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What do people refer to when they talk about browser fingerprinting?

What is the difference between browser fingerprinting and linkability?

Why is it important?

For what is it used?

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Browser fingerprinting is the idea to have a unique set of property which can identify a browser. When you are visiting a website your browser identifies to the webserver. The site Whats my user agent shows you this information. Usually you'll find the operating system (Windows, Linux, Mac, Android etc.), the name of your browser plus the version and maybe installed plugins. This gives a first good idea about the used browser. However with techniques like JavaScript etc. it is possible to read a list of installed plugins, the fonts, the screen size and other things. All this information gives a more or less unique view of the browser. EFF'S Panopticlick site does extensive fingerprinting. When you use a fresh installed browser it can identify the browser from one in three million.

Linkability tries to chain different information with the goal to deanonymize you or to find more information about you. Assume you use a browser where you sometimes use Tor and visit this site (Tor.SE). The site also loads contents from Google Analytics and Quantcast. If you're visiting any other site which also loads those components, regardless if you use Tor or not, Google Analytics as well as Quantcast might be able to link this information. This means they can say user X first visited Tor.SE and later went to site XYZ. In between those sessions he toggled Tor.

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A browser fingerprint is a collection of info about a browser. When you visit a website, you send a variety of information, like screen size, installed fonts, and software versions; the site can also ask you for much more, as demonstrated by BrowserSpy. Like human fingerprints, browser fingerprints are usually unique. With your browser fingerprint governments and advertising companies can track you across the web, making a list of the things you've looked at, the places you've been, and what you've said and done.

Linkability is the ability to link two or more profiles, identities, or pieces of data. For example, imagine you have a Gmail account and a Facebook account; you have them registered under different names, and believe nobody can trace them back to you, or to each other. In reality, the advertisers and government agencies have linked you to both of them by using your browser fingerprint to follow you across the web.

The Tor Browser Bundle helps to prevent this kind of attack by making its fingerprint the same as everyone else's -- among other things -- thus making it more difficult to track you, identify you, and link your various identities to one person. This is, like most things, imperfect. Most websites use other companies to host their data and offer services and ads. Every time you visit one of these sites, you are by extension visiting all the others. This leaves a very messy footprint; after visiting just two sites, you may actually have notified dozens. There is a browser extension that makes a very neat visualization of this. A good way to avoid this is to block content: installing AdBlock would prevent ads from being downloaded, and thus those companies/sites would not be aware of you. The same goes for Flash, JavaScript, and even images.

Interesting links:

Panopticlick

BrowserSpy

Wikipedia Article

Device Fingerprinting (Under the JS Display menu, cursor position is among the reported data.)

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