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319 lines
12 KiB
319 lines
12 KiB
+++
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author = "Maik de Kruif"
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title = "Final Battle"
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subtitle = "Challenge 24 - AdventOfCTF"
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date = 2021-09-22T12:12:12+01:00
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description = "A writeup for challenge 24 of AdventOfCTF."
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cover = "img/writeups/adventofctf/2020/b915cb528c4b3d6fc4644f73ba8b829d.png"
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tags = [
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"AdventOfCTF",
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"challenge",
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"ctf",
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"hacking",
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"writeup",
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"web",
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"python",
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"serialization",
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"blockchain",
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]
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categories = [
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"ctf",
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"writeups",
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"hacking",
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]
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aliases = [
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"challenge_24"
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]
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+++
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- Points: 2400
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## Description
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The final battle! The elves want revenge for their lost game! They have enhanced the tic-tac-toe game with blockchain technology. Cyber Security on the Blockchain will revolutionize everything, but most importantly ensure they will win this time. No cheating Santa!
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Visit <https://24.adventofctf.com> to start the challenge.
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## Recon
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When opening the page, we're greeted with what looks like the same screen as we had on [challenge 20]({{< ref "challenge_20.md" >}}). The only noticeable difference is the addition of `blockchain? True` in the footer.
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If we take a look at the source, we also find a comment with the following python code:
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```py
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# <!-- Development notes: Do not let santa see!
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def hash_string(string):
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return hashlib.md5(string.encode('utf-8')).hexdigest()
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def hash_row(row):
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conv = lambda i : i or ' '
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res = [conv(i) for i in row]
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return hash_string(' '.join(res))
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def hash_board(board):
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acc = ""
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for row in board:
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acc += hash_row(row)
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return acc
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def verify_chain(game):
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board=game["board"]
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chain = game["chain"]
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if len(chain) > 0:
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if board != chain[-1]["board"]:
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return False
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for i in range(len(chain)):
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block=chain[i]
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h = hash_board(block["board"])
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h = hash_string(h + block["prev"])
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if h != block["hash"]:
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return False
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return True
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# -->
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```
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Just like in challenge 20, we alse find a game cookie:
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```text
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game=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```
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## Finding the vulnerability
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Using the same method as in challenge 20, we can take a look at the board:
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```py
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In [1]: import pickle
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In [2]: import base64
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In [3]: pickle.loads(base64.b64decode("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"))
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```
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This time our board is quite a bit larger:
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{{< code language="py" title="Board" >}}
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```py
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{
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"blockchain": True,
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"board": [
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["O", "O", None],
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["O", "X", "X"],
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[None, "X", "X"]
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],
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"chain": [
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{
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"board": [
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[None, None, None],
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[None, None, None],
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[None, None, "X"]
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],
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"hash": "e7dc8e1f7a6788bc0cb6841538b216e8",
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"prev": "cef215c5be8cf63fcf3d43ecf2510b33"
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},
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{
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"board": [
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["O", None, None],
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[None, None, None],
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[None, None, "X"]
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],
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"hash": "fc93236b5eea5f1d55e2b5b308d67390",
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"prev": "e7dc8e1f7a6788bc0cb6841538b216e8"
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},
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{
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"board": [
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["O", None, None],
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[None, "X", None],
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[None, None, "X"]
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],
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"hash": "a8dc0d3da290d1e894eaaffcb98398c9",
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"prev": "fc93236b5eea5f1d55e2b5b308d67390"
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},
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{
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"board": [
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["O", "O", None],
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[None, "X", None],
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[None, None, "X"]
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],
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"hash": "e74f5b22f5213ba4c2449759ce91c2aa",
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"prev": "a8dc0d3da290d1e894eaaffcb98398c9"
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},
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{
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"board": [
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["O", "O", None],
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[None, "X", "X"],
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[None, None, "X"]
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],
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"hash": "ef25514dffbf8247cff6063be90f2d54",
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"prev": "e74f5b22f5213ba4c2449759ce91c2aa"
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},
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{
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"board": [
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["O", "O", None],
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["O", "X", "X"],
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[None, None, "X"]
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],
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"hash": "e3a4c037bdf154b344ed9bd1643a629d",
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"prev": "ef25514dffbf8247cff6063be90f2d54"
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},
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{
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"board": [
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["O", "O", None],
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["O", "X", "X"],
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[None, "X", "X"]
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],
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"hash": "c240fa161737c967ece5fd94672ab0f8",
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"prev": "e3a4c037bdf154b344ed9bd1643a629d"
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}
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],
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"finished": False,
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"sane": True,
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"turn": "O",
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"winner": ""
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}
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```
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{{< /code >}}
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We can see that a `chain` value has been added. From the title of this challenge, we can say that this is the blockchain that we likely have to bypass.
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## Exploit
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Cracking this shouldn't be too hard. Let's start by copying our script from challenge 20 and resetting the chain:
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```py
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import base64
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import pickle
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board_b64 = "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"
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game = pickle.loads(base64.b64decode(board_b64))
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game["board"] = [['X', 'X', 'X'], ['O', None, 'O'], [None, None, None]]
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game["chain"] = []
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game["finished"] = True
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game["winner"] = 'X'
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game["turn"] = 'O'
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print(base64.b64encode(pickle.dumps(data)))
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```
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Sadly, just submitting the result of this script doesn't work. We have to fill the chain.
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To find out how the chain works, let's take another look at the `verify_chain()` function:
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```py
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def verify_chain(game):
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board=game["board"]
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chain = game["chain"]
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if len(chain) > 0:
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if board != chain[-1]["board"]:
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return False
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for i in range(len(chain)):
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block=chain[i]
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h = hash_board(block["board"])
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h = hash_string(h + block["prev"])
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if h != block["hash"]:
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return False
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return True
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```
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We can see that, to verify the game, the function loops through all the blocks in the chain. It then calculates a hash based on the board, and the previous hash and compares that to the actual hash in the block (the user provided one).
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To crack this, we can simply reverse this algorithm. To do this, I grabbed the script from the HTML source, and added a crack method like so:
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{{< code language="py" title="solve.py" >}}
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```python
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import hashlib
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import pickle
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import base64
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def load_base64(base64_string):
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return pickle.loads(base64.b64decode(base64_string))
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def export_base64(game):
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return base64.b64encode(pickle.dumps(game)).decode()
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def hash_string(string):
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return hashlib.md5(string.encode('utf-8')).hexdigest()
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def hash_row(row):
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def conv(i): return i or ' '
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res = [conv(i) for i in row]
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return hash_string(' '.join(res))
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def hash_board(board):
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acc = ""
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for row in board:
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acc += hash_row(row)
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return acc
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def verify_chain(game):
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board = game["board"]
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chain = game["chain"]
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if len(chain) > 0:
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if board != chain[-1]["board"]:
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return False
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for block in chain:
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h = hash_board(block["board"])
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h = hash_string(h + block["prev"])
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if h != block["hash"]:
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return False
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return True
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def crack(game):
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game["board"] = [['X', 'X', 'X'], ['O', None, 'O'], [None, None, None]]
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game["chain"] = []
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game["finished"] = True
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game["winner"] = 'X'
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game["turn"] = 'O'
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board = game["board"]
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h = hash_board(board)
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h = hash_string(h)
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game["chain"].append({
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"board": board,
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"hash": h,
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"prev": ""
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})
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if __name__ == "__main__":
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game = load_base64("gAN9cQAoWAUAAABib2FyZHEBXXECKF1xAyhYAQAAAE9xBGgETmVdcQUoaARYAQAAAFhxBmgGZV1xByhOaAZoBmVlWAQAAAB0dXJucQhoBFgIAAAAZmluaXNoZWRxCYlYBgAAAHdpbm5lcnEKWAAAAABxC1gEAAAAc2FuZXEMiFgKAAAAYmxvY2tjaGFpbnENiFgFAAAAY2hhaW5xDl1xDyh9cRAoaAFdcREoXXESKE5OTmVdcRMoTk5OZV1xFChOTmgGZWVYBAAAAHByZXZxFVggAAAAY2VmMjE1YzViZThjZjYzZmNmM2Q0M2VjZjI1MTBiMzNxFlgEAAAAaGFzaHEXWCAAAABlN2RjOGUxZjdhNjc4OGJjMGNiNjg0MTUzOGIyMTZlOHEYdX1xGShoAV1xGihdcRsoaAROTmVdcRwoTk5OZV1xHShOTmgGZWVoFWgYaBdYIAAAAGZjOTMyMzZiNWVlYTVmMWQ1NWUyYjViMzA4ZDY3MzkwcR51fXEfKGgBXXEgKF1xIShoBE5OZV1xIihOaAZOZV1xIyhOTmgGZWVoFWgeaBdYIAAAAGE4ZGMwZDNkYTI5MGQxZTg5NGVhYWZmY2I5ODM5OGM5cSR1fXElKGgBXXEmKF1xJyhoBGgETmVdcSgoTmgGTmVdcSkoTk5oBmVlaBVoJGgXWCAAAABlNzRmNWIyMmY1MjEzYmE0YzI0NDk3NTljZTkxYzJhYXEqdX1xKyhoAV1xLChdcS0oaARoBE5lXXEuKE5oBmgGZV1xLyhOTmgGZWVoFWgqaBdYIAAAAGVmMjU1MTRkZmZiZjgyNDdjZmY2MDYzYmU5MGYyZDU0cTB1fXExKGgBXXEyKF1xMyhoBGgETmVdcTQoaARoBmgGZV1xNShOTmgGZWVoFWgwaBdYIAAAAGUzYTRjMDM3YmRmMTU0YjM0NGVkOWJkMTY0M2E2MjlkcTZ1fXE3KGgBXXE4KF1xOShoBGgETmVdcTooaARoBmgGZV1xOyhOaAZoBmVlaBVoNmgXWCAAAABjMjQwZmExNjE3MzdjOTY3ZWNlNWZkOTQ2NzJhYjBmOHE8dWV1Lg==")
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crack(game)
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print(export_base64(game))
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```
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{{< /code >}}
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After running this script, we get the following result:
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```text
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gASVsgAAAAAAAAB9lCiMBWJvYXJklF2UKF2UKIwBWJRoBGgEZV2UKIwBT5ROaAZlXZQoTk5OZWWMBHR1cm6UaAaMCGZpbmlzaGVklIiMBndpbm5lcpRoBIwEc2FuZZSIjApibG9ja2NoYWlulIiMBWNoYWlulF2UfZQojAVib2FyZJRoAowEaGFzaJSMIDZkZTM2NDM0OTAwZTI4YTdlYWYwNDhhYjBhY2JlNjA0lIwEcHJldpSMAJR1YXUu
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```
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When we set this and refresh the page, we get the following message: `"Game goes to: X NOVI{blockchain_cyb3r_security} Thank you for playing Advent of CTF! I hope you have a great christmas!"`.
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## Solution
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We got the flag! It is `NOVI{blockchain_cyb3r_security}`.
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