[cap-talk] Scope/span of capability systems

Marcus Brinkmann marcus.brinkmann at ruhr-uni-bochum.de
Tue Mar 3 07:04:48 EST 2009

David-Sarah Hopwood wrote:
> Marcus Brinkmann wrote:
>> Bill Frantz wrote:
>>> marcus.brinkmann at ruhr-uni-bochum.de (Marcus Brinkmann) on Thursday, February 26, 2009 wrote:
>>>> Capabilities can only survive in an isolated, homogeneous environment.  I
>>>> think that this is a serious limitation, which in my opinion severely
>>>> restricts the applicability of capability theory.
>>> This statement is wrong on the face of it. Any data-as-capability (e.g.
>>> WebKeys, SPKI authorizations, etc.) can be securely passed through systems,
>>> such as encrypted email, that are completely unaware of capabilities, let
>>> alone the precise capability system they represent.
>> That is just a transport issue, and not what I meant.  If you send me a capability-as-data
>> over any channel, what can I do with it?  Nothing useful, until I feed it back into a system that
>> accepts the data as a valid capability for anything.  For that to happen, the system must
>> somehow be in rather intimate contact (and if only by following the same P2P protocol) with the
>> system from which the capability originated.  It does not need to be the same system, but surely
>> all such systems form a common domain.  This domain is the isolated, homogeneous environment I am
>> talking about.
> Your definition of an "isolated, homogenous environment" would apply just as
> well to the world-wide web. I don't think many people would agree that this
> is a useful definition; it seems quite misleading to me.

I accept that it applies to the web.  The web would be utterly useless without
the standards that hold it together.  And we have some experience with how
those standards work.  The more low-level the standard is, the better is
compliance.  Compliance to TCP/IP, DNS, etc is very good.  With SMTP we
already see some divisions, and HTTP compliance of browsers is the fabric of
legends.  But it only gets really nasty once you enter the world of
application services, where nothing interoperates and even the users have no
control over what happens to their data.  Take for example OpenID: Everybody
wants to be a service provider, but nobody wants to accept other service
providers.  Why should the situation be different if capabilities are
introduced into the mix?  The incentives just are not there.

In a world where we can't even agree on compatible data formats, I see little
chance to agree on compatible capability interfaces, which have potentially a
much higher degree of complexity.


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