Across V3 – L2BEAT

Website	across.to
App	across.to
Repository	github.com/across-protocol/contracts-v2
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Detailed description

Across V3 is a cross-chain optimistic bridge that uses actors called Relayers to fulfill user transfer requests on the destination chain.

Relayers are later reimbursed by providing a proof of their action to an Optimistic Oracle on Ethereum. Relayer reimbursements over a specific block range are bundled and posted on-chain as merkle roots which uniquely identify the set of all repayments and rebalance instructions. The architecture leverages a single liquidity pool on Ethereum and separate deposit/reimburse pools on destination chains that are rebalanced using canonical bridges.

Risk summary

Principle of operation

This bridge performs cross-chain swaps by borrowing liquidity from a network of Relayers who are then reimbursed on a chain of their choosing from a common liquidity pool (which consists of user deposits and deposits of independent Liquidity Providers). Specifically, when a user deposits funds for a swap into a dedicated pool on origin chain, a Relayer first pays the user on the requested destination chain and then shows proof of that deposit to Optimistic Oracle on Ethereum to be reimbursed. Liquidity used for reimbursements is rebalanced between a main pool on Ethereum (called Hub Pool) and pools on destination chains (called Spoke Pools) via native chain bridges.

Funds can be frozen if owner pauses the Hub Pool contract, or changes bond, routes, or fees parameters in such a way as to make the escrow inoperable.
Funds can be lost if owner invokes a "haircut" functionality, dedicated for irrecoverable loss of funds on L2. Calling the haircutReserves() function, the owner can decrease the token utilizedReserves on L1, decreasing the amount of funds in the bridge expected to flow from L2 to L1.
Funds can be lost if third-party bridge infrastructure is compromised, such as canonical messaging services, Linea USDC bridge, and USDC Cross-Chain Transfer Protocol (CCTP) infrastructure.

Across V3 Architecture

Validation via Optimistic Oracle

Money from the liquidity pool is used to reimburse Relayers based on a proof of deposit on a destination chain that is provided to an Optimistic Oracle on Ethereum. The proof can be disputed in a configured time period.

Funds can be stolen if a false claim to the Optimistic Oracle is not disputed in time.
Funds can be lost if a re-org occurs on destination chain after the Optimistic Oracle dispute time passes.

Across V3 Optimistic Oracle documentation

Destination tokens

Only tokens that have been bridged using native chain bridges are supported.

Permissions

The system uses the following set of permissioned addresses:

HubPool Multisig 0xB524…3715

Can invoke admin functions of HubPool contract, and by implication of other contracts. This is a Gnosis Safe with 3 / 5 threshold.

HubPool Multisig participants 0x1d93…9F05 0x8372…c5fe 0x9962…65e9 0xcc40…e37d 0x868C…FD34

Those are the participants of the HubPool Multisig.

BondToken transfer proposers 0xf7bA…997c

Allowed to propose BondToken transfers.

Smart contracts

The system consists of the following smart contracts:

HubPool 0xc186…bEda

Escrow contract for ERC20 tokens and administration of other contracts. There is a 1h delay before a bundle proposal is considered finalized. This contract stores the following tokens: USDC, WETH, WBTC, DAI, BAL, UMA, BOBA, USDT, ACX.

BondToken 0xee1D…02ea

Token (ABT) used to bond the data worker for proposing Relayer refund bundles. It also used as a required bond to dispute a bundle proposal. Currently, the bond amount is set to 0.45 ABT.

UMAOptimisticOracle 0xeE3A…7c24

UMA Optimistic Oracle smart contract. It registers dispute requests, status of disputes, and dispute settlement.

GovernorV2 0x7b29…3fa8

Owner of the Optimistic Oracle. This contract is used to execute a proposed UMA governance action that has been approved by UMA token holders.

VotingToken 0x04Fa…F828

Token used to vote on UMA Optimistic Oracle governance proposals.

LpTokenFactory 0x7dB6…fD9d

Factory to deploy new LP tokens for L1 tokens, used to represent a liquidity provider position in the HubPool.

Finder 0x40f9…77c3

Provides addresses of the live contracts implementing certain interfaces, such as the whitelist interface for setting the Bond Token.

Arbitrum_Adapter 0x2952…543F

Base_Adapter 0x2d8B…046E

Boba_Adapter 0x33B0…5Af3

Ethereum_Adapter 0x527E…5084

Linea_Adapter 0x7Ea0…8EfE

Optimism_Adapter 0xAd1b…246F

Polygon_Adapter 0x3E94…CE1A

ZkSync_Adapter 0xE233…47FB

Ethereum_SpokePool 0x5c7B…35C5 Implementation (Upgradable)Admin

Contract enabling depositors to transfer assets from Ethereum to L2s, and relayers to fulfill transfer from L2s to Ethereum. Deposit orders are fulfilled by off-chain relayers with the fillV3Relay() function. Relayers are later refunded with destination token out of this contract when the data worker submits a proof that the relayer correctly submitted a relay on this SpokePool. This contract stores the following tokens: USDC, WETH, WBTC, DAI, BAL, UMA, BOBA, USDT, ACX.

PolygonTokenBridger 0x48d9…2c65

Contract deployed on Ethereum and Polygon PoS to facilitate token transfers from Polygon to the HubPool.

AcrossConfigStore 0x3B03…43f5

Contract storing configurations such as token rate models and minimum transfer thresholds, meant for off-chain consumption.

Value Locked is calculated based on these smart contracts and tokens:

Escrow for USDC, WETH, WBTC, DAI, BAL, UMA, BOBA, USDT, ACX 0xc186…bEda

Escrow for USDC, WETH, WBTC, DAI, BAL, UMA, BOBA, USDT, ACX 0x5c7B…35C5 Implementation (Upgradable)Admin

The current deployment carries some associated risks:

Funds can be stolen if a Spoke Pool contract receives a malicious code upgrade. There is no delay on code upgrades (CRITICAL).

Knowledge Nuggets

Across deep dive

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Choose token

Funds can be stolen if