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Password-capabilities: their evolution from the password-capability system into Walnut and beyond

Since we first devised and defined password capabilities as a new technique for building capability-based operating systems, a number of research systems around the world have used them as the bases for a variety of operating systems. Our original Password-Capability System was implemented on custom built hardware with a novel address translation and protection scheme specifically designed to support password-capabilities. The password-capability concept later formed the basis of Opal developed at the University of Washington, and Mungi from the University of New South Wales, both of which used commercially available hardware. A second generation password-capability based system, Walnut, was developed at Monash University in the 1990s. Walnut was designed to run on commercially available hardware. It addressed some shortcomings of the original Password-Capability System but had to sacrifice some features that depended on hardware support. A third generation system that will extend Walnut to support mandatory security policies and other advanced features is currently being considered. This paper analyses the evolution of the Password-Capability System into Walnut, examines the shortcomings of the systems, and identifies issues to be addressed in the new system.