DumbWallet.sol

// WHATEVER YOU DO, DON'T USE THIS.
// This is a bit of demo code posted to illustrate a principle. It is unaudited,
// probably full of bugs, and definintely not production ready.
// https://twitter.com/nicksdjohnson/status/1041642345467404291

contract DumbWallet {
    mapping(address=>bool) public authorised;
    
    event Authorised(address indexed target, bool value);
    event Call(address indexed target, uint value);
    event Received(address indexed sender, uint value);
    
    modifier authorisedOnly {
        require(authorised[msg.sender]);
        _;
    }
    
    constructor() {
        authorised[msg.sender] = true;
    }
    
    function setAuthorised(address target, bool value) authorisedOnly {
        require(target != msg.sender);
        authorised[target] = value;
        Authorised(target, value);
    }
    
    function invoke(address target, uint value, bytes data) payable authorisedOnly {
        require(target.call.value(value)(data));
    }
    
    function() payable {
        require(msg.data.length == 0 && msg.value > 0);
        Received(msg.sender, msg.value);
    }
}