Ugly python code to decrpyt header2 from tjc.tft files

tjc3224T132_header2_decrypter.py

import argparse
from pathlib import Path
import struct

NONCE = struct.pack("III", 0x92930f5e, 0x924f6584, 0xfbad3bbf)
KEY = struct.pack("II", 0x582659b6, 0xbd829c9c)

if __name__ == "__main__":
    desc = """TFT header2 decoder
              Developped by Clark Scheff, licensed under GPLv3"""
    parser = argparse.ArgumentParser(description=desc)
    group = parser.add_mutually_exclusive_group(required=True)
    group.add_argument("-i", "--input", metavar="TFT_FILE", type=str,
                        help="Path to the TFT file")

args = parser.parse_args()

tftPath = Path(args.input)
if not tftPath.exists():
    parser.error("Invalid source file!")

header2 = None

with open(tftPath, 'rb') as f:
    f.seek(200)
    header2 = f.read(200)

if header2 is not None:
    header_size = len(header2)
    word_idx = 0
    nonce_bytes = bytearray(NONCE)
    while word_idx < header_size:
        header_word = struct.unpack("I", header2[word_idx:word_idx+4])[0]
        rk_idx = int(word_idx / 4) % len(KEY)
        key_byte = KEY[rk_idx]
        start_nonce1, start_nonce2, start_nonce3 = struct.unpack("III",  nonce_bytes)

        nonce1 = (start_nonce1 << 1) & 0xffffffff
        nonce_bytes[0:4] = struct.pack("I", nonce1)
        nonce_byte = nonce_bytes[header_size & 0x03]
        nonce_bytes[header_size & 0x03] = ((key_byte ^ 0xbc) + nonce_byte) & 0xff
        nonce1 = struct.unpack("III", nonce_bytes)[0]

        nonce2 = (start_nonce2 << 2) & 0xffffffff
        nonce_bytes[4:8] = struct.pack("I", nonce2)
        b_idx = 3 - (key_byte & 0x03)
        nonce_byte = nonce_bytes[4 + b_idx]
        nonce_bytes[4 + b_idx] = ((key_byte ^ 0x58) + nonce_byte) & 0xff
        nonce2 = struct.unpack("III", nonce_bytes)[1]

        new_nonce = ~(((nonce1 + nonce2) ^ start_nonce3) + word_idx + 8) & 0xffffffff
        new_nonce2 = (new_nonce + start_nonce3) & 0xffffffff
        new_header_word = (header_word - new_nonce2) & 0xffffffff
        new_header_word = (new_header_word ^ new_nonce) & 0xffffffff

        nonce3 = (start_nonce3 << 3) & 0xffffffff
        nonce_bytes[8:12] = struct.pack("I", nonce3)
        b_idx = 3 - (key_byte & 0x03)
        nonce_byte = nonce_bytes[8 + b_idx]
        nonce_bytes[8 + b_idx] = ((key_byte ^ 0x7a) + nonce_byte) & 0xff
        nonce3 = struct.unpack("III", nonce_bytes)[2]
        
        result1 = (start_nonce2 + start_nonce3 + nonce1) & 0xffffffff
        result = (result1 + start_nonce2) & 0xffffffff
        new_header_word = ((new_header_word - result) ^ result1) & 0xffffffff

        result1 = (start_nonce1 + start_nonce2) & 0xffffffff
        b = (key_byte & 0x07)
        result1 = ((start_nonce3 << b) + result1) & 0xffffffff
        result = (result1 + start_nonce1) & 0xffffffff

        new_header_word = ((new_header_word - result) ^ result1) & 0xffffffff

        print(f'0x{word_idx+0xc8:03x}: 0x{header_word:08x} --> 0x{new_header_word:08x}')
        word_idx += 4