resign a kwz for flipnote studio 3d. handles signature and all crc32 checksums.

kwzSignature.py

# usage: python3 kwzSignature.py <privkey.pem> <target.kwz>

# rsa module installed with:
# pip3 install rsa
# documentation here:
# https://stuvel.eu/files/python-rsa-doc/usage.html

import struct
import rsa 
import zlib
from sys import argv

with open(argv[1], "rb") as pem, open(argv[2], "rb+") as flipnote:
  key = rsa.PrivateKey.load_pkcs1(pem.read())

  flipnote.seek(0, 2)
  body_length = flipnote.tell() - 256
  flipnote.seek(0)

  offset = 0
  while offset < body_length:
    flipnote.seek(offset)
    magic, length = struct.unpack("<3sxI", flipnote.read(8))
    magic = str(magic, "utf-8")

    if magic in ["KFH", "KTN", "KMC"]:
      flipnote.seek(4, 1)
      content = flipnote.read(length - 4)
      checksum = zlib.crc32(content)
      flipnote.seek(-length, 1)
      flipnote.write(struct.pack("<I", checksum))

    if magic == "KSN":
      # skip section header + sound meta
      flipnote.seek(offset + 8 + 28)
      # read content excluding meta
      content = flipnote.read(length - 28)
      # recalc checksum
      flipnote.seek(offset + 8 + 24)
      checksum = zlib.crc32(content)
      flipnote.write(struct.pack("<I", checksum))

    offset += length + 8

  flipnote.seek(0)
  body = flipnote.read(body_length)
  signature = rsa.sign(body, key, "SHA-256")
  flipnote.write(signature)