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We participated in UoTCTF 2025 as 0nePadding and placed 43rd out of 897 teams. (Though I personally didn’t solve many challenges this time.)
\nSince this file appeared to be a .NET PE file, I proceeded to analyze it with ILSpy.
\nHowever, for some reason, ILSpy was unable to analyze parts of the code.
\nSo I tried using dnSpy instead, which allowed me to view the decompiled results of the challenge binary as shown below. (The reason ILSpy failed to analyze it is unclear.)
\nWith this, I was able to decompress the encrypted RAR file and extract a file called flag.py.
\nThis flag.py appears to encrypt the flag with AES and save it as flag.enc.
\nfrom Crypto.Cipher import AES\nfrom Crypto.Util.Padding import pad\nfrom Crypto.Hash import SHA256\nfrom time import time\nimport random\nrandom.seed(int(time()))\nKEY = SHA256.new(str(random.getrandbits(256)).encode()).digest()\nFLAG = \"uoftctf{fake_flag}\"\n\ndef encrypt_flag(flag, key):\n cipher = AES.new(key, AES.MODE_EAX)\n ciphertext, tag = cipher.encrypt_and_digest(flag.encode())\n return cipher.nonce + ciphertext\n\ndef main():\n encrypted_flag = encrypt_flag(FLAG, KEY)\n with open(\"flag.enc\", \"wb\") as f:\n f.write(encrypted_flag)\n\nif __name__ == \"__main__\":\n main()When searching for the flag.enc file that contains the encrypted correct flag, I found that it was stored as an Alternate Data Stream (ADS) of flag.py.
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