code: def aead_filename(aead_dir, key_handle, public_id):
      
      parts = [aead_dir, key_handle] + pyhsm.util.group(public_id, 2)
      path = os.path.join(*parts)
  
      if not os.path.isdir(path):
          os.makedirs(path)
  
      return os.path.join(path, public_id)
docs: Return the filename of the AEAD for this public_id,
      and create any missing directorys.