Google Analytics uses only a first party cookie to identify visitors. That means your id is different on each site you visit. You therefore cannot track a unique visitor across the web using GA cookies.

Google Analytics requests are also only unencrypted if the site itself is unencrypted, so the fact that the GA request includes the referrer doesn't seem relevant (since the referrer would have already been transferred in the clear in the Referer header on the initial HTTP request.)