Capture The Ether WRITEUP

Writeup of the different challenges of Capture the ether

Warmup

2) Call Me

Pretty simple, you just call the function callMe().

3) Choose a nickname

Again the challenge is simple, you call the function setNickame() with your nickname. 2 subtleties:

One, you need to call setNickname() on the CaptureTheEther contract at address 0x71c46Ed333C35e4E6c62D32dc7C8F00D125b4fee and not the address given on the left.
Two, you need to be sure to encode your nickname correctly. The contract check the leftmost character to see if its 0 or not so you need to be sure that your nickname is save as 0xYOURNICKNAMEINHEX00000000000000000000...0 instead of 0x00000000000000000000...0YOURNICKNAMEINHEX.

Lotteries

4) Guess the number

Simply call guess() with 42 as argument and msg.value equal to 1 ether.

5) Guess the secret number

We need to find which uint8 has a keccak256 hash equal to the one save in the contract. We can easily brute force the solution with a loop:

for i in range(2 ** 8):
    # if the hash matches, guess the number
    if Web3.solidityKeccak(["uint8"], [i]).hex().upper() == answerHash.upper():
        contract.guess(i, {"from": account, "value": ONE})

Then we send the transcation like the previous challenge.

6) Guess the random number

We need to find the number saved during the creation of the contract. Since the variable doesn't have the public visibility modifer, solidity didn't create a getter answer(), but we can still look at the contract storage and find the number:

answer = int(w3.eth.get_storage_at(contract_address, 0).hex(), base=16)

Then we send the transcation like the previous challenge.

7) Guess the new number

Similar to the previous challenges, we need to find the right number, only now, the number is generated at the same time we are trying to guess it.

uint8 answer = uint8(keccak256(block.blockhash(block.number - 1), now));

To guess this number, we create an attack contract that will do the same calculation as the target contract and will then "guess" the correct answer. Since the calculations are done during the same transaction, they will have the same block.number and timestamp:

uint8 answer = uint8(
            uint256(
                keccak256(
                    abi.encodePacked(
                        blockhash(block.number - 1),
                        block.timestamp
                    )
                )
            )
        );
GuessTheNewNumber(target_contract).guess{value: msg.value}(answer);
payable(msg.sender).transfer(address(this).balance);

We need to slightly modify the syntax since we are using solidity version 0.8.0 so now becomes block.timestamp and keccak(x,y) become keccak256(abi.encodePacked(x,y)) Then our attack() function call the guess() function with a msg.value of 1 ether and we then transfer the received ether to our EOA attacker address. We also need to make sure our attack contract has a receive() function to receive the ether from the target contract:

receive() external payable {}

8) Predict the future

We need to guess a number between 0 and 9 (because of the % 10) and call lockInGuess(uint8 n). Then with a different transaction, check wheter our guess was correct or not by calling settle(). The problem is, if the guess was wrong, we need to guess again and each guess cost us 1 ether:

function lockInGuess(uint8 n) public payable {
    require(guesser == 0);
    require(msg.value == 1 ether); // need to pay 1 ether to guess

    guesser = msg.sender;
    guess = n;
    settlementBlockNumber = block.number + 1; 
}

function settle() public {
    require(msg.sender == guesser); // need to call lockInGuess() each time we settle
    require(block.number > settlementBlockNumber);// has to be a different transaction (and in a different block)

    uint8 answer = uint8(
        keccak256(block.blockhash(block.number - 1), now)
    ) % 10; // calculate the answer

    guesser = 0; //reset, so if the next line is wrong, we need to call lockInGuess() again
    if (guess == answer) {
        msg.sender.transfer(2 ether);
    }
}

The trick is to randomly guess a number, say 8, and then with a different transaction check whether that transaction will be in a block where the answer will be equal to our guess. If it is, we call settle() and win, if it's not, we simply abort the transaction, lost some gas fee, and try again.

function attack() public returns (bool) {
    uint8 answer = uint8(
        uint256(
            keccak256(
                abi.encodePacked(
                    blockhash(block.number - 1),
                    block.timestamp
                )
            )
        )
    ) % 10; // compute the answer just like the target contract will, with the same params (`block.number` and `block.timestamp`)
    require(answer == 8); // make sure our answer is correct, if not revert
    PredictTheFutureChallenge(target).settle(); // if its correct, we call settle()
    payable(msg.sender).transfer(address(this).balance);// and transfer our ether from our contract to our attacker address
}

Now we just need to send the same transaction where we call attack() until the tx goes through and we recover our ethers. We only lose some gas fee.

9) Predict the block hash

Same challenge as before, except we now need to guess the hash of the current block instead of a number modulo 8. Another difference is that the answer check the hash of the block our guess transaction was in (the one in which we call lockInGuess(bytes32 hash)) and not the current one. If we read the solidity docs on blockhash, we see that they are only stored for 256 blocks, for scalability reasons, and after that blockhash(blockNumber) will return 0. so we know that the answer will be zero in the future, we can just guess zero, wait until the block number has increased by more than 256 (it takes about an hour) and make the contract check our answer.

Math

10) Token sale

Here we need to create an overflow in the buy() function in order to increase our balance on the contract without sending that much ethers:

require(msg.value == numTokens * PRICE_PER_TOKEN); // PRICE_PER_TOKEN = 10^18

We need to set numTokens to a value that will overflow numTokens * PRICE_PER_TOKEN so msg.value can be lower that what the contract think we sent. We need to overflow the righthand side of this require. We know that for an uint256 an overflow happens at 2^256, which become zero. If we divide that number by 10^18 (since numTokens will be mul by 1 ether) we get a value x that will be multiply by 10^18 and cause an overflow. BUT, since we first divided by 10^18, we get a float number that will be floored when given to the contract so when we multiply that number by 10^18, we'll get a smaller number that before and there will be no overflow. So before sending that x number to the contract, we need to add 1 to it, so x time 10^18 will be bigger than 2^256. Here are the numbers:

# 2^256 = 115,792,089,237,316,195,423,570,985,008,687,907,853,269,984,665,640,564,039,457,584,007,913,129,639,936
# /10^18 = 115,792,089,237,316,195,423,570,985,008,687,907,853,269,984,665,640,564,039,457.584007913129639936
# take the floor = 11,579,208,923,731,619,542,357,098,500,868,790,785,326,998,466,564,056,403,9457 -> `x`
# +1 = 11,579,208,923,731,619,542,357,098,500,868,790,785,326,998,466,564,056,403,9458 -> `NumTokens`
# *10^18 = 115,792,089,237,316,195,423,570,985,008,687,907,853,269,984,665,640,564,039,458,000,000,000,000,000,000 -> what will happen in the require (will overflow) and need to be equal to msg.value
# MOD = 415,992,086,870,360,064 -> `msg.value`

11) Token whale

We can see that the private function _transfer_() does something strange. It never take an argument for the from of the transfer and assumes that the transfer comes from the msg.sender. This cause a problem in transferFrom() since it calls _transfer() which means the transfer is between the msg.sender and the _to, but the msg.sender balance is never checked for underflow. So when it reduces the balance of msg.sender, it can underflow and the msg.sender ends up with a huge balance.

function transferFrom(
    address from,
    address to,
    uint256 value
) public {
    require(balanceOf[from] >= value); // check for underflow for `from` and not `msg.sender`
    require(balanceOf[to] + value >= balanceOf[to]);
    require(allowance[from][msg.sender] >= value);

    allowance[from][msg.sender] -= value;
    _transfer(to, value);
}

12) Retirement Fund

Staying on the same theme, we need to create an underflow in the withdrawn variable:

function collectPenalty() public {
    require(msg.sender == beneficiary);

    uint256 withdrawn = startBalance - address(this).balance;

    // an early withdrawal occurred
    require(withdrawn > 0);

    // penalty is what's left
    msg.sender.transfer(address(this).balance);
}

In order to have an underflow, we need to increase the ether balance of the contract. Since there is no receive() function, we can use selfdestruct() to force the contract to receive some ethers.

13) Mapping

We need to change the isComplete variable to true. There are no setter for this variable but we have one for the dynamic array. We just need to find which key will access the storage slot of isComplete and set its value to 1. This will overwrite the bool variable. Dynamic array variable are stored like that:

The slot# corresponding to the variable (slot 1) contains the length of the array
The values are then store at (keccak(slot#) + i) We know that at slot keccak(1) will be stored map[0], at slot keccak(1) + 1 will be stored map[1], at slot keccak(1) + i will be stored map[i], at slot 2^256 -1 will be stored map[2^256 -1 -keccak(1)], and at slot 0 will be stored map[2^256 -1 -keccak(1) +1]

So we call set(2^256 -1 -keccak(1) +1, 1)

MikeSpa / capturetheether