Recently, a non-programmer friend pointed out that there is no
simple explanation of capabilities available on the web. Less
than a week later, another friend -- this one a senior operating
system developer for a major UNIX software company -- said to me
that he really didn't understand what I was working on
(capabilities). If neither the lay reader nor the experts
understand what these things are, an introductory description is
definitely needed, so I set out to write one. I quickly
discovered that doing so isn't easy. Capabilities themselves are
very simple, but they turn everything people ``know'' about
security upside down. A lot of people look at them and say
``this is too simple to possibly work.'' This essay is a basic
introduction to capabilities and capability-based systems.
Rather than try to give an exhaustive discussion, I have taken
some editorial liberties in the interest of clarity: This essay
will tell you what the term access control covers, what a
capability is, and provide some examples of