decrypt_flag.py

"""
decrypt_flag.py — AES-256 Encrypted Zip Decryption
Project Helix CTF · TCM Security
Author: Dibyanshu Sekhar

Decrypts flag.zip using the RNA-decoded password.
Standard unzip can't handle AES-256 (PK compat v5.1) — pyzipper can.

Usage:
    python decrypt_flag.py --zip /tmp/flag.zip [--password "|RNAPolymeraseSlip|"]

Dependencies:
    pip install pyzipper
"""

import argparse
import pyzipper


# Passwords tried before arriving at the correct one.
# Documented for educational purposes — specifically the "delimiters are data" lesson.
PASSWORD_GRAVEYARD = [
    b'eraseSlip||RNAPolym',    # Wrong-framed single-cycle decode
    b'eraseSlipRNAPolym',      # Without pipes
    b'RNAPolymeraseSlippage',  # Full biology term
    b'RNAPolymeraseSlip',      # Right text, missing delimiters ← SO CLOSE
    b'3817.3',                 # The frequency
    b'helix',                  # Machine name
    b'HELIX',
    b'drowens',                # Username
    b'xKeyJTOPm78=',           # Registry red herring
]

CORRECT_PASSWORD = b'|RNAPolymeraseSlip|'


def attempt_decrypt(zip_path: str, password: bytes, verbose: bool = False) -> str | None:
    """Attempt to decrypt and read flag.txt from the zip."""
    try:
        with pyzipper.AESZipFile(zip_path) as z:
            z.setpassword(password)
            content = z.read('flag.txt')
            return content.decode('utf-8').strip()
    except (RuntimeError, pyzipper.BadZipFile, KeyError) as e:
        if verbose:
            print(f"  [-] Failed: {e}")
        return None


def main():
    parser = argparse.ArgumentParser(description='Decrypt flag.zip — Project Helix')
    parser.add_argument('--zip', default='/tmp/flag.zip', help='Path to flag.zip')
    parser.add_argument('--password', default=None, help='Password to try (default: correct one)')
    parser.add_argument('--graveyard', action='store_true', help='Also try the failed passwords first')
    args = parser.parse_args()

    print(f"[*] Target: {args.zip}\n")

    if args.graveyard:
        print("[*] Trying password graveyard first (for the lols)...")
        for pwd in PASSWORD_GRAVEYARD:
            result = attempt_decrypt(args.zip, pwd)
            status = f"✓ {result}" if result else "✗"
            print(f"  {pwd.decode()!r:<35} → {status}")
        print()

    password = args.password.encode() if args.password else CORRECT_PASSWORD

    print(f"[*] Trying: {password.decode()!r}")
    flag = attempt_decrypt(args.zip, password, verbose=True)

    if flag:
        print(f"\n[+] ══════════════════════════════════")
        print(f"[+]  FLAG: {flag}")
        print(f"[+] ══════════════════════════════════")
        leet = flag.replace('0', 'O').replace('1', 'I').replace('4', 'A').replace('7', 'T').replace('3', 'E')
        print(f"[+]  Leet: {leet}")
    else:
        print(f"[-] Decryption failed with password: {password.decode()!r}")
        print("    Double-check the pipes — they're part of the key.")


if __name__ == '__main__':
    main()