docs: def __read_master_key(self):
          '''
          Read in the rotating master authentication key
          '''
          key_user = self.salt_user
          if key_user == 'root':
              if self.opts.get('user', 'root') != 'root':
                  key_user = self.opts.get('user', 'root')
          if key_user.startswith('sudo_'):
              key_user = self.opts.get('user', 'root')
          if salt.utils.platform.is_windows():
              # The username may contain '\' if it is in Windows
              # 'DOMAIN\username' format. Fix this for the keyfile path.
              key_user = key_user.replace('\\', '_')
          keyfile = os.path.join(self.opts['cachedir'],
                                 '.{0}_key'.format(key_user))
          try:
              # Make sure all key parent directories are accessible
              salt.utils.verify.check_path_traversal(self.opts['cachedir'],
                                                     key_user,
                                                     self.skip_perm_errors)
              with salt.utils.files.fopen(keyfile, 'r') as key:
                  return salt.utils.stringutils.to_unicode(key.read())
          except (OSError, IOError, SaltClientError):
              # Fall back to eauth
              return ''