ERC20 order-based exchange

Project built in the context of the Solidity Underhanded contest 2022. Won 2nd place.

Simple order-based exchange for selling ERC20 tokens in exchange for ETH. Orders are stored off-chain and signed by the sellers. Buyers submit the order to execute to the contract, along with the seller's signature and the ETH to purchase the tokens.

Orders can be EXACT or PARTIAL. Exact orders need to be fulfilled in a single operation, whereas partial orders can be fulfilled in multiple purchases from multiple buyers.

Orders can also optionally define a referrer address, who will receive 1% of the value in ETH of the purchase. Can be used to reward the app that stores the orders off-chain. Does not allocate a fee if set to the zero address.

Reference

struct Order {
  address referrer;     // optional referrer field that takes a 1% of the amount paid in ETH
  address token;        // token being offered in this order
  uint128 rate;         // number of tokens per eth sold (times RATE_DENOMINATOR)
  uint24 nonce;         // nonce for differentiating two otherwise identical orders
  uint256 amount;       // amount of tokens offered in this order
  uint8 orderType;      // type of order (EXACT=0, PARTIAL=1)
}

TODO

• Allow sellers to cancel an order
• Add cross-chain replay attack protection