Single Signed Approval Flow Using ApprovalTarget

Magicians Primordial Soup
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Here is a draft proposal for monkeypatching EIP-2612 support on ERC-20. This is by no means a complete proposal.

Summary

ApprovalTarget is an immutable contract that accepts token approvals from users across various ERC-20s, then enables anyone with a valid user signature to transfer those tokens. You can think of it like monkeypatching EIP-2612 support on all ERC-20 compliant tokens.

Abstract

To use ERC-20 tokens in dApps, approving is needed, which is expensive.
Design a ApprovalTarget contract to create a single signed approval flow for ERC-20 tokens. This allows users to send approval transaction only once.

Motivation

EIP-2612 allows abstraction in the ERC-20 approve method. But some of ERC-20 tokens do not have a permit function. Introducing ApprovalTarget indirectly allows approval by signature for all ERC-20 compliant tokens.

In addition ApprovalTarget replaces approve transaction with a signed approve and will improves UX.

Specification

ApprovalTarget has three functions.

function nonces(address owner) external view returns (uint)

function PERMIT_AND_TRANSFER_FROM_TYPEHASH() external view returns (bytes32)

function permitAndTransferFrom(address erc20, address owner, address recipient, uint256 value, uint256 deadline, uint8 v, bytes32 r, bytes32 s) external

The semantics of which are as follows:

For all addresses erc20, owner, recipient, uint256s value, deadline and nonce, uint8 v, bytes32 r and s, a call to permitAndTransferFrom(erc20, owner, recipient, value, deadline, v, r, s) will call transferFrom(owner, recipient, value), increment nonces[owner] by 1. If and only if the following conditions are met:

  • The current blocktime is less than or equal to deadline.
  • erc20 is not the zero address. this address must be ERC-20 compliant token address.
  • owner is not the zero address.
  • nonces[owner] (before the state update) is equal to nonce.
  • r, s and v is a valid secp256k1 signature from owner of the message:

NOTE: spender in the provided signature must be the same as msg.sender. recipient isn’t attested by a signature from owner, and is instead chosen by msg.sender, attested in the signature as spender.

If any of these conditions are not met, the permitAndTransferFrom call must revert.

keccak256(abi.encodePacked(
   hex"1901",
   DOMAIN_SEPARATOR,
   keccak256(abi.encode(
            keccak256("PermitAndTransferFrom(address erc20,address owner,address spender,value,uint256 nonce,uint256 deadline)"),
            erc20,
            owner,
            msg.sender, // NOTE: spender
            value,
            nonce,
            deadline))
))

where DOMAIN_SEPARATOR is defined according to EIP-712.

In summary the caller of the permitAndTransferFrom function must be spender. spender can choose recipient.

Rationale

The spender is not provided in permitAndTransferFrom paramters. If spender is not msg.sender, the transaction will revert.

The recipient is provided by not the owner but the spender. So, the recipient isn’t attested by a signature.

More details here GitHub - massun-onibakuchi/approval-target