code: def authenticate(self, req_data, identifier: Optional[str]=None,
                       signature: Optional[str]=None, threshold: Optional[int] = None,
                       verifier: Verifier=DidVerifier):
          
          to_serialize = {k: v for k, v in req_data.items()
                          if k not in self.excluded_from_signing}
          if req_data.get(f.SIG.nm) is None and \
                  req_data.get(f.SIGS.nm) is None and \
                  signature is None:
              raise MissingSignature
          if req_data.get(f.IDENTIFIER.nm) and (req_data.get(f.SIG.nm) or
                                                signature):
              try:
                  # if not identifier:
                  identifier = identifier or self._extract_identifier(req_data)
  
                  # if not signature:
                  signature = signature or self._extract_signature(req_data)
  
                  signatures = {identifier: signature}
              except Exception as ex:
                  if ex in (MissingSignature, EmptySignature, MissingIdentifier,
                            EmptyIdentifier):
                      ex = ex(req_data.get(f.IDENTIFIER.nm), req_data.get(f.SIG.nm))
                  raise ex
          else:
              signatures = req_data.get(f.SIGS.nm, None)
          return self.authenticate_multi(to_serialize, signatures=signatures,
                                         threshold=threshold, verifier=verifier)
docs: Prepares the data to be serialised for signing and then verifies the
          signature
          :param req_data:
          :param identifier:
          :param signature:
          :param verifier:
          :return: