Yahoo is now defaulting all their search results over SSL, which is secure search but also means that pesky [not provided] figure will increase.
Danny Sullivan broke the story at Search Engine Land yesterday. In short, if you do a search at Yahoo.com or search.yahoo.com, the results will direct to https://search.yahoo.com/. When a searcher clicks from the search results to your site, you won't be getting the referrer data, nor the query data. Well, that is if your site is not secure itself.
Google shows not provided for 90%+ of the search referrers. Google is specifically not going with protocol, they are wiping out the search referrer data even if the searcher is going from HTTPS to HTTPS, i.e. from Google to your secure site.
Yahoo does not do that, Yahoo will pass the referrer and query data if your site is secure.
Bing just added SSL support to search but you need to actively go to the HTTPS version of Bing to see it. When you do, the referrer data is not passed, like Yahoo, to an unsecure site. Which again is protocol. But from HTTPS to HTTPS, it does pass, unlike Google.
Danny summed it up well, if you have a standard HTTP site, then:
- Google: secure search is the default, Google referrer passed but search terms stripped, except for advertisers.
- Bing: secure search is optional, no referrers passed
- Yahoo: secure search is the default, no referrers passed
I decided to launch a What Is My Secure Referrer? site, so when Google, Bing and Yahoo index it, you can search for What Is My Secure Referrer? and click on the https://referrer.rustybrick.com URL and it will report back your referrer. There are plenty of sites that show the non HTTPS referrer path.
Forum discussion at Google+.