Re: OpenID/Debian PRNG/DNS Cache poisoning advisory

[Dan Kaminsky <dan () doxpara com>]

Eric Rescorla wrote:
> 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...
Funnily enough I was just working on this -- and found that we'd end up 
adding a couple megabytes to every browser.  #DEFINE NONSTARTER.  I am 
curious about the feasibility of a large bloom filter that fails back to 
online checking though.  This has side effects but perhaps they can be 
made statistically very unlikely, without blowing out the size of a browser.

Updating the filter could then be something we do on a 24 hour 
autoupdate basis.  Doing either this, or doing revocation checking over 
DNS (seriously), is not necessarily a bad idea.  We need to do better 
than we've been.

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