Flow EVM Bridge

This repo contains contracts enabling bridging of fungible & non-fungible tokens between Cadence and EVM on Flow.

Deployments

The bridge contracts in this repo are deployed to the following addresses:

And below are the bridge escrow's EVM addresses. These addresses are CadenceOwnedAccounts (COA) and they are stored stored in the same Flow account as you'll find the Cadence contracts (see above).

Interacting with the bridge

:information_source: All bridging activity in either direction is orchestrated via Cadence on CadenceOwnedAccount (COA) resources. This means that all bridging activity must be initiated via a Cadence transaction, not an EVM transaction regardless of the directionality of the bridge request. For more information on the interplay between Cadence and EVM, see EVM Integration FLIP #223

Overview

The Flow EVM bridge allows both fungible and non-fungible tokens to move atomically between Cadence and EVM. In the context of EVM, fungible tokens are defined as ERC20 tokens, and non-fungible tokens as ERC721 tokens. In Cadence, fungible tokens are defined by contracts implementing FungibleToken and non-fungible tokens the NonFungibleToken standard contract interfaces.

Like all operations on Flow, there are native fees associated with both computation and storage. To prevent spam and sustain the bridge account's storage consumption, fees are charged for both onboarding assets and bridging assets. In the case where storage consumption is expected, fees are charges based on the storage consumed at the current network rates. In all cases, there is a flat-rate fee in addition to any storage fees.

Onboarding

Since a contract must define the asset in the target VM, an asset must be "onboarded" to the bridge before requests can be fulfilled. Moving from Cadence to EVM, onboarding can occur on the fly, deploying a template contract in the same transaction as the asset is bridged to EVM if the transaction so specifies. Moving from EVM to Cadence, however, requires that onboarding occur in a separate transaction due to the fact that a Cadence contract is initialized at the end of a transaction and isn't available in the runtime until after the transaction has executed.

Below are transactions relevant to onboarding assets:

• onboard_by_type.cdc
• onboard_by_evm_address.cdc

Bridging

Once an asset has been onboarded, either by its Cadence type or EVM contract address, it can be bridged in either direction referred to by its Cadence type. For Cadence-native assets, this is simply its native type. For EVM-native assets, this is in most cases a templated Cadence contract deployed to the bridge account, the name of which is derived from the EVM contract address. For instance, an ERC721 contract at address 0x1234 would be onboarded to the bridge as EVMVMBridgedNFT_0x1234, making its type identifier A.<BRIDGE_ADDRESS>.EVMVMBridgedNFT_0x1234.NFT.

However, the derivation of these identifiers can be abstracted within transactions. For example, calling applications can provide the defining contract address and name of the bridged asset (see bridge_nft_to_evm.cdc). Alternatively, the defining EVM contract could be provided, etc - this flexibility is thanks to Cadence's scripted transactions.

NFTs

Any Cadence NFTs bridging to EVM are escrowed in the bridge account and either minted in a bridge-deployed ERC721 contract or transferred from escrow to the calling COA in EVM. On the return trip, NFTs are escrowed in EVM - owned by the bridge's COA - and either unlocked from escrow if locked or minted from a bridge-owned NFT contract.

Below are transactions relevant to bridging NFTs:

• bridge_nft_to_evm.cdc
• bridge_nft_from_evm.cdc

Fungible Tokens

Any Cadence fungible tokens bridging to EVM are escrowed in the bridge account only if they are Cadence-native. If the bridge defines the tokens, they are burned. On the return trip the pattern is similar, with the bridge burning bridge-defined tokens or escrowing them if they are EVM-native. In all cases, if the bridge has authority to mint on one side, it must escrow on the other as the native VM contract is owned by an external party.

With fungible tokens in particular, there may be some cases where the Cadence contract is not deployed to the bridge account, but the bridge still follows a mint/burn pattern in Cadence. These cases are handled via TokenHandler implementations. Also know that moving $FLOW to EVM is built into the EVMAddress object so any requests bridging $FLOW to EVM will simply leverage this interface; however, moving $FLOW from EVM to Cadence must be done through the COA resource.

Below are transactions relevant to bridging fungible tokens:

• bridge_tokens_to_evm.cdc
• bridge_tokens_from_evm.cdc

Prep Your Assets for Bridging

Context

To maximize utility to the ecosystem, this bridge is permissionless and open to any fungible or non-fungible token as defined by the respective Cadence standards and limited to ERC20 and ERC721 Solidity standards. Ultimately, a project does not have to do anything for users to be able to bridge their assets between VMs. However, there are some considerations developers may take to enhance the representation of their assets in non-native VMs. These largely relate to asset metadata and ensuring that bridging does not compromise critical user assumptions about asset ownership.

EVMBridgedMetadata

Proposed in @onflow/flow-nft/pull/203, the EVMBridgedMetadata view presents a mechanism to both represent metadata from bridged EVM assets as well as enable Cadence-native projects to specify the representation of their assets in EVM. Implementing this view is not required for assets to be bridged, but the bridge does default to it when available as a way to provide projects greater control over their EVM asset definitions within the scope of ERC20 and ERC721 standards.

The interface for this view is as follows:

access(all) struct URI: MetadataViews.File {
    /// The base URI prefix, if any. Not needed for all URIs, but helpful
    /// for some use cases For example, updating a whole NFT collection's
    /// image host easily
    access(all) let baseURI: String?
    /// The URI string value
    /// NOTE: this is set on init as a concatenation of the baseURI and the
    /// value if baseURI != nil
    access(self) let value: String

    access(all) view fun uri(): String

}

access(all) struct EVMBridgedMetadata {
    access(all) let name: String
    access(all) let symbol: String

    access(all) let uri: {MetadataViews.File}
}

This uri value could be a pointer to some offchain metadata if you expect your metadata to be static. Or you could couple the uri() method with the utility contract below to serialize the onchain metadata on the fly.

SerializeMetadata

The key consideration with respect to metadata is the distinct metadata storage patterns between ecosystem. It's critical for NFT utility that the metadata be bridged in addition to the representation of the NFTs ownership. However, it's commonplace for Cadence NFTs to store metadata onchain while EVM NFTs often store an onchain pointer to metadata stored offchain. In order for Cadence NFTs to be properly represented in EVM platforms, the metadata must be bridged in a format expected by those platforms and be done in a manner that also preserves the atomicity of bridge requests. The path forward on this was decided to be a commitment of serialized Cadence NFT metadata into formats popular in the EVM ecosystem.

For assets that do not implement EVMBridgedMetadata, the bridge will attempt to serialize the metadata of the asset as a JSON data URL string. This is done via the SerializeMetadata contract which serializes metadata values into a JSON blob compatible with the OpenSea metadata standard. The serialized metadata is then committed as the ERC721 tokenURI upon bridging Cadence-native NFTs to EVM. Since Cadence NFTs can easily update onchain metadata either by field or by the ownership of sub-NFTs, this serialization pattern enables token URI updates on subsequent bridge requests.

Opting Out

It's also recognized that the logic of some use cases may actually be compromised by the act of bridging, particularly in such a unique runtime environment. These would be cases that do not maintain ownership assumptions implicit to ecosystem standards. For instance, an ERC721 implementation may reclaim a user's assets after a month of inactivity time period. In such a case, bridging that ERC721 to Cadence would decouple the representation of ownership of the bridged NFT from the actual ownership in the definining ERC721 contract after the token had been reclaimed - there would be no NFT in escrow for the bridge to transfer on fulfillment of the NFT back to EVM. In such cases, projects may choose to opt-out of bridging, but importantly must do so before the asset has been onboarded to the bridge.

For Solidity contracts, opting out is as simple as extending the BridgePermissions.sol abstract contract which defaults allowsBridging() to false. The bridge explicitly checks for the implementation of IBridgePermissions and the value of allowsBridging() to validate that the contract has not opted out of bridging.

Similarly, Cadence contracts can implement the IBridgePermissions.cdc contract interface. This contract has a single method allowsBridging() with a default implementation returning false. Again, the bridge explicitly checks for the implementation of IBridgePermissions and the value of allowsBridging() to validate that the contract has not opted out of bridging. Should you later choose to enable bridging, you can simply override the default implementation and return true.

In both cases, allowsBridging() gates onboarding to the bridge. Once onboarded - a permissionless operation anyone can execute - the value of allowsBridging() is irrelevant and assets can move between VMs permissionlessly.

Under the Hood

For an in-depth look at the high-level architecture of the bridge, see [FLIP

237](https://github.com/onflow/flips/blob/main/application/20231222-evm-vm-bridge.md)

Local Development

The contracts in this repo are not yet included in the Flow emulator. For local development against the bridge, follow the steps below to stand up a local Flow emulator instance and deploy the bridge contracts:

Prerequisites

• Install Flow CLI on your machine. For instructions, see the Flow CLI documentation.
• Download and install Go. For instructions, see the Go documentation.

Ensure both are installed with:

flow version

and go with:

go version

Start your local emulator

Start the Flow emulator with the following command:

flow emulator

Run the deployment script

In a separate terminal window, run the deployment script to deploy the bridge contracts to your local emulator:

go run main.go

If all is successful, you should see a long flow of event and transaction logs in your terminal with a final line resulting in:

SETUP COMPLETE! Bridge is now unpaused and ready for use.

Interact with the bridge

You're now ready to interact with the bridge!

Additional Resources

For the current state of Flow EVM across various task paths, see the following resources:

• Flow EVM Equivalence forum post
• EVM Integration FLIP #223
• Gateway & JSON RPC FLIP #235