Why does Google +1 record my mouse movements?
This is only on pages with a Google +1 box on my website:
It seems to be firing off an event on every mouse move. Anyone know what it is doing? I searched on Google (perhaps I should try Bing for once on this one!) but no one seems to have written about it. Is it recording information about my visitors browsing habits? Is it some sort of CAPTCHA to detect human like behviour?
Example URL, press F12 in chrome, go to timeline and press record, then move your mouse around this page (it plus ones this question, don't worry):
For what it's worth (I can see this is going to be a popular question), I don't think there is anything sinister behind it, it might even be a useless artifact/bug, but if it is doing some sort of tracking, well, it seems a little deceptive to me.
Google +1 privacy policy
http://www.google.com/intl/en/privacy/plusone/
Google +1 Button Privacy Policy
June 28, 2011
The Google Privacy Policy describes how we treat personal information when you use Google’s products and services, including information provided when you use the Google +1 button. In addition, the following describes our additional privacy practices specific to your use of the +1 button.
Information we collect and how it is shared
The Google +1 button is a way for you to share information publicly with the world. The Google +1 button helps you and others receive personalized content from Google and our partners. The fact that you +1’d something will be recorded by Google, along with information about the page you were viewing when you clicked on the +1 button. Your +1’s may appear to others as an annotation with your profile name and photo in Google services (such as in search results or on your Google Profile) or elsewhere on websites and ads on the Internet.
We will record information about your +1 activity in order to provide you and other users with a better experience on Google services.
In order to use the Google +1 button, you need to have a public Google Profile visible to the world, which at a minimum includes the name you chose for the profile. That name will be used across Google services and in some cases it may replace another name you’ve used when sharing content under your Google Account. We may display your Google Profile identity to people who have your email address or other identifying information.
Use of the collected information
In addition to the above-described uses, the information you provide to us is used subject to our main Google Privacy Policy.
We may share aggregate statistics related to users’ +1 activity with the public, our users, and partners, such as publishers, advertisers, or connected sites. For example, we may tell a publisher that “10% of the people who +1’d this page are in Tacoma, Washington.”
Your choices
You may view the list of items you have +1’d on the +1 tab on your Profile. You can remove individual items from that list.
You may opt out of seeing +1 recommendations on third-party websites (including on ads on third-party sites) from people you know.
We will store data (such as your recent +1’s) locally in your browser. You may be able to access and clear this information in your browser settings.
More information
Google adheres to the U.S. Safe Harbor privacy principles. For more information about the Safe Harbor framework or our registration, see the Department of Commerce’s website.
Answer
It appears to be seeding a random number generator with your mouse movements.
The mouse move handler itself does something along the lines of the following:
var b = ((event.X << 16) + event.Y) * (new Date().getTime() % 1000000);
c = c * b % d;
if (previousMouseMoveHandler) previousMouseMoveHandler.call(arguments);
d is (screen.width * screen.width + screen.height) * 1000000, and c is a variable that starts out as 1.
All of this is wrapped in the scope of an anonymous function, which itself is immediately evaluated to return a function that is assigned to a property named "random". That returned function looks something like this:
var b = c;
b += parseInt(hash.substr(0,20), 16);
hash = MD5(hash);
return b / (d + Math.pow(16, 20));
hash, BTW, is a variable that starts out as the MD5 hash of the page's cookies, location, the new Date().getTime(), and Math.random().
(Note, of course, that Google may change the script returned at any time and hence invalidate this analysis)