scriptCTF{mY_buck37_1s_l34k1ng!} Challenge
I am from the future! I accidentally forgot to link chall.zip! Surely you can find it and solve it right?
Solution
The missing archive could still be downloaded from the same S3 path used by the other challenges. It contained chall.py and enc.txt.
The encryption code seeded Python’s random module with the current Unix timestamp, then used random.randbytes(32) as an AES key:
random.seed(int(time.time()))
key = random.randbytes(32)
enc = AES.new(key, AES.MODE_ECB).encrypt(pad(flag, 16))This makes the key predictable if the encryption time is known. The ZIP metadata for enc.txt contained the UTC timestamp 2069-11-30 11:39:00, matching the hint about being from the future.
I tried the nearby timestamps, regenerated each key, and decrypted the ciphertext. The correct seed was 33 seconds after the stored minute:
2069-11-30 11:39:33 UTCFlag
scriptCTF{mY_buck37_1s_l34k1ng!}
Verification and Defensive Perspective
The search space was deliberately kept narrow: parse the ZIP timestamp as UTC, test nearby epoch seconds, regenerate random.randbytes(32) for each candidate, decrypt with AES-ECB, and accept only correctly padded plaintext beginning with the known scriptCTF{ prefix. This makes the recovery deterministic and avoids relying on visual inspection of random plaintext.
The cryptographic primitive was not broken; key generation was. Python’s random module is deterministic and is not suitable for secrets. A secure implementation should use secrets.token_bytes(32) or os.urandom(32), use an authenticated mode such as AES-GCM, and never derive a key solely from public or guessable time metadata.