code: def signature_type(self):
          
          if not self.mardata.signatures:
              return None
  
          for sig in self.mardata.signatures.sigs:
              if sig.algorithm_id == 1:
                  return 'sha1'
              elif sig.algorithm_id == 2:
                  return 'sha384'
          else:
              return 'unknown'
docs: Return the signature type used in this MAR.
  
          Returns:
              One of None, 'unknown', 'sha1', or 'sha384'