Secure Protocol for Native Meta Transactions

Abstract

This EIP defines a contract-level protocol for Recipient contracts to accept meta-transactions through trusted Forwarder contracts. No protocol changes are made. Recipient contracts are sent the effective msg.sender (referred to as _msgSender()) and msg.data (referred to as _msgData()) by appending additional calldata.

Motivation

There is a growing interest in making it possible for Ethereum contracts to accept calls from externally owned accounts that do not have ETH to pay for gas. Solutions that allow for third parties to pay for gas costs are called meta transactions. For the purposes of this EIP, meta transactions are transactions that have been authorized by a Transaction Signer and relayed by an untrusted third party that pays for the gas (the Gas Relay).

Specification

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Definitions

Transaction Signer: Signs & sends transactions to a Gas Relay

Gas Relay: Receives signed requests off-chain from Transaction Signers and pays gas to turn it into a valid transaction that goes through a Trusted Forwarder

Trusted Forwarder: A contract trusted by the Recipient to correctly verify signatures and nonces before forwarding the request from Transaction Signers

Recipient: A contract that accepts meta-transactions through a Trusted Forwarder

Example Flow

Extracting The Transaction Signer address

The Trusted Forwarder is responsible for calling the Recipient contract and MUST append the address of the Transaction Signer (20 bytes of data) to the end of the call data.

For example :

(bool success, bytes memory returnData) = to.call.value(value)(abi.encodePacked(data, from));

The Recipient contract can then extract the Transaction Signer address by performing 3 operations:

1. Check that the Forwarder is trusted. How this is implemented is out of the scope of this proposal.
2. Extract the Transaction Signer address from the last 20 bytes of the call data and use that as the original sender of the transaction (instead of msg.sender)
3. If the msg.sender is not a trusted forwarder (or if the msg.data is shorter than 20 bytes), then return the original msg.sender as it is.

The Recipient MUST check that it trusts the Forwarder to prevent it from extracting address data appended from an untrusted contract. This could result in a forged address.

Protocol Support Discovery Mechanism

Unless a Recipient contract is being used by a particular frontend that knows that this contract has support for native meta transactions, it would not be possible to offer the user the choice of using meta-transaction to interact with the contract. We thus need a mechanism by which the Recipient can let the world know that it supports meta transactions.

This is especially important for meta transactions to be supported at the Web3 wallet level. Such wallets may not necessarily know anything about the Recipient contract users may wish to interact with.

As a Recipient could trust forwarders with different interfaces and capabilities (e.g., transaction batching, different message signing formats), we need to allow wallets to discover which Forwarder is trusted.

To provide this discovery mechanism a Recipient contract MUST implement this function:

function isTrustedForwarder(address forwarder) external view returns(bool);

isTrustedForwarder MUST return true if the forwarder is trusted by the Recipient, otherwise it MUST return false. isTrustedForwarder MUST NOT revert.

Internally, the Recipient MUST then accept a request from forwarder.

isTrustedForwarder function MAY be called on-chain, and as such gas restrictions MUST be put in place. It SHOULD NOT consume more than 50,000 gas

Rationale

• Make it easy for contract developers to add support for meta transactions by standardizing the simplest viable contract interface.
• Without support for meta transactions in the recipient contract, an externally owned account can not use meta transactions to interact with the recipient contract.
• Without a standard contract interface, there is no standard way for a client to discover whether a recipient supports meta transactions.
• Without a standard contract interface, there is no standard way to send a meta transaction to a recipient.
• Without the ability to leverage a trusted forwarder every recipient contract has to internally implement the logic required to accept meta transactions securely.
• Without a discovery protocol, there is no mechanism for a client to discover whether a recipient supports a specific forwarder.
• Making the contract interface agnostic to the internal implementation details of the trusted forwarder, makes it possible for a recipient contract to support multiple forwarders with no change to code.
• msg.sender is a transaction parameter that can be inspected by a contract to determine who signed the transaction. The integrity of this parameter is guaranteed by the Ethereum EVM, but for a meta transaction securing msg.sender is insufficient.
  • The problem is that for a contract that is not natively aware of meta transactions, the msg.sender of the transaction will make it appear to be coming from the Gas Relay and not the Transaction Signer. A secure protocol for a contract to accept meta transactions needs to prevent the Gas Relay from forging, modifying or duplicating requests by the Transaction Signer.

Reference Implementation

Recipient Example

contract RecipientExample {

    function purchaseItem(uint256 itemId) external {
        address sender = _msgSender();
        // ... perform the purchase for sender
    }

    address immutable _trustedForwarder;
    constructor(address trustedForwarder) internal {
        _trustedForwarder = trustedForwarder;
    }

    function isTrustedForwarder(address forwarder) public returns(bool) {
        return forwarder == _trustedForwarder;
    }

    function _msgSender() internal view returns (address payable signer) {
        signer = msg.sender;
        if (msg.data.length>=20 && isTrustedForwarder(signer)) {
            assembly {
                signer := shr(96,calldataload(sub(calldatasize(),20)))
            }
        }    
    }

}

Security Considerations

A malicious forwarder may forge the value of _msgSender() and effectively send transactions from any address. Therefore, Recipient contracts must be very careful in trusting forwarders. If a forwarder is upgradeable, then one must also trust that the contract won't perform a malicious upgrade.

In addition, modifying which forwarders are trusted must be restricted, since an attacker could "trust" their own address to forward transactions, and therefore be able to forge transactions. It is recommended to have the list of trusted forwarders be immutable, and if this is not feasible, then only trusted contract owners should be able to modify it.

Copyright

Copyright and related rights waived via CC0.