Extract Hikvision DAV file

from binascii import hexlify, unhexlify
from itertools import izip, cycle
import struct
from sys import argv

filename = argv[1]

with open(filename,'rb') as fware:
    header_structure = '<iiiiii'
    header = fware.read(struct.calcsize(header_structure))

    short_key = unhexlify('BACDBCFED6CADDD3BAB9A3ABBFCBB5BE')

    # The longer key just rotates the short_key left by 1 byte each time
    key = ''
    for i in range(0,16):
        key += short_key
        short_key = short_key[1:16] + short_key[0]

    # XOR decode the header
    header = ''.join(chr(ord(c1) ^ ord(c2)) for c1, c2 in izip(header, cycle(key)))

    magic_number, header_checksum, total_length, file_number, language, device_class = struct.unpack_from(header_structure, header, 0)

    print ('Magic number:    %08x' % magic_number)
    print ('Header checksum: %08x' % header_checksum)
    print ('Header length:   %08x' % total_length)
    print ('File number:     %08x' % file_number)
    print ('Language:        %08x' % language)
    print ('Device class:    %08x' % device_class)

    fware.seek(0)
    header = fware.read(total_length)

    header = ''.join(chr(ord(c1) ^ ord(c2)) for c1, c2 in izip(header, cycle(key)))

    # Header checksum is just a byte sum from the checksum to length
    print ('Calced checksum: %08x' % sum(ord(b) for b in header[12:]))

    # Extract the filenames and locations in the file.
    index = 64
    for _ in range(0, file_number):
        file_structure = '32siii'
        file_structure_size = struct.calcsize(file_structure)

        print hexlify(header[index:index+file_structure_size])
        file_name, file_start, file_length,checksum = struct.unpack_from('32siii', header, index)

        file_name = file_name.strip('\0')
        print ('File name:          %s' % file_name)
        print ('Start:              %08x' % file_start)
        print ('Length:             %08x' % file_length)
        print ('Checksum:           %08x' % checksum)

        index += file_structure_size

        fware.seek(file_start)

        file = fware.read(file_length)

        print ('Calced checksum:    %08x' % sum(ord(b) for b in file))

        with open(file_name,'wb') as outfile:
            outfile.write(file)