code: def _verify_signature(message, signature, certs):
      
      for pem in certs:
          verifier = Verifier.from_string(pem, is_x509_cert=True)
          if verifier.verify(message, signature):
              return
  
      # If we have not returned, no certificate confirms the signature.
      raise AppIdentityError('Invalid token signature')
docs: Verifies signed content using a list of certificates.
  
      Args:
          message: string or bytes, The message to verify.
          signature: string or bytes, The signature on the message.
          certs: iterable, certificates in PEM format.
  
      Raises:
          AppIdentityError: If none of the certificates can verify the message
                            against the signature.