docs: def __update_in_toto_layout_pubkeys(self):
          '''
          NOTE: We assume that all the public keys needed to verify any in-toto
          root layout, or sublayout, metadata file has been directly signed by
          the top-level TUF targets role using *OFFLINE* keys. This is a
          reasonable assumption, as TUF does not offer meaningful security
          guarantees if _ALL_ targets were signed using _online_ keys.
          '''
  
          target_relpaths = []
          targets = self.__updater.targets_of_role('targets')
  
          for target in targets:
              target_relpath = target['filepath']
  
              # Download this target only if it _looks_ like a public key.
              if target_relpath.endswith('.pub'):
                  # NOTE: Avoid recursively downloading in-toto metadata for
                  # in-toto root layout pubkeys themselves, and so on ad
                  # infinitum.
                  self.__get_target(target_relpath, download_in_toto_metadata=False)
                  target_relpaths.append(target_relpath)
  
          return target_relpaths