def _from_cookie(self):
     z = read_user_cookie(self._cookie)
     try:
         d = simplejson.loads(decrypt(z))
         return dict((k, v) for k, v in d.iteritems() if k in self.allowed_prefs)
     except ValueError:
         return {}

    def _from_cookie(self):
        cookie = c.cookies.get(self.COOKIE_NAME)
        if not cookie:
            return {}

        try:
            return self._decode_json(cookie.value)
        except ValueError:
            # old-style _options cookies are encrypted
            try:
                plaintext = decrypt(cookie.value)
                values = self._decode_json(plaintext)
            except (TypeError, ValueError):
                # this cookie is totally invalid, delete it
                c.cookies[self.COOKIE_NAME] = Cookie(value="", expires=DELETE)
                return {}
            else:
                self._to_cookie(values)  # upgrade the cookie
                return values

 def test_encryption_roundtrip(self):
     tested = tracking.decrypt(tracking.encrypt(MESSAGE))
     self.assertEquals(MESSAGE, tested)