Subject: OCSP and Nonces

From: "Miguel Rodriguez" <[email-masked]>
Date: Fri, 26 Sep 2003 12:21:34 -0500

The use of nonces to prevent replay attacks is incompatible with the use
of cached responses.
 
From the client side, what is the difference between not including a
nonce at all and accepting responses without it?
 
From the server side, as Ryan Hurst said, the presence of a nonce can be
viewed as an indication that the client is looking for a “fresh”
response. What if the responder can’t provide one? 
 
Cached responses can be treated as CRLs (checked for time validity using
the ThisUpdate and NextUpdate fields). We don’t worry about replay
attacks when using CRLs. 
 
If we want to enable the client to work with both fresh and cached
responses we could include a nonce in the request and accept responses
without it. Of course this means that we are giving up prevention
against replay attacks. This prevention only makes sense for fresh
responses. The bad scenario is a replay attack for fresh responses, how
can we avoid it? Maybe by having a way to distinguish fresh responses
from cached ones. Then the client can reject a fresh response without
the nonce included in the request. 
 
Opinions are welcomed,
 
Miguel A. Rodriguez
Software Engineer
SeguriDATA
Panzacola 62, Piso 1
Colonia Villa Coyoacán
04000 - México, D.F.
Phone: +52 (55) 5339-0158
Fax: +52 (55) 5339 5829