Re: OpenID/Debian PRNG/DNS Cache poisoning advisory

From: Eric Rescorla <ekr@networkresonance.com>
To: Dave Korn <dave.korn@artimi.com>
Cc: 'Eric Rescorla' <ekr@networkresonance.com>,'Ben Laurie' <benl@google.com>,bugtraq@securityfocus.com,security@openid.net,'OpenID List' <general@openid.net>,cryptography@metzdowd.com,full-disclosure@lists.grok.org.uk
Subject: Re: OpenID/Debian PRNG/DNS Cache poisoning advisory
Date:


At Fri, 8 Aug 2008 17:31:15 +0100,
Dave Korn wrote:
> 
> Eric Rescorla wrote on 08 August 2008 16:06:
> 
> > At Fri, 8 Aug 2008 11:50:59 +0100,
> > Ben Laurie wrote:
> >> However, since the CRLs will almost certainly not be checked, this
> >> means the site will still be vulnerable to attack for the lifetime of
> >> the certificate (and perhaps beyond, depending on user
> >> behaviour). Note that shutting down the site DOES NOT prevent the attack.
> >> 
> >> Therefore mitigation falls to other parties.
> >> 
> >> 1. Browsers must check CRLs by default.
> > 
> > Isn't this a good argument for blacklisting the keys on the client
> > side?
> 
>   Isn't that exactly what "Browsers must check CRLs" means in this context
> anyway?  What alternative client-side blacklisting mechanism do you suggest?

It's easy to compute all the public keys that will be generated
by the broken PRNG. The clients could embed that list and refuse
to accept any certificate containing one of them. So, this
is distinct from CRLs in that it doesn't require knowing 
which servers have which cert...

-Ekr





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