After google started [using https for signed-in users](http://googleblog.blogspot.com/2011/10/making-search-more-secure.html), Google Analytics started [showing "(not proviced)" as the keyword](http://analytics.blogspot.com/2011/10/making-search-more-secure-accessing.html) when a user enters a site via https organic search. For example:!["(not provided)"](https://i.stack.imgur.com/yGg9e.png)

Given that the referrer is not available when a user is using HTTPS, how would Gooogle Analytics be able to tell that an HTTPS user came from a search engine?