':return:
A list of unicode strings that are URLs that should contain either
an individual DER-encoded X.509 certificate, or a DER-encoded CMS
message containing multiple certificates'

@property
def issuer_cert_urls(self):

    if (self._issuer_cert_urls is None):
        self._issuer_cert_urls = []
        if self.authority_information_access_value:
            for entry in self.authority_information_access_value:
                if (entry[u'access_method'].native == u'ca_issuers'):
                    location = entry[u'access_location']
                    if (location.name != u'uniform_resource_identifier'):
                        continue
                    url = location.native
                    if (url.lower()[0:7] == u'http://'):
                        self._issuer_cert_urls.append(url)
    return self._issuer_cert_urls
