code: def _get_cert_url(self):
          
          cert_url = self._data.get('SigningCertURL')
          if cert_url:
              if cert_url.startswith('https://'):
                  url_obj = urlparse(cert_url)
                  for trusted_domain in settings.BOUNCE_CERT_DOMAINS:
                      parts = trusted_domain.split('.')
                      if url_obj.netloc.split('.')[-len(parts):] == parts:
                          return cert_url
              logger.warning(u'Untrusted certificate URL: "%s"', cert_url)
          else:
              logger.warning(u'No signing certificate URL: "%s"', cert_url)
          return None
docs: Get the signing certificate URL.
          Only accept urls that match the domains set in the
          AWS_SNS_BOUNCE_CERT_TRUSTED_DOMAINS setting. Sub-domains
          are allowed. i.e. if amazonaws.com is in the trusted domains
          then sns.us-east-1.amazonaws.com will match.