SignedSwap

A Solidity EIP-712 intent settlement protocol for non-custodial ERC-20 swaps with cumulative partial fills, nonce cancellation, protocol fees, and Foundry invariant testing.

Features

• EIP-712 typed data signing for human-readable order authorization
• ECDSA signature verification with OpenZeppelin
• Non-custodial settlement — contract never holds user funds
• Cumulative partial fills with mathematically correct rounding
• Per-maker nonce binding — each nonce locks to exactly one order hash
• Individual nonce cancellation and bulk invalidation via floor
• Immutable protocol fee paid by taker on top of maker's amount
• Reentrancy protection with checks-effects-interactions and ReentrancyGuard
• SafeERC20 for compatibility with non-standard tokens
• Foundry fuzz and invariant tests proving accounting correctness

Architecture

Single contract design:

src/SignedSwap.sol    — Core protocol (~200 LOC)

Order Model

struct Order {
    address maker;      // Seller
    address taker;      // Authorized buyer (address(0) = anyone)
    address sellToken;  // Token maker sells
    address buyToken;   // Token maker receives
    uint256 sellAmount; // Total to sell
    uint256 buyAmount;  // Total to receive
    uint256 nonce;      // Unique per-maker identifier
    uint256 deadline;   // Expiration timestamp
}

Partial Fill Arithmetic

SignedSwap uses cumulative accounting to guarantee consistent rounding regardless of fill fragmentation.

For an order with sellAmount = S and buyAmount = B:

sellReleased(filledBuy) = floor(filledBuy × S / B)

Each partial fill calculates sell output as:

sellFill = sellReleased(filledAfter) − sellReleased(filledBefore)

This ensures that multiple small fills produce exactly the same total as a single large fill.

Fees use the same cumulative approach:

fee(filledBuy) = floor(filledBuy × feeBps / 10_000)

Security Properties

• EIP-712 domain separation prevents cross-chain and cross-contract replay
• Nonce-order binding ensures each nonce commits to exactly one order
• Cumulative accounting eliminates fragmentation-based rounding exploits
• Checks-effects-interactions pattern throughout
• ReentrancyGuard on all state-changing external functions
• SafeERC20 handles non-standard token return values
• Immutable fee rate prevents admin manipulation

Unsupported Token Behaviors

The protocol assumes exact-transfer ERC-20 semantics:

• Fee-on-transfer tokens are not supported
• Rebasing tokens are not supported
• Native ETH is not supported (wrap to WETH)

Commands

# Build
forge build

# Run all tests
forge test

# Run fuzz tests
forge test --match-contract SignedSwapFuzzTest

# Run invariant tests
forge test --match-contract SignedSwapInvariantTest

# Check formatting
forge fmt --check

# Deploy (local)
forge script script/DeploySignedSwap.s.sol --broadcast

Environment Variables (Deploy)

Variable	Description	Default
FEE_BPS	Fee in basis points (max 100)	30
FEE_RECIPIENT	Address to receive fees	deployer

License

MIT