Since the beginning of March there has been a huge increase of traffic sources that show KEYWORD: (not provided) in the overview of Google Analytics.
The same problem exists when you are using Piwik web analytics, where it says Keyword not defined instead.
This is the result of an improvement(?) Google announced back in October to “increase the privacy and security of your web searches”. Users that are signed in to Google are now automatically forwarded to the SSL protected version https://www.google.com. Unfortunately this site does not submit the search terms to your website and your analytics software therefore does not know what keywords the user entered to find your page.
This is not a technical limitation of https but something Google intentionally choose to block on its SSL encrypted version. Funnily enough the search terms are not being blocked when you click on an ad. So much for your privacy.
What you can do to solve this? Absolutely nothing unfortunately. Google recommends to use Google Webmaster Tools which will show you the top 1000 search queries for your site but this only gives you information about your whole domain and no detailed information about single pages. I expect that in the future search term referrer data will be less and less available with new laws and regulations and users becoming more aware of privacy issues.
This is written from the view of a website provider of course – As a user it is a good thing to use an SSL encrypted search engine and to not have your search terms submitted to other websites. However, it leaves a bad taste in my mouth that advertisers still have access to search queries while other website owners don’t. I also wonder if Google will integrate the data which it still collects and partly shows in Webmaster Tools into Google Analytics while leaving other analytics software in the dark.