Bridges
Rely upon a central entity or system for his or her operations. Function utilizing decentralized programs reminiscent of good contracts with embedded algorithms.
Belief assumptions regarding funding custody and bridge safety. Customers largely depend on the bridge operator’s repute. The safety of the bridge is similar as that of the underlying blockchain.
Customers want to surrender management of their crypto property. Allow customers to stay answerable for their funds by good contracts.

Inside each units of belief assumptions one can distinguish completely different, widespread kinds of bridge designs:

  • Lock, mint, and burn token bridges: Prompt assured finality as minting property on the vacation spot blockchain can happen each time required with out the opportunity of a failed transaction. Customers obtain an artificial, usually referred to as a wrapped asset, on the vacation spot blockchain, not the native asset.
  • Liquidity networks with swimming pools of native property with unified liquidity: A single asset pool on one blockchain is related with extra asset swimming pools on different blockchains with shared entry to at least one one other’s liquidity. This strategy doesn’t allow instantaneous, assured finality since transactions can fail if there’s a lack of liquidity within the shared swimming pools.

Nonetheless, all designs, and below no matter belief assumptions, have to handle 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:

  • Prompt Assured Finality: Assure to obtain property 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 property between supply and goal blockchains.
  • Native Property: Obtain goal blockchain property as an alternative of property 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: Capacity to attach completely different blockchains.
  • Generalizability: Permits for arbitrary knowledge messaging

Apart from the trilemmas which will 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 essentially, a bridge between blockchains is barely as safe because the least safe blockchain used within the (chain of) bridge(s) for an asset. Nonetheless, this latter concern just isn’t an issue for bridges between Layer 2 platforms which might 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 now have not particularly talked about L2 platforms which might 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, nevertheless, 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 completely different L2s.

The rationale why bridges between L2s are necessary is similar as for L1s: L2 property are searching for capital effectivity on different L2s, in addition to portability and different functionalities.

The distinction in native belief assumptions on L2 platforms will be overcome if bridged L2s are, as remarked already, anchored on the identical L1. And that the bridge doesn’t require extra belief assumptions. Nonetheless, variations in L2 transaction finality on the anchor L1 make it difficult to bridge property 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 should be abstracted away from every L2 protocol they interface with by an abstraction layer – loose-coupling paradigm.
  • Purchasers should be capable to confirm that the information 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 diverse panorama of L2 bridges with a reputation, temporary abstract, and bridge design sort:

Bridge Title Description Sort of Design
Hop Trade Rollup-to-rollup basic 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.trade/whitepaper.pdf Liquidity Community (utilizing an Automated Market Maker)
Stargate Composable native asset bridge, and dApp constructed on LayerZero. DeFi customers can swap native property cross-chain on Stargate inside a single transaction. Purposes 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. https://www.dropbox.com/s/gf3606jedromp61/Delta-Fixing.The.Bridging-Trilemma.pdf?dl=0 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 referred to 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 might be rebalanced utilizing canonical bridges. Liquidity Community
Beamer Allows 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 straightforward to make use of as doable for the tip consumer. That is achieved by separating two completely different issues: the service supplied to the tip consumer, and the reclaiming of funds by the liquidity supplier. The service is supplied 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 good contract-based wallets for customers to work together with liquidity suppliers to switch tokens between completely 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 important part. The SLL aggregates liquidity throughout a number of bridges and DEXs and likewise permits for P2P settlements. That is completely different from a Liquidity Pool community since this single meta-bridge permits dynamical choice and routing of funds through one of the best bridge as per a consumer’s preferences reminiscent of 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 prime 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 completely different blockchains. Liquidity Community
Connext Dispatches and handles messages associated to sending funds throughout chains. Custodies funds for canonical property, quick liquidity, and steady swaps. The Connext contract makes use of the Diamond sample so it includes a set of Aspects that act as logical boundaries for teams of capabilities. Aspects share contract storage and will be upgraded individually. Hybrid (Token Bridge/Liquidity Community)
Elk Finance Makes use of ElkNet with options reminiscent of 

  • Cross-chain utility token for worth switch ($ELK)
  • Secure 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 related blockchains
  • Impermanent Loss Safety (ILP) for our liquidity suppliers
  • Non-Fungible Tokens (Moose NFTs) with distinctive skills 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 out there on the API/Contract stage by an SDK, or as an embeddable widget in a dApp  Liquidity Pool Aggregator
LayerSwap Bridge for tokens from a centralized trade 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 permit for easy one-time affirmation transactions with a deliberate Fuel Station fixing the fuel payment demand on every chain. Liquidity Pool Aggregator
Orbiter A decentralized cross-rollup bridge for transferring Ethereum-native property. 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 common course of, the ‘Sender’ sends property 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 property between completely 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 consists of verifying block headers and transactions through Merkle proofs. A number of the good contracts utilized by the bridge infrastructure will not be verified on Etherscan. Token Bridge
Voyager (Router Protocol) The Router Protocol makes use of a pathfinder algorithm to seek out probably the most optimum route to maneuver property from the supply chain to the vacation spot chain using the Router community which is analogous to Cosmos’ IBC. Liquidity Community
Umbria Community Umbria has three main protocols working collectively:

  • A Cross-chain Asset Bridge; enabling the switch of property 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 good contracts, ruled totally on-chain.

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

Liquidity Community (utilizing an Automated Market Maker)
By way of 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 completely different blockchains
  • Make one transaction by decentralized bridges which have built-in DEXs
  • Make one transaction by 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-Get together 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 challenge. It’s a cross-chain bridge that enables 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 will not be exactly locked and can be utilized in DeFi protocols by Orbit Farm. Accrued curiosity just isn’t handed on to token depositors. Bridge contract implementation and farm contract supply code will not be verified on Etherscan. Token Bridge
Portal (Wormhole) Portal Token Bridge is constructed on prime 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 challenge maintains an lively checklist of bridges related to L2s with the Whole-Worth-Locked (TVL) within the bridge in addition to an outline and temporary threat evaluation, if out there.

L2 Bridges Danger Profiles

Lastly, when customers make the most of L2 Bridges, actually, any bridge, care must be taken, and the next dangers must 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 will not be 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 comprises malicious code or performance that may be abused by a contract proprietor/administrator
  • Token Bridge homeowners 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 checklist just isn’t exhaustive, it offers an excellent 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 threat components and tackle the 2 bridge trilemmas. Specifically, the usage of 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 will 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 now have 1-of-N sincere 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 comparable validation schemes will not be wanted 
  • Extensible as a result of purposes can retrieve zkp-verified block headers, and execute application-specific verification and performance 
  • Environment friendly due to new, optimized proof schemes with quick proof technology and quick proof verification instances

Albeit early, all these 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 property and purposes throughout the ecosystem. 
  • L2 bridges used on L2s anchored on the identical L1, reminiscent of Ethereum Mainnet, are safer than bridges between L1s – assuming the supply code is protected, which is commonly a giant if.
  • As with all distributed system architectures, there are important tradeoffs to be made, as expressed within the two posited Trilemmas – Bridging Trilemma and Interoperability Trilemma.
  • L2 Bridges have very completely different belief assumptions, e.g., trusted vs. trustless bridges, and really completely different design selections, 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 one of the best risk-reward profile for his or her wants.
  • There are new developments underway utilizing latest zkp-technologies which might be successfully addressing the 2 bridge trilemmas, and assist to extend the safety of bridges total.

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

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

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