picoCTF: unpackme.py
Another challenge from picoCTF 2022, worth 200 points.
This time we need to reverse-engineer a Python script.
Description:
Can you get the flag? Reverse engineer this Python program. (No further hints given.)
This is the Python program that has to be reverse engineered:
import base64
from cryptography.fernet import Fernet
payload = b'gAAAAABiMD1Dt87s50caSunQlHoZqPOwtGNaQXexN-gjKwZeuLEN_-v6UcFJHVXOT4B6DcD1SB7cnd6yTcHg9e9LZCAeJY2cJ0r0sfyGPRiH60F-WbkyUjlAdDywI8RPdTpDYLuBmpZ_Kun-kHyTzMjeKR6R26Z4JITUS8n7Dj9X--9eNLajH6UuYD4GkjRACpsidl_8z33DlB86u_xDCMMm7HFK2oJTs8HG1fBex6enQsu0frUAJbx56DxhRvWawAysDMtLE50vaohrzkVV7Yaz4ClilwgfjQ=='
key_str = 'correctstaplecorrectstaplecorrec'
key_base64 = base64.b64encode(key_str.encode())
f = Fernet(key_base64)
plain = f.decrypt(payload)
exec(plain.decode())
This code imports the cryptography library Fernet from Python’s standard library. Fernet is a module that can be used to implement a symmetric encryption:
https://www.geeksforgeeks.org/fernet-symmetric-encryption-using-cryptography-module-in-python/
Perhaps the variable key_str contains the secret key to decrypt the payload, which looks like base64 encoded binary data.
The last line of the program executes a piece of code that is handed over as a parameter, which is the result of plain.decode(). If we can just see this string before the exec() function is called, we may already be able to get the flag.
To get access to the sought string, I have simply executed every line of the short program in the Python interpreter. (For longer programs one would use a debugger and just set a stop point before the exec command. Spyder could be used in such a case.)
# Python 3.7.3 (default, Mar 27 2019, 22:11:17)
# [GCC 7.3.0] :: Anaconda, Inc. on linux
# Type "help", "copyright", "credits" or "license" for more information.
# >>>
# >>>
# >>> import base64
# >>> from cryptography.fernet import Fernet
# >>>
# >>> payload = b'gAAAAABiMD1Dt87s50caSunQlHoZqPOwtGNaQXexN-gjKwZeuLEN_-v6UcFJHVXOT4B6DcD1SB7cnd6yTcHg9e9LZCAeJY2cJ0r0sfyGPRiH60F-WbkyUjlAdDywI8RPdTpDYLuBmpZ_Kun-kHyTzMjeKR6R26Z4JITUS8n7Dj9X--9eNLajH6UuYD4GkjRACpsidl_8z33DlB86u_xDCMMm7HFK2oJTs8HG1fBex6enQsu0frUAJbx56DxhRvWawAysDMtLE50vaohrzkVV7Yaz4ClilwgfjQ=='
# >>>
# >>> key_str = 'correctstaplecorrectstaplecorrec'
# >>>
# >>> key_base64 = base64.b64encode(key_str.encode())
# >>> f = Fernet(key_base64)
# >>> f
# <cryptography.fernet.Fernet object at 0x7ff1628fb9b0>
# >>>
# >>> plain = f.decrypt(payload)
# >>> plain.decode()
# "\npw = input('What\\'s the password? ')\n\nif pw == 'batteryhorse':\n print('picoCTF{175_chr157m45_8aef58d2}')\nelse:\n print('That password is incorrect.')\n\n"
# >>>
Now we can already see the flag in the last output. If you execute the script and enter batteryhorse as the password, you get the following flag:
picoCTF{175_chr157m45_8aef58d2}