18. Recovery.md

Recovery

difficulty 6/10

A contract creator has built a very simple token factory contract. Anyone can create new tokens with ease. After deploying the first token contract, the creator sent 0.5 ether to obtain more tokens. They have since lost the contract address.

This level will be completed if you can recover (or remove) the 0.5 ether from the lost contract address.

Sources

pragma solidity ^0.4.23;

import 'openzeppelin-solidity/contracts/math/SafeMath.sol';

contract Recovery {

  //generate tokens
  function generateToken(string _name, uint256 _initialSupply) public {
    new SimpleToken(_name, msg.sender, _initialSupply);
  
  }
}

contract SimpleToken {

  using SafeMath for uint256;
  // public variables
  string public name;
  mapping (address => uint) public balances;

  // constructor
  constructor(string _name, address _creator, uint256 _initialSupply) public {
    name = _name;
    balances[_creator] = _initialSupply;
  }

  // collect ether in return for tokens
  function() public payable {
    balances[msg.sender] = msg.value.mul(10);
  }

  // allow transfers of tokens
  function transfer(address _to, uint _amount) public { 
    require(balances[msg.sender] >= _amount);
    balances[msg.sender] = balances[msg.sender].sub(_amount);
    balances[_to] = _amount;
  }

  // clean up after ourselves
  function destroy(address _to) public {
    selfdestruct(_to);
  }
}

Level author

Adrian Manning
[email protected]
https://github.com/AgeManning

Solution

1. On ropsten.etherscan.io find the list of transactions from the instance contract. Find the missing Contract Creation transaction and note the address.

2. Using the provided source, import it to Remix and create a Token contract instance at that address.

3. Invoke destroy method using the player address as parameter. When destroying a contract, all remaining Ether will be sent to provided address.

4. Submit instance 🎉

Lesson

Contract addresses are deterministic and are calculated by keccack256(address, nonce) where the address is the address of the contract (or ethereum address that created the transaction) and nonce is the number of contracts the spawning contract has created (or the transaction nonce, for regular transactions).

Because of this, one can send ether to a pre-determined address (which has no private key) and later create a contract at that address which recovers the ether. This is a non-intuitive and somewhat secretive way to (dangerously) store ether without holding a private key.

An interesting blog post by Martin Swende details potential use cases of this.

If you're going to implement this technique, make sure you don't miss the nonce, or your funds will be lost forever.