Subject: Re: Operational Protocols - LDAPv3

From: "David P. Kemp" <[email-masked]>
Date: Mon, 10 Jul 2000 11:31:58 -0400 (EDT)

> From: "David Chadwick" <[email-masked]>
> >
> >    [dpk] ... A CRL does not
> >    become invalid when the next CRL is issued, although some
> >    applications behave as though it does. 
> 
> We are playing with fine semantics here. Is a superceded CRL 
> invalid or not? No, its not technically invalid, as everything it contains 
> is good and true, but it is certainly incomplete (in the scenario we 
> are discussing). Is an incomplete CRL invalid or not? I would say 
> from a trust perspective yes, given that another CRL has been 
> issued and therefore one or more other certificates have been 
> revoked and I dont know which ones they are, therefore I cannot 
> rely on the superceded CRL, therefore it might as well be invalid. In 
> other words, for certificate X, where X is not in the list of revoked 
> certificates in the superceded CRL, using the superceded CRL or 
> using an invalid CRL give me exactly the same answer, viz: I dont 
> know if X has been revoked or not. The superceded CRL can only 
> be regarded as being valid, if X is in the list of revoked certificates. 
> 
> Conclusion. The superceded CRL is valid if it contains the certificate 
> we are checking, and is invalid otherwise.

This conclusion depends on some assumptions: all clients have the same
revocation freshness requirements, those requirements don't depend on
the value of the information being processed at the time, and the CA
is the controlling authority for how often clients must check CRLs.

In a world where communication is instantaneous, free, and 100%
reliable, there would be no reason for a client to ever use anything
but the most up-to-date revocation information from a CA, and your
conclusion would be correct.  But in a world where the cost of
communicating must be traded off against the cost of not communicating
(relying on a superceded CRL), I believe local administrators are in a
better position than a CA to make the tradeoff.

Putting the CA in control of CRL "validity" creates a perverse
incentive against improving service - the more frequently the CA issues
scheduled CRLs, the more the clients are going to complain about the
cost of retrieving them.  I prefer an environment where the CA is
incentivized to improve service - to advertise "status updates every hour",
"every 15 minutes" or "every minute", and fastest would be unequivocably
best.

Putting the CA in control of CRL "validity" turns what would normally
be a pull model into a push model - clients have to take the latest
information that has been generated, or else.  In my view, that isn't
the best way to improve security.

Regards,
Dave

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From: "David Chadwick" <[email-masked]>
Date: Mon, 10 Jul 2000 20:07:49 +0100

> > Conclusion. The superceded CRL is valid if it contains the
> > certificate we are checking, and is invalid otherwise.
> 
> 

Sorry David, I disagree with you on the above. They are irrelevant 
in my opinion to the statement I made. However, they may be 
relevant to the client (who is choosing to ignore the fact that a later 
CRL is available, but that is its choice. The fact still stands that the 
old CRL is not fully trustworthy (or valid) in an absolute sense. But it 
might be good enough for the client.)

David

> the best way to improve security.

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David Chadwick
IS Institute, University of Salford, Salford M5 4WT
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