transaction.sol

    
pragma solidity >=0.7.0 <0.9.0;

import "github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.3/contracts/cryptography/ECDSA.sol";
import "github.com/OpenZeppelin/openzeppelin-contracts/blob/release-v3.3/contracts/utils/ReentrancyGuard.sol";

contract UniDirectionalPaymentChannel is ReentrancyGuard {

    // ...
    
    function send(uint _amount, bytes memory _sig) external nonReentrant {
        //  verifing signature and sender != receiver
        require(msg.sender == receiver, "sender must be different than receiver");
        require(_verify(_amount, _sig), "invalid signature");
        
        (bool sent, ) = receiver.call{value: _amount}("");
        require(sent, "Failed to send Ether");
        selfdestruct(sender);
    }
    
    // ...
 }