The goal of this level is to get our token balance to 0. NaughtCoin mints us the entire supply of its coin contingent upon the fact that we cannot transfer the tokens until a 10 year lockout period passes. This is achieved with the lockTokens() modifier:
modifier lockTokens() {
if (msg.sender == player) {
require(block.timestamp > timeLock);
_;
} else {
_;
}
}
This modifier checks that the msg.sender is not equal to the player - our address. Thus, while we are not allowed to interact with the transfer() function, other EOAs or contracts can passing the lockTokens() modifier. The trick to this level is that Naught Coin is an ERC20 token.
ERC20 tokens follow the ERC-20 Token Standard, a token standard that implements an API for tokens within smart contracts. The fact that Naught Coin is an ERC20 token is important is that it contains certain methods such as:
function approve(address _spender, uint256 _value) public returns (bool success);
function transferFrom(address _from, address _to, uint256 _value) public returns (bool success);
The approve() function is useful insofar you can approve other EOAs and contracts to spend a set number of tokens on your behalf. The transferFrom() function is useful insofar you can transfer tokens from any one address to another. Coupled with the approve() function, we could approve a smart contract to spend all of our Naught Coin and then have that smart contract transfer the tokens out of our wallet, using the transferFrom() function. This would satisfy the lockTokens() modifier and would result in us having a balance of zero.
To pass this level, deploy AttackNaughtCoin. After, deploy the IERC20 interface at the Naught Coin address and call the approve() function approving the total supply of Naught Coin. Then call the remove() function to transfer the Naught Coin out of our wallet.