document: An XPath-based preference language for P3P The Platform for Privacy Preferences ( P3P ) is the most significant effort currently underway to enable web users to gain control over their private information . The designers of P3P simultaneously designed a preference language called APPEL to allow users to express their privacy preferences , thus enabling automatic matching of privacy preferences against P3P policies . Unfortunately subtle interactions between P3P and APPEL result in serious problems when using APPEL : Users can only directly specify what is unacceptable in a policy , not what is acceptable ; simple preferences are hard to express ; and writing APPEL preferences is error prone . We show that these problems follow from a fundamental design choice made by APPEL , and can not be solved without completely redesigning the language . Therefore we explore alternatives to APPEL that can overcome these problems . In particular , we show that XPath serves quite nicely as a preference language and solves all the above problems . We identify the minimal subset of XPath that is needed , thus allowing matching programs to potentially use a smaller memory footprint . We also give an APPEL to XPath translator that shows that XPath is as expressive as APPEL .
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