There is broad-based demand for standardization of cross-chain transfers of ERC-20 tokens. (See for example this proposal from Optimism.) We at Axelar are seeking feedback on a proposal targetting the following goals:
- Simplicity and flexibility.
- This standard should be abstract enough to facilitate token transfer to both EVM and non-EVM chains.
- The caller doesn’t need to know how cross-chain transfers are implemented.
The idea is to add a transferXChain method that behaves like a conventional transfer except it also accepts a destChain argument. Similarly, we add allowanceXChain, approveXChain, transferFromXChain methods and accompanying events TransferXChain, ApprovalXChain. Taking inspiration from ERC-223, we also add a generic auxiliaryData argument to these new methods so as to facilitate additional features at the discretion of the implementer.
With these items in mind, the new interface might look something like:
interface IERC20XChain is IERC20 {
function transferXChain(
string recipient,
uint256 amount,
string recipientChain, // new
bytes auxiliaryData // new
) external returns (bool);
function approveXChain(
string spender,
uint256 amount,
string spenderChain, // new
) external returns (bool);
function allowanceXChain(
address owner,
string spender,
string spenderChain, // new
) external view returns (uint256);
function transferFromXChain(
string sender,
string recipient,
uint256 amount,
string recipientChain, // new
string senderChain, // new
bytes auxiliaryData, // new
) external returns (bool);
event TransferXChain(
string indexed from,
string indexed to,
uint256 value,
string toChain, // new
string fromChain, // new
bytes auxiliaryData // new
);
event ApprovalXChain(
address indexed owner,
string indexed spender,
uint256 value,
string spenderChain, // new
);
}
Notes
- The implementer has complete flexibility in how to interpret
recipientChainand how to execute the cross-chain transfer. For example, the implementer could use a cross-chain platform such as Axelar. - The
recipientChainisstringto afford maximum flexibility and to play nice with block explorers. We recommend a separate standard such as CAIP-2 on a universal convention for whichstringsequences correspond to which chains. - The
recipientis nowstringto afford maximum flexibility for the address format in the destination chain. (For most chains an address fits into abytes32but we do not wish to preclude chains with longer addresses.) -
recipientChainneed not be different from the current chain. Presumably, implementers would optimize their implementation for this special case so as not to spam cross-chain infrastructure. -
approveXChainandallowanceXChainhave a newspenderChainargument so that the spender’s address can reside on a different chain.- Example: Alice holds token T on chain A, approves Bob on chain B. Bob can post a tx to chain B that sends Alice’s tokens from chain A to Charlie on another chain C. Alice’s approval of Bob on chain B should be restricted only to chain B—Bob should not be able to use this approval to send tokens on Alice’s behalf from any chain other than B.
- It is the responsibility of the implementer to execute and enforce this behaviour.
- Possible uses of
auxiliaryData:- Define the unlocking condition for a token returning to its native chain.
- Execute functions elsewhere outside of this token transfer.
- Point to a specific on-chain mapping
chain string -> chain data.
Community call
We want your feedback! You’re invited to a community call on Wednesday, July 13, 2022 at 13:00–13:30 UTC to discuss this proposal. [zoom link]
Anyone who has read this far is welcome to attend. Explicit invitations for the following forum members who have expressed interest in this topic in the past: @maurelian @TransmissionsDev @Dzack23 @fredlacs @luzius @akolotov @frangio