How P3P Works

The Platform for Privacy Preferences (P3P) is designed to let Internet users control the way their data is collected and used by the Web sites they visit. P3P can be built into Web browsers, proxy servers, or browser plug-ins.

Personal Data Sources

P3P compares a site’s privacy policy with the privacy preferences that a user has specified in her browser. If the policy and preferences match, the Web site can request that data about the user be transmitted to the Web site. That data comes from two sources: cookies and data that the user explicitly enters into the browser.

Cookies are text fragments that a Web site stores on a visitor’s computer. They allow Web sites to preserve “state” information, such as which items are in a user’s shopping cart. While they can contain personal information, such as an encrypted user ID, cookies can’t be used to view any personal information that the user has not volunteered to the site.

The user can also store data, such as her e-mail address and full name, in settings that the browser can access. Microsoft’s Internet Explorer 6 (IE6), one of the few client programs to implement P3P, can use a profile stored in the Windows address book.