EVM IBC hooks

Overview

EVM hooks, implemented as IBC middleware, play a critical role in facilitating cross-chain contract calls that involve token transfers. This capability is particularly crucial for cross-chain swaps, providing a robust mechanism for decentralized trading across different blockchain networks. The key to this functionality is the memo field in the ICS20 and ICS721 transfer packets, as introduced in IBC v3.4.0.

EVM Contract Execution Format

Before we dive into the IBC metadata format, let's take a look at the hook data format and address which fields we need to be setting. The EVM MsgCall is defined here and other types are defined here.

// HookData defines a wrapper for evm execute message
// and async callback.
type HookData struct {
 // Message is a evm execute message which will be executed
 // at `OnRecvPacket` of receiver chain.
 Message evmtypes.MsgCall `json:"message"`

 // AsyncCallback is a callback message which will be executed
 // at `OnTimeoutPacket` and `OnAcknowledgementPacket` of
 // sender chain.
 AsyncCallback *AsyncCallback `json:"async_callback,omitempty"`
}

// AsyncCallback is data wrapper which is required
// when we implement async callback.
type AsyncCallback struct {
 // callback id should be issued form the executor contract
 Id            uint64 `json:"id"`
 ContractAddr  string `json:"contract_addr"`
}

// MsgCall is a message to call an Ethereum contract.
type MsgCall struct {
 // Sender is the that actor that signed the messages
 Sender string `protobuf:"bytes,1,opt,name=sender,proto3" json:"sender,omitempty"`
 // ContractAddr is the contract address to be executed.
 // It can be cosmos address or hex encoded address.
 ContractAddr string `protobuf:"bytes,2,opt,name=contract_addr,json=contractAddr,proto3" json:"contract_addr,omitempty"`
 // Hex encoded execution input bytes.
 Input string `protobuf:"bytes,3,opt,name=input,proto3" json:"input,omitempty"`
}

So we detail where we want to get each of these fields from:

• Sender: We cannot trust the sender of an IBC packet, the counter-party chain has full ability to lie about it. We cannot risk this sender being confused for a particular user or module address on Initia. So we replace the sender with an account to represent the sender prefixed by the channel and a evm module prefix. This is done by setting the sender to Bech32(Hash(Hash("ibc-evm-hook-intermediary") + channelID/sender)), where the channelId is the channel id on the local chain.

• ContractAddr: This field should be directly obtained from the ICS-20 packet metadata

• Input: This field should be directly obtained from the ICS-20 packet metadata.

So our constructed evm call message that we execute will look like:

msg := MsgCall{
 // Sender is the that actor that signed the messages
 Sender: "init1-hash-of-channel-and-sender",
 // ContractAddr is the contract address to be executed.
 // It can be cosmos address or hex encoded address.
 ContractAddr: packet.data.memo["evm"]["message"]["contract_addr"],
 // Hex encoded execution input bytes.
 Input: packet.data.memo["evm"]["message"]["input"],
}

ICS20 packet structure

So given the details above, we propogate the implied ICS20 packet data structure. ICS20 is JSON native, so we use JSON for the memo format.

{
  //... other ibc fields that we don't care about
  "data": {
    "denom": "denom on counterparty chain (e.g. uatom)", // will be transformed to the local denom (ibc/...)
    "amount": "1000",
    "sender": "addr on counterparty chain", // will be transformed
    "receiver": "ModuleAddr::ModuleName::FunctionName",
    "memo": {
      "evm": {
        // execute message on receive packet
        "message": {
          "contract_addr": "0x1",
          "input": "hex encoded byte string",
        },
        // optional field to get async callback (ack and timeout)
        "async_callback": {
          "id": 1,
          "contract_addr": "0x1"
        }
      }
    }
  }
}

An ICS20 packet is formatted correctly for evmhooks iff the following all hold:

• memo is not blank

• memo is valid JSON

• memo has at least one key, with value "evm"

• memo["evm"]["message"] has exactly five entries, "contract_addr" and "input"

• receiver == "" || receiver == "module_address::module_name::function_name"

We consider an ICS20 packet as directed towards evmhooks iff all of the following hold:

• memo is not blank

• memo is valid JSON

• memo has at least one key, with name "evm"

If an ICS20 packet is not directed towards evmhooks, evmhooks doesn't do anything. If an ICS20 packet is directed towards evmhooks, and is formatted incorrectly, then evmhooks returns an error.

Execution flow

Pre evm hooks:

• Ensure the incoming IBC packet is cryptogaphically valid

• Ensure the incoming IBC packet is not timed out.

In evm hooks, pre packet execution:

• Ensure the packet is correctly formatted (as defined above)

• Edit the receiver to be the hardcoded IBC module account

In evm hooks, post packet execution:

• Construct evm message as defined before

• Execute evm message

• if evm message has error, return ErrAck

• otherwise continue through middleware

Async Callback

A contract that sends an IBC transfer, may need to listen for the ACK from that packet. To allow contracts to listen on the ack of specific packets, we provide Ack callbacks. The contract, which wants to receive ack callback, have to implement two functions.

• ibc_ack

• ibc_timeout

interface IIBCAsyncCallback {
    function ibc_ack(uint64 callback_id, bool success) external;
    function ibc_timeout(uint64 callback_id) external;
}

Also when a contract make IBC transfer request, it should provide async callback data through memo field.

• memo['evm']['async_callback']['id']: the async callback id is assigned from the contract. so later it will be passed as argument of ibc_ack and ibc_timeout.

• memo['evm']['async_callback']['contract_addr']: The address of module which defines the callback function.

Conclusion

The EVM hook represents an advancement in interoperability, enabling seamless cross-chain functionalities and enhancing the utility of token transfers. By leveraging the EVM hook, developers can initiate complex contract interactions across different chains, fostering new use cases and applications. This development not only simplifies the technical barriers previously associated with cross-chain communications but also paves the way for more integrated and efficient blockchain ecosystems. As this technology matures, it will undoubtedly unlock further potential for decentralized applications, contributing to the broader adoption and operational effectiveness of blockchain technologies.