As computing environments become both mobile and pervasive, the need for robust and flexible access control systems comes to the fore. Instead of relying simply on identity-based mechanisms or multi-level classifications, modern information systems must incorporate contextual factors into the access control decision. Examples of these factors include the user's location at the time of the request, the unique instance of the hardware device, and the history of previous accesses.;Designing and implementing such contextual access control mechanisms requires addressing a number of interesting challenges. First, one must be able to determine when the required policy conditions are satisfied. For instance, in the realm of spatially aware access control, the system must be able to validate user's claims to a particular location at a given time. Next, contextual mechanisms must be able to detect and react to changes in the environmental conditions, such as when a connection becomes disrupted. Finally, the integrity of the execution environment must be ensured, despite the complexity of modern computing systems.;To address these challenges, we have examined the creation of trusted enforcement mechanisms that are built on a combination of secure hardware, cryptographic protocols, virtual machine monitors, and randomized execution environments. We have developed a number of prototypes using NFC, PUFs, VMMs, and a microkernel OS to demonstrate the feasibility of our approaches to a number of contextual settings. Our experimental evaluation and security analyses demonstrate that robust mechanisms can be deployed for a minimal amount of computational expense.
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