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comment on distributed capabilities

From: Jonathan Rees <jar@creativecommons.org>
Date: Fri, 12 Feb 2010 10:14:50 -0500
Message-ID: <760bcb2a1002120714x1d2a8d0sede2ad5f45a1253e@mail.gmail.com>
To: noah_mendelsohn@us.ibm.com
Cc: www-tag@w3.org
I wanted to recognize and address a point you raised on the call
yesterday, which is that "distributed capabilities" in the web-key
sense are not the same as "distributed capabilities" in the sense used
in some capability systems from the 1970s and 1980s. This is true. I
think you were referring to systems in which each node in the network
has a trusted capability kernel that all other nodes can trust. In
this situation it is possible to some extent to limit copying by
treating the right to copy as one of the rights controlled by
capability discipline. Clearly this regime cannot apply in a web
context where such general trust isn't available: an attacker can just
run a kernel that ignores the directive not to copy.

Even where you have trusted kernels, copy prevention may help prevent
accidental leaks, but it can't really prevent attacks. (Suppose A
shares a copy-limited capability X with an attacker B. B can share X
with crony C just by setting up a proxy Y that forwards messages to
and from X, and sharing Y with C.) Limits on copying only have
significant effect in the total absence of side communication channels
that would let B and C communicate, and that kind of confinement is
too... confining to be useful in the computing contexts we've been
talking about. Limits on the ability to copy individual capabilities
have fallen out of favor in the capability community, with attention
instead being shifted to more general mechanisms for leak control.
Because of the de-emphasis of confinement and copy limitation, many
people have been happy to drop the distinction between traditional
capabilities and string-representable keys and use "capability"
generically.

We could argue about what is the proper application of the term
"capability" but that's not important. I don't think anyone is trying
to pull a fast one by using the word "capability", but if it's a
sticking point for you we can agree to say that secret URIs such as
web-keys are used analogously with capabilities (as opposed to being
capabilities), or that the secret URI pattern is analogous to the
capability pattern (as opposed to being an instance of it).

The question of how easy it is to copy a key, either by mistake or by
attack or a combination, is relevant and we'll continue talking about
it.

Jonathan
Received on Friday, 12 February 2010 15:15:24 GMT

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