Setting Up ERC-4337 Paymasters for Gasless Token Swaps in DeFi Protocols

ERC-4337 Paymaster Contract for Gasless Token Swaps

In ERC-4337 account abstraction, a Paymaster contract enables gasless transactions by sponsoring UserOperation gas costs. For DeFi token swaps, the Paymaster can selectively validate operations that invoke swap functions on trusted DEX routers, enhancing user experience while mitigating abuse risks through targeted checks.

```solidity
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.19;

import {IPaymaster, PackedUserOperation, PostOpMode} from "@account-abstraction/contracts/v0.7/interfaces/IPaymaster.sol";
import {IEntryPoint} from "@account-abstraction/contracts/v0.7/interfaces/IEntryPoint.sol";

/// @title TokenSwapPaymaster
/// @notice ERC-4337 Paymaster for sponsoring gasless token swaps in DeFi protocols
/// @dev Validates UserOperations targeting token swap functions on trusted DEX routers
contract TokenSwapPaymaster is IPaymaster {

    IEntryPoint private immutable _entryPoint;
    address public immutable owner;

    /// @notice Selector for Uniswap V2 Router swapExactTokensForTokens
    bytes4 private constant SWAP_SELECTOR = 0x38ed1739;
    /// @notice Example Uniswap V2 Router address (update for mainnet)
    address private constant UNISWAP_V2_ROUTER = 0x7a250d5630B4cF539739dF2C5dAcb4c659F2488D;

    constructor(IEntryPoint entryPoint) {
        _entryPoint = entryPoint;
        owner = msg.sender;
    }

    function validatePaymasterUserOp(
        PackedUserOperation calldata userOp,
        bytes32 /* userOpHash */,
        uint256 /* maxCost */
    ) external override returns (bytes memory context, uint256 validationData) {
        // Ensure only EntryPoint can call
        require(msg.sender == address(_entryPoint), "Paymaster: caller is not EntryPoint");

        // Decode callData to check if it's a token swap
        // Assumes account executes external call to DEX router with swap selector
        (address target, uint256 value, bytes memory callData) = _decodeCall(userOp.callData);

        require(target == UNISWAP_V2_ROUTER, "Paymaster: invalid DEX router");
        require(bytes4(callData) == SWAP_SELECTOR, "Paymaster: not a token swap");

        // Additional checks: e.g., sufficient balance, whitelisted sender, etc.
        // For demo, always sponsor if swap detected

        return ("", 0);
    }

    function postOp(
        PostOpMode /* mode */,
        bytes calldata /* context */,
        uint256 /* actualGasCost */
    ) external override {
        require(msg.sender == address(_entryPoint), "Paymaster: caller is not EntryPoint");

        // Handle refunds, token transfers for gas reimbursement, etc.
        // e.g., transfer tokens from user to paymaster pre-swap
    }

    /// @dev Simplified decoder for account's callData (assumes standard multicall format)
    function _decodeCall(bytes memory callData)
        private
        pure
        returns (address target, uint256 value, bytes memory data)
    {
        // Placeholder: parse based on account implementation
        // In practice, use ABI decode for your account's exec format
        target = address(0xdead); // Replace with actual decoding
        value = 0;
        data = "";
    }
}
```

This example demonstrates core validation logic: verifying the DEX target and swap selector from the UserOperation's callData. Production deployments should incorporate robust safeguards, such as minimum token amounts for gas reimbursement, sender whitelisting, and rate limiting. Integrate with your smart account implementation by adjusting the _decodeCall function to match its execution format.