Certificates: limiting your liability with reuse limitations

Suppose I am a CA.  I am worried that by issuing a certificate with a 
lifespan of more than 2 milliseconds I am opening myself up to unlimited 
liability if for some reason, despite my best efforts, I issue an 
erroneous certificate.

I know I can write disclaimers, but that's not reliable since courts 
often ignore them, and anyway it scares off customers.

I know I can put an expiration date on the certificate, but that's not 
enough.  I can accumulate a lot of exposure in a few seconds, much less 
weeks.

I know I can put a reliance limit in the X.509 ver 3 certificate, but 
that's not enough.  Even a $1 limit could be used many millions of times.

Is it feasabile to say: Can only be relied on once per day/week/month?  
Is this something the relying parties can reasonably be expected to monitor?

It seems to me that this sort of a limit is essential if a CA is to feel 
comfortable outside Utah....

A. Michael Froomkin        | +1 (305) 284-4285; +1 (305) 284-6506 (fax)
Associate Professor of Law | 
U. Miami School of Law     | froomkin@law.miami.edu
P.O. Box 248087            | http://www.law.miami.edu/~froomkin
Coral Gables, FL 33124 USA | It's warm here.

You write:

>Suppose I am a CA.  I am worried that by issuing a certificate with a 
>lifespan of more than 2 milliseconds I am opening myself up to unlimited 
>liability if for some reason, despite my best efforts, I issue an 
>erroneous certificate.

How do notaries public get around this liability problem?  It seems to me
that the checking done for a certificate might be similar to the checking
done by a notary - a glance at a driver's license, say.  Are they subject
to liability if they are fooled by fake ID?

Hal

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A. Michael Froomkin writes:
> I know I can put an expiration date on the certificate, but that's not 
> enough.  I can accumulate a lot of exposure in a few seconds, much less 
> weeks.
> 
> I know I can put a reliance limit in the X.509 ver 3 certificate, but 
> that's not enough.  Even a $1 limit could be used many millions of times.
> 
> Is it feasabile to say: Can only be relied on once per day/week/month?  

This sounds like it would present the same exposure problems as an expiration
date, but perhaps be more difficult to impose. As you said above, you can
assume huge liability in a few seconds, even if you're only given a few
seconds a week. Also, I don't immediately see a way to arrange this on the
technical side that doesn't reduce to using something that expires and
replacing/refreshing it periodically. Of course, the net is in some ways
excellent for that sort of application.

How about combining value limits with time limits ?  Over the wire, using
low value limits and replacing them frequently might be a workable solution.

> Is this something the relying parties can reasonably be expected to monitor?

This sounds like a legal question, so I don't think I can offer a useful 
response. 

Futplex <futplex@pseudonym.com>       "I think every player in the NFL should 
	    	have to go through grad school. It would be a great humbler." 
	    -Matt Miller, Cleveland Browns 1979-1983, Ph.D. Georgia Tech 1993

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A. Michael Froomkin writes:
> I know I can put an expiration date on the certificate, but that's not
> enough.  I can accumulate a lot of exposure in a few seconds, much less
> weeks.
>
> I know I can put a reliance limit in the X.509 ver 3 certificate, but
> that's not enough.  Even a $1 limit could be used many millions of times.
>
> Is it feasabile to say: Can only be relied on once per day/week/month?

	Undeniable digital signatures.  They're not 'undeniable'
differently from normal digital signatures, but they do require the
cooperation of the signer to confirm the signature.  Thus, a KCA could
decide only to verify a signature 50 times, or once per day (or once
per being paid the $10 signature verification fee.)

	Schneier has a decent amount on undeniable digital signatures.

Adam


-- 
"It is seldom that liberty of any kind is lost all at once."
					               -Hume