def ocsp_url(self):
        """
        Location of the OCSP responder for this certificate. Will be one of the
        following types:

         - None for no OCSP responder
         - A unicode string of the URL to the OCSP responder
        """

        if self._authority_information_access is None:
            return None

        for ad in self._authority_information_access:
            if ad['access_method'].native == 'ocsp' and ad['access_location'].name == 'uniform_resource_identifier':
                return ad['access_location'].chosen.native

        return None