bugtraq - RE: OpenID/Debian PRNG/DNS Cache poisoning advisory

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Date: Fri, 8 Aug 2008 18:08:03 +0100
From: "Dave Korn" <dave.korn@...imi.com>
To: "'Eric Rescorla'" <ekr@...workresonance.com>
Cc: "'Ben Laurie'" <benl@...gle.com>, <bugtraq@...urityfocus.com>,
	<security@...nid.net>, "'OpenID List'" <general@...nid.net>,
	<cryptography@...zdowd.com>, <full-disclosure@...ts.grok.org.uk>
Subject: RE: OpenID/Debian PRNG/DNS Cache poisoning advisory

Eric Rescorla wrote on 08 August 2008 17:58:

> 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...

<scurries off to read CRL format in RFC>

  Oh, you can't specify them solely by key, you have to have all the
associated metadata.  That's annoying, yes, I understand your point now.

  IIRC various of the vendors' sshd updates released in the immediate wake
of the Debian catastrophe do indeed block all the weak keys.


    cheers,
      DaveK
-- 
Can't think of a witty .sigline today....