README.md

TupleCoin

Description

TupleCoin is launching their eponymous cryptocurrency today, and their flamboyant and capricious CEO is challenging you to bring home a bug bounty. Can you crack the crypto?! tuplecoin.cha.hackpack.club

Exploration

Heading over to that site I see a website where a user can create an account and transfer coins to and from accounts.

There is a bounty page asking hunters to try and steal coins from Tuco

Info:

• His account number is 314159265, and you can't have it.
• He absolutely hates robots, especially ones from Silicon Valley.
• He never found a sack of gold, or crypto, that wasn't worth almost dying for until he thought better of it.

Lets claim an account.

Number - #452139554 (3.14159 TuCo)

Transferring Money

I see we can Transfer money. Lets Transfer to Tuco's account and check out the request.

It made 2 Post Methods: 1:

url = 'https://tuplecoin.cha.hackpack.club/api/transaction/certify'
body = {"from_acct":452139554,"to_acct":314159265,"num_tuco":1}
resp = {
"transaction": {
 "from_acct": 452139554,
 "to_acct": 314159265,
 "num_tuco": 1
}
}

2:

url = 'https://tuplecoin.cha.hackpack.club/api/transaction/commit'
body = {
"auth_tag": "d4f7afe3dd729d535319a72bda14951ffbc822ce10c91b8fd8a4d7d87f97498b",
"transaction": {
 "from_acct": 452139554,
 "num_tuco": 1,
 "to_acct": 314159265
}
}
resp = "OK"

So i'll try and do the request myself as it is the UI that stops me from entering an account that is not mine as the from_acct/

Change the Requests

I'll look to make the requests in Python. I Try the following:

import requests

json_data = {
    'from_acct': 314159265,
    'to_acct': 452139554,
    'num_tuco': 1,
}

response = requests.post('https://tuplecoin.cha.hackpack.club/api/transaction/certify', json=json_data)

print(response.content)

and get

b'{"detail":"Ha! You think you can steal from Tuco so easily?!!"}'

So evidently there is some authentication going on.

Lets look into this auth_tag more...

Decoding the Auth Tag

I get the following Auth Tag - d4f7afe3dd729d535319a72bda14951ffbc822ce10c91b8fd8a4d7d87f97498b Maybe there is something I can do to fake it.

Lets spin up cyberchef and try and decode what looks like hex. Decoding it in Hex doesn't seem to yield me anything useful.

Own Tuco's Account

Maybe i can look to see if I can own Tuco's Account somehow.

Lets go back to the accounts page and check what the request is when I create an account.

curl 'https://tuplecoin.cha.hackpack.club/api/account/claim?acct_num=5464654'

response - {
 "balance": {
  "acct_num": 5464654,
  "num_tuco": 3.141592653589793
 },
 "auth_tag": "2876e7121c1cc941ac25edb58e3ca970bec3c79d547739b51f8e78d47dc37231"
}

So a claim sends a post reques with the account number in the parameters and then returns the account numnber, balance and auth_tag.

Lets see what happens if i add Tuco's account number.

import requests

params = {
    'acct_num': '314159265',
}

response = requests.post('https://tuplecoin.cha.hackpack.club/api/account/claim', params=params)
print(response.content)

b'{"detail":"That\'s Tuco\'s account number! Don\'t make Tuco mad!"}'

There seems to be some entrypoint i'm not getting here. Lets wait until the writeups - my thought is that it is to do with reverse engineering the auth_tag however I'm not sure where to start as it doesnt decode to anything useful.

Conclusion

So I wasn't able to solve tuple coin - however looking at articles since there are some key takeaways.

The first missed clue was that there was a page - robots.txt - from this I should have been able to find the source code to the backend. Specifically this python function,

 59 class Transaction(BaseModel):
 60    from_acct: int
 61    to_acct: int
 62    num_tuco: float
 63
 64    def serialize(self) -> bytes:
 65        return (str(self.from_acct) + str(self.to_acct) + str(self.num_tuco)).encode()
 66
 67    def sign(self, secret_key: bytes) -> AuthenticatedTransaction:
 68        tuco_smash = self.serialize()
 69        tuco_hash = hmac.new(secret_key, tuco_smash, "sha256").hexdigest()
 70        
 71        return CertifiedTransaction.parse_obj({
 72            "transaction": {
 73                "from_acct": self.from_acct,
 74                "to_acct": self.to_acct,
 75                "num_tuco": self.num_tuco
 76            },
 77            "auth_tag": tuco_hash,
 78        })
 79

Here you can see that the serialise function just joins the accounts and the number of tuco and then does a has on it. Key think here is that it is without a delimiter. So this means we can reverse engineer!

If we were to in the certify request just put tuco's 1st half of his account as the from, then the second half of his account as the too - and then the number of coins as -> <to_acc><num_coine> we will then get the correct hash to validate our send!

Thanks to https://ev1lm0rty.com/posts/writeup-hackpack-2022/ for the guidance here!