decrypt_dyn.py

import sys
import os

# Check if we have an argument.
if(len(sys.argv) <= 1):
    print('Usage: {} file.dyn'.format(sys.argv[0]))
    sys.exit(1)

# File names
input_filename = sys.argv[1]
output_filename = os.path.splitext(input_filename)[0] + '.zip'

# XOR key and shuffle index arrays for decryption.
CHUNK_SIZE = 8
key = bytearray([(0x8D + i) for i in range(CHUNK_SIZE)])
indices = [5, 3, 6, 7, 4, 2, 0, 1]

# Decrypt...
with open(input_filename, 'rb') as input_file, open(output_filename, 'wb') as output_file:
    while True:
        # Read in a chunk of raw data
        chunk = input_file.read(CHUNK_SIZE)
        if not chunk:
            break

        # Need a full 8-byte chunk to decrypt.
        if(len(chunk) == CHUNK_SIZE):
            output = bytearray(CHUNK_SIZE)

            # Decrypt the chunk.
            for i in range(CHUNK_SIZE):
                output[i] = chunk[indices[i]] ^ key[i]
        else:
            output = chunk

        output_file.write(output)


print('Done.')