Access Control (Capabilities)

Access control is central to computer security. Traditionally, we wish to restrict
the user to exactly what he should be able to do, no more and no less.
You might think that this only applies to legitimate users: where do attackers
fit into this worldview? Of course, an attacker is a user whose access should be
limited just like any other. Increasingly, of course, computers expose services
that are available to anyone – in other words, anyone can be a a legitimate user.
As well as users there are also programs we would like to control. For
example, the program that keeps the clock correctly set on my machine should
be allowed to set the clock and talk to other time-keeping programs on the
Internet, and probably nothing else1
Increasingly we are moving towards an environment where users choose what
is installed on their machines, where their trust in what is installed is highly
variable2 and where “installation” of software is an increasingly fluid concept,
particularly in the context of the Web, where merely viewing a page can cause
code to run.
In this paper I explore an alternative to the traditional mechanisms of roles
and access control lists. Although I focus on the use case of web pages, mashups
and gadgets, the technology is applicable to all access control.



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