HashedTimelockERC20.sol

pragma solidity ^0.4.23;
pragma experimental "v0.5.0";

import "openzeppelin-solidity/contracts/token/ERC20/ERC20.sol";

/**
* @title Hashed Timelock Contracts (HTLCs) on Ethereum ERC20 tokens.
*
* This contract provides a way to create and keep HTLCs for ERC20 tokens.
*
* See HashedTimelock.sol for a contract that provides the same functions 
* for the native ETH token.
*
* Protocol:
*
*  1) newContract(receiver, hashlock, timelock, tokenContract, amount) - a 
*      sender calls this to create a new HTLC on a given token (tokenContract) 
*       for a given amount. A 32 byte contract id is returned
*  2) withdraw(contractId, preimage) - once the receiver knows the preimage of
*      the hashlock hash they can claim the tokens with this function
*  3) refund() - after timelock has expired and if the receiver did not 
*      withdraw the tokens the sender / creater of the HTLC can get their tokens 
*      back with this function.
 */
contract HashedTimelockERC20 {

    event LogHTLCERC20New(
        bytes32 indexed contractId,
        address indexed sender,
        address indexed receiver,
        address tokenContract,
        uint amount,
        bytes32 hashlock,
        uint timelock
    );
    event LogHTLCERC20Withdraw(bytes32 indexed contractId);
    event LogHTLCERC20Refund(bytes32 indexed contractId);

    struct LockContract {
        address sender;
        address receiver;
        address tokenContract;
        uint amount;
        bytes32 hashlock;
        uint timelock; // UNIX timestamp seconds - locked UNTIL this time
        bool withdrawn;
        bool refunded;
        bytes32 preimage;
    }

    modifier tokensTransferable(address _token, address _sender, uint _amount) {
        require(_amount > 0);
        require(ERC20(_token).allowance(_sender, this) >= _amount);
        _;
    }
    modifier futureTimelock(uint _time) {
        // only requirement is the timelock time is after the last blocktime (now).
        // probably want something a bit further in the future then this.
        // but this is still a useful sanity check:
        require(_time > now);
        _;
    }
    modifier contractExists(bytes32 _contractId) {
        require(haveContract(_contractId));
        _;
    }
    modifier hashlockMatches(bytes32 _contractId, bytes32 _x) {
        require(contracts[_contractId].hashlock == sha256(_x));
        _;
    }
    modifier withdrawable(bytes32 _contractId) {
        require(contracts[_contractId].receiver == msg.sender);
        require(contracts[_contractId].withdrawn == false);
        require(contracts[_contractId].timelock > now);
        _;
    }
    modifier refundable(bytes32 _contractId) {
        require(contracts[_contractId].sender == msg.sender);
        require(contracts[_contractId].refunded == false);
        require(contracts[_contractId].withdrawn == false);
        require(contracts[_contractId].timelock <= now);
        _;
    }

    mapping (bytes32 => LockContract) contracts;

    /**
     * @dev Sender / Payer sets up a new hash time lock contract depositing the
     * funds and providing the reciever and terms.
     *
     * NOTE: _receiver must first call approve() on the token contract. 
     *       See allowance check in tokensTransferable modifier.

     * @param _receiver Receiver of the tokens.
     * @param _hashlock A sha-2 sha256 hash hashlock.
     * @param _timelock UNIX epoch seconds time that the lock expires at. 
     *                  Refunds can be made after this time.
     * @param _tokenContract ERC20 Token contract address.
     * @param _amount Amount of the token to lock up.
     * @return contractId Id of the new HTLC. This is needed for subsequent 
     *                    calls.
     */
    function newContract(
        address _receiver,
        bytes32 _hashlock,
        uint _timelock,
        address _tokenContract,
        uint _amount
    )
        external
        tokensTransferable(_tokenContract, msg.sender, _amount)
        futureTimelock(_timelock)
        returns (bytes32 contractId)
    {
        contractId = sha256(
            msg.sender,
            _receiver,
            _tokenContract,
            _amount,
            _hashlock,
            _timelock
        );

        // Reject if a contract already exists with the same parameters. The
        // sender must change one of these parameters (ideally providing a
        // different _hashlock).
        if (haveContract(contractId))
            revert();

        // This contract becomes the temporary owner of the tokens
        if (!ERC20(_tokenContract).transferFrom(msg.sender, this, _amount))
            revert();
            
        contracts[contractId] = LockContract(
            msg.sender,
            _receiver,
            _tokenContract,
            _amount,
            _hashlock,
            _timelock,
            false,
            false,
            0x0
        );

        emit LogHTLCERC20New(
            contractId,
            msg.sender,
            _receiver,
            _tokenContract,
            _amount,
            _hashlock,
            _timelock
        );
    }

    /**
    * @dev Called by the receiver once they know the preimage of the hashlock.
    * This will transfer ownership of the locked tokens to their address.
    *
    * @param _contractId Id of the HTLC.
    * @param _preimage sha256(_preimage) should equal the contract hashlock.
    * @return bool true on success
     */
    function withdraw(bytes32 _contractId, bytes32 _preimage)
        external
        contractExists(_contractId)
        hashlockMatches(_contractId, _preimage)
        withdrawable(_contractId)
        returns (bool)
    {
        LockContract storage c = contracts[_contractId];
        c.preimage = _preimage;
        c.withdrawn = true;
        ERC20(c.tokenContract).transfer(c.receiver, c.amount);
        emit LogHTLCERC20Withdraw(_contractId);
        return true;
    }

    /**
     * @dev Called by the sender if there was no withdraw AND the time lock has
     * expired. This will restore ownership of the tokens to the sender.
     *
     * @param _contractId Id of HTLC to refund from.
     * @return bool true on success
     */
    function refund(bytes32 _contractId)
        external
        contractExists(_contractId)
        refundable(_contractId)
        returns (bool)
    {
        LockContract storage c = contracts[_contractId];
        c.refunded = true;
        ERC20(c.tokenContract).transfer(c.sender, c.amount);
        emit LogHTLCERC20Refund(_contractId);
        return true;
    }

    /**
     * @dev Get contract details.
     * @param _contractId HTLC contract id
     * @return All parameters in struct LockContract for _contractId HTLC
     */
    function getContract(bytes32 _contractId)
        public
        view
        returns (
            address sender,
            address receiver,
            address tokenContract,
            uint amount,
            bytes32 hashlock,
            uint timelock,
            bool withdrawn,
            bool refunded,
            bytes32 preimage
        )
    {
        if (haveContract(_contractId) == false)
            return;
        LockContract storage c = contracts[_contractId];
        return (
            c.sender,
            c.receiver,
            c.tokenContract,
            c.amount,
            c.hashlock,
            c.timelock,
            c.withdrawn,
            c.refunded,
            c.preimage
        );
    }

    /**
     * @dev Is there a contract with id _contractId.
     * @param _contractId Id into contracts mapping.
     */
    function haveContract(bytes32 _contractId)
        internal
        view
        returns (bool exists)
    {
        exists = (contracts[_contractId].sender != address(0));
    }

}