docs: def iter_transform(filename, key):
      """Generate encrypted file with given key.
  
      This generator function reads the file
      in chunks and encrypts them using AES-CTR,
      with the specified key.
  
      :param filename: The name of the file to encrypt.
      :type filename: str
      :param key: The key used to encrypt the file.
      :type key: str
      :returns: A generator that produces encrypted file chunks.
      :rtype: generator
      """
      # We are not specifying the IV here.
      aes = AES.new(key, AES.MODE_CTR, counter=Counter.new(128))
  
      with open(filename, 'rb+') as f:
          for chunk in iter(lambda: f.read(CHUNK_SIZE), b''):
              yield aes.encrypt(chunk), f