Gasless Way to Buy ETH with USDC: Building the Gas Broker Contract

In the rapidly evolving world of decentralized finance (DeFi), user experience remains a critical bottleneck—especially when it comes to gas fees. What if users could swap stablecoins like USDC for ETH without needing ETH in their wallet to pay for gas? This is not only possible but practical using a gasless transaction mechanism powered by smart contracts.

This article dives into the core component of such a system: the Gas Broker contract, a trustless middleware that enables gasless swaps between users (signers) and gas providers. We’ll explore its architecture, security considerations, and implementation details using Solidity, with a focus on ERC-2612 permit, EIP-712 signatures, and replay attack prevention.

Core Components of the Gas Broker System

The Gas Broker contract acts as an intermediary between two parties:

Customer (Signer): Initiates the swap by signing a permit and reward message.
Gas Provider (Sender): Executes the transaction, pays gas, and receives tokens in return.

The central function—swap()—enables this interaction without requiring the customer to hold ETH for gas.

Key Functional Requirements

Allow customers to approve token transfers off-chain via permit.
Enable gas providers to front ETH for execution.
Ensure atomicity: all steps succeed or fail together.
Prevent replay attacks through cryptographic linking of signatures.

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Designing the Swap Interface

The swap() function must accept all necessary data to validate both the token allowance and reward claim. Here's the interface:

function swap(
    address signer,
    address token,
    uint256 value,
    uint256 deadline,
    uint256 reward,
    uint8 permitV,
    bytes32 permitR,
    bytes32 permitS,
    uint8 rewardV,
    bytes32 rewardR,
    bytes32 rewardS
) external payable

Each signature (permit and reward) is split into v, r, s components for optimal gas efficiency. The function is payable because the gas provider must send ETH to cover the output amount sent to the customer.

Why Split Signatures?

While Ethereum natively supports signature recovery via ecrecover, passing signatures as three separate values avoids extra encoding/decoding costs, reducing gas consumption during execution.

How the Swap Logic Works

When the gas provider calls swap(), the following sequence occurs:

Validate Reward Amount: Ensure value > reward to prevent negative outputs.
Verify Reward Signature: Confirm the customer signed the reward message.
Execute Permit: Use ERC-2612’s permit() to grant the contract rights to transfer USDC.
Transfer Tokens: Pull USDC from the customer to the contract.
Calculate ETH Output: Query a price oracle for the equivalent ETH amount.
Send ETH to Customer: Transfer calculated ETH from msg.value.
Refund Excess ETH: Return any leftover ETH to the gas provider.
Deliver USDC: Transfer full token amount to the gas provider as compensation.

Security-critical operations like signature validation are handled internally, ensuring no external tampering.

Securing Against Replay Attacks with EIP-712

A major threat in off-chain signing systems is replay attacks. Without proper safeguards, a malicious actor could reuse old signatures to extract more rewards than intended.

The Attack Scenario

Imagine:

A user signs a swap with a 10 USDC reward.
Later, they initiate another swap with only 5 USDC reward.
A gas provider replays the earlier, higher-value signature to claim extra funds.

To prevent this, we bind each reward signature to its corresponding permit signature.

Solution: Linking Signatures Cryptographically

We define a Reward struct:

struct Reward {
    uint256 value;
    bytes32 permitHash; // keccak256 of (v,r,s) from permit signature
}

This ensures that a reward signature is only valid if used with the exact permit signature it was paired with.

Additionally, since ERC-2612 already includes a nonce, the permit itself cannot be replayed—adding another layer of protection.

Implementing EIP-712 for Human-Readable Messages

EIP-712 allows wallets like MetaMask to display structured, readable messages instead of raw byte strings when signing.

This increases user trust and reduces phishing risks.

Domain Separator

Each contract defines a unique domain separator:

bytes32 DOMAIN_SEPARATOR = keccak256(
    abi.encode(
        keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
        keccak256(bytes("Gas broker")),
        keccak256("1"),
        chainId,
        address(this)
    )
);

This binds signatures to the specific contract, chain, and version—preventing cross-contract misuse.

Hashing and Verification

We compute the typed hash of the reward message:

function hashReward(Reward memory reward) private view returns (bytes32) {
    return keccak256(
        abi.encodePacked(
            "\x19\x01",
            DOMAIN_SEPARATOR,
            keccak256(
                abi.encode(
                    keccak256("Reward(uint256 value,bytes32 permitHash)"),
                    reward.value,
                    reward.permitHash
                )
            )
        )
    );
}

Then verify it using ecrecover:

function verifyReward(...) private view returns (bool) {
    return signer == ecrecover(hashReward(reward), sigV, sigR, sigS);
}

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Complete Gas Broker Contract Code

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol";
import "@openzeppelin/contracts/utils/Address.sol";

interface IPriceOracle {
    function getPriceInEth(address token, uint amount) external view returns (uint256);
}

interface IERC2612 {
    function permit(address owner, address spender, uint value, uint deadline, uint8 v, bytes32 r, bytes32 s) external;
}

struct Reward {
    uint256 value;
    bytes32 permitHash;
}

contract GasBroker {
    using Address for address payable;

    bytes32 public immutable DOMAIN_SEPARATOR;
    IPriceOracle immutable priceOracle;

    constructor(uint256 chainId, address _priceOracle) {
        priceOracle = IPriceOracle(_priceOracle);
        DOMAIN_SEPARATOR = keccak256(
            abi.encode(
                keccak256("EIP712Domain(string name,string version,uint256 chainId,address verifyingContract)"),
                keccak256(bytes("Gas broker")),
                keccak256("1"),
                chainId,
                address(this)
            )
        );
    }

    function swap(
        address signer,
        address token,
        uint256 value,
        uint256 deadline,
        uint256 reward,
        uint8 permitV,
        bytes32 permitR,
        bytes32 permitS,
        uint8 rewardV,
        bytes32 rewardR,
        bytes32 rewardS
    ) external payable {
        require(value > reward, "Reward could not exceed value");
        bytes32 permitHash = keccak256(abi.encodePacked(permitV, permitR, permitS));

        require(
            verifyReward(signer, Reward({value: reward, permitHash: permitHash}), rewardV, rewardR, rewardS),
            "Reward signature is invalid"
        );

        IERC2612(token).permit(signer, address(this), value, deadline, permitV, permitR, permitS);
        SafeERC20.safeTransferFrom(IERC20(token), signer, address(this), value);

        uint256 ethAmount = _getEthAmount(token, value - reward);
        require(msg.value >= ethAmount, "Not enough ETH provided");

        payable(signer).sendValue(ethAmount);
        if (msg.value > ethAmount) {
            payable(msg.sender).sendValue(msg.value - ethAmount);
        }

        SafeERC20.safeTransfer(IERC20(token), msg.sender, value);
    }

    function hashReward(Reward memory reward) private view returns (bytes32) {
        return keccak256(
            abi.encodePacked(
                "\x19\x01",
                DOMAIN_SEPARATOR,
                keccak256(
                    abi.encode(
                        keccak256("Reward(uint256 value,bytes32 permitHash)"),
                        reward.value,
                        reward.permitHash
                    )
                )
            )
        );
    }

    function verifyReward(
        address signer,
        Reward memory reward,
        uint8 sigV,
        bytes32 sigR,
        bytes32 sigS
    ) private view returns (bool) {
        return signer == ecrecover(hashReward(reward), sigV, sigR, sigS);
    }

    function _getEthAmount(address token, uint256 amount) internal view returns (uint256 ethAmount) {
        ethAmount = priceOracle.getPriceInEth(address(token), amount);
    }

    function getEthAmount(address token, uint256 amount) external view returns (uint256 ethAmount) {
        ethAmount = _getEthAmount(token, amount);
    }
}

Frequently Asked Questions (FAQ)

Q: What does "gasless" mean in this context?

A: "Gasless" means the end user doesn’t need to hold ETH to pay gas fees. Instead, a third party (gas provider) pays the gas and gets compensated in tokens.

Q: How do users approve token transfers without paying gas?

A: Using ERC-2612 permit, users sign a message off-chain. This signature is submitted on-chain by the gas provider, granting approval without a blockchain transaction from the user.

Q: Can anyone act as a gas provider?

A: Yes—anyone with ETH can call swap() and front gas. They are incentivized by receiving the full token amount minus the ETH payout to the user.

Q: Is this system trustless?

A: Yes. All logic is enforced by smart contracts. No party needs to trust another; cryptographic proofs ensure correctness.

Q: How are prices determined?

A: A price oracle (IPriceOracle) feeds real-time exchange rates. The contract uses this to calculate how much ETH should be sent to the user.

Q: Can this be used with tokens other than USDC?

A: Yes! As long as the token supports ERC-20 + ERC-2612, it can be integrated into this system.

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Conclusion

The Gas Broker contract represents a significant step toward frictionless DeFi interactions. By decoupling transaction execution from payment responsibility, it opens doors for new onboarding models where users can trade instantly—without pre-funding gas.

With robust defenses against replay attacks and clear UX via EIP-712, this design balances security, efficiency, and usability.

As we move toward broader adoption, solutions like this will play a crucial role in making blockchain accessible to everyone—not just those who already hold ETH.

Core Keywords: gasless swap, USDC to ETH, ERC-2612 permit, EIP-712 signature, DeFi transactions, smart contract security, off-chain signing