Subject: RE: OCSP request

From: Simon Tardell <[email-masked]>
Date: Tue, 25 Jul 2000 16:06:34 +0200

> > Why isn't it sufficient to get a fresh OCSP response? The OCSP response
> > itself contains either a revocation date or "good". The  first you can
> > compare against your time interval, and in the latter case you have the
> > validity period of the certificate... .
> 
> If I understand everything correctly, Karl is after verifying 
> the signature of a
> 
> document that was signed, say, 3 months ago.  2 months ago, the
certificate
> got compromised and is therefore not "good" anymore, but it may still be
> interesting to check if the certificate was valid at the time of
signature, and
> from what I understand item 2 above, Karl would like to see 
> the possibility to do that kind of check with OCSP.

If you don't allow for certificate suspension there is no problem. If the
certificate revocation date is less than two months ago, it's ok. If the
certificate is expired now, you also have to know if and for how long the
status of the certificate will be kept in the database. Certificate
suspension is not allowed under German signature law, and perhaps also not
Austrian, so if we're talking SigG compliant certs there is no such need.

Now, if a key gets compromised, you cannot trust *any* signatures by that
key (before or after compromise), because you cannot trust the signer's
assertion of what time it was. A practical system has to protect itself from
that somehow. Enters time stamping. A time stamping service would probably
check the revocation status of the signatory cert and store that together
with the time stamp. 

If you are willing to take the consequences reverse time effects of key
compromise (you don't do timestamping), and if you allow for certificate
suspension, the question "was this certificate valid at this time" makes
sense.

Simon

Simon Tardell, Software Architect, iD2 Technologies
voice +46 8 7755225, fax +46 8 7267912, cell +46 70 3198319
[email-masked], http://www.iD2tech.com
Securing the Internet economy

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From: Denis Pinkas <[email-masked]>
Date: Thu, 27 Jul 2000 11:12:41 +0200

Simon,

Bob picked up the same sentence in one of your E-mails and replied.
I would like to comment on the same sentence.

(text deleted) 

> assertion of what time it was. 

This is untrue. See the paper I co-wrote with Hans Nilson about the
rational for using a time stamp. If the key of the TSA is not
compromised, then you can prove that a given digital signature was
done after the key compromise.

> A practical system has to protect itself from

This is certainly not the definition of a time stamping service
within the PKIX WG. The TSA does NOT "check the revocation status of
the signatory cert and store that together with the time stamp".
 
Denis