Table of contents
This challenge is marked as a 4/5 difficulty level problem but would fail to meet your expectations. If you remember the Vault - Level 8, you pretty much understand the background required to get your hands dirty and solve the problem.
Objective: The creator of this contract was careful enough to protect the sensitive areas of its storage.
Unlock this contract to beat the level.
Background
The main concept behind the challenge is that nothing stored in the blockchain is truly private, the private variables are readable by others and also the data is stored in slots, similar to my article on vault: Read here.
One more concept you should be aware of is downcasting and explicit conversions.
When we downcast a value you will lose information, since naturally, you cannot fit something in a box that is bigger than the box.
bytes32 random32 = '0x6632a4653dd0709532bbaca095a70d32f28a9669466f9be1c9d9b4665b778e71'
//total 64 characters, so technically 66 including 0x
bytes16 random16 = bytes16(random32) //downcasting.
//random16 --> '0x6632a4653dd0709532bbaca095a70d32f2' --> total 32 characters, so technically 34 including 0x.
Note: One question that a lot of beginners ask is, how a 32-byte hash can accommodate 64 digits. To do that you need to understand and learn hexadecimal and binary formats.
One byte is made up of 8 bits i.e. the range is 00000000 - 11111111 in binary, similarly, in hexadecimal, the range for one byte is 0x00 - 0xFF. One byte is represented in hex as a 2-character string. Therefore, a 32-byte hex string is 64 characters long.
Before reading ahead, I hope you understand what is slot packing and what are private variables.
Code of Importance
Okay, so we know that for statically-sized variables, solidity tries to pack multiple variables into a single slot (size 32 bytes) to optimize storage.
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;
contract Privacy {
bool public locked = true; //slot 0, takes 1 Byte, but cannot pack because of the next variable.
uint256 public ID = block.timestamp; //slot 1
uint8 private flattening = 10; //slot 2
uint8 private denomination = 255; //slot 2
uint16 private awkwardness = uint16(block.timestamp); //slot 2
bytes32[3] private data; //slot 3, 4, 5, 6 --> this is an array and each element takes one slot.
//data[0] => slot 3
//data[1] => slot 4
//data[2] => slot 5
//data[3] => slot 6
constructor(bytes32[3] memory _data) {
data = _data; //initialises data
}
function unlock(bytes16 _key) public {
require(_key == bytes16(data[2])); //key is in slot 5.
locked = false; //if we can fetch and enter the key, we are done, as locked will be false.
}
//this represents the famous rainbow kitty image, I guess.
/*
A bunch of super advanced solidity algorithms...
,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`
.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,
*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^ ,---/V\
`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*. ~|__(o.o)
^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*'^`*.,*' UU UU
*/
}
Solutions
Create a new level instance and fire up the console.
const data32 = await web3.eth.getStorageAt(contract.address, 5) //gets the data in slot 5 const data16 = data32.slice(0,34) //downcasts to 16 bytes //the reason we take 34 instead of 32 is to, add the inital `0x`. await contract.unlock(data16) //unlocks await contract.locked() //check if this returns false.Submit Instance
Privacy🔏