We describe the design, implementation, and performance of a new system for access control on the web. To achieve greater exibility in forming access-control policies - in particular, to allow better interoperability across administrative boundaries - we base our system on the ideas of proof-carrying authorization (PCA). We extend PCA with the notion of goals and sessions, and add a module system to the proof language. Our access-control system makes it possible to locate and use pieces of the security policy that have been distributed across arbitrary hosts. We provide a mechanism which allows pieces of the security policy to be hidden from unauthorized clients. Our system is implemented as modules that extend a standard web server and web browser to use proof-carrying authorization to control access to web pages.


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