docs: def create_key(file_):
      """
      Create a key and save it into ``file_``.
  
      Note that ``file`` must be opened in binary mode.
      """
      pkey = crypto.PKey()
      pkey.generate_key(crypto.TYPE_RSA, 2048)
  
      file_.write(crypto.dump_privatekey(crypto.FILETYPE_PEM, pkey))
      file_.flush()