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Delegation and Policy
I offer these examples to address questions raised by some regarding delegation,
tags-as-capabilities, tags-as-attributes, needs for policy,and more. If I have it wrong, please feel free to point this out.
If a certificate is issued with delegation, it is implied that
the keyholder may use their key to certify other key/keyholders.
That is, for a set of tags T, assuming may-delegate,
KX!(KY,T) validates KY!(KZ,T') for T' = subset(T).
The nature of the issuer, subject and tags make some delegations natural and
others absurd. "Subject, may-login" seems a natural candidate to delegate, and looks like a capability. "Subject, has scar on left cheek" appears more like
an attribute, and delegation seems out of the question. Unfortunately this distinction likely cannot be codified.
A related case that kept nagging at me involved delegation of capability,
where the last-line certifiers must apply this capability to certify what
almost seems the same, but is not. Consider this example, a plausible
future where my personal key asserts my license-to-drive.
Let X represent the root authority for the department of
motor vehicles (to issue driver's licenses). As X cannot
possibly deal with the mass of people who need licenses,
they delegate this capability to regional offices Y, where
cadres of examiners Z to do the real work.
Let T represent the tag "may issue license-to-drive".
Assume KX!(KY,T) [may-delegate] and KY!(KZ,T) [no-delegate].
Z examines me, determines I am fit to drive, and certifies
such by signing my key KI with tag ... what? Certainly not
with any derivative of "may issue license-to-drive", which
they are not allowed to delegate in any case.
Somewhere, my key must gain the tag "HAS licence-to-drive".
One scenario would have Z simply issue me KZ!(KI,"has-license").
When I later get pulled over for speeding, I hand my certificate
(in a picture-ID smartcard) to the officer, whose reader transmits
it to X (the service) for a validation check. But this means that
X must collapse the Z-Y-X chain every time I get pulled over, and
also relies on the persistence of the Z-Y-X chain.
A reasonable scenario would have Z still form the certificate, but
immediately submit it to X for validation. The result could be a
CRCert that reads KX!(KI,"has-license") directly. But this means
the chain must be collapsed whenever Z issues a license.
Clearly, Z would submit their own certificate KY!(KZ,"may-issue")
to X when they joined the staff, cutting Y out of the chain and
gaining the CRCert KX!(KZ,"may-issue"), but with no delegation.
(Note that the intersections occur on the "may-issue-license",
tag, not on any "has-license" tag.)
Now X has a cert-conjunction policy, to wit:
WHERE: KX!((sub1),"may-issue") AND (sub1)!((sub2),"has-license")
Does this capture the right idea?
A more interesting question is how SPKI might facilitate the manner whereby the
"Authority-To-Issue-License-To-Drive" mesh gets reliably and securely linked
to the "Authority-To-Pull-People-Over-For-Speeding" mesh. Or more generally,
how do distinct authority-meshes benefit from each others certifications?
____TONY____ (just stirring the pot).