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

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Date: Sat, 9 Aug 2008 00:29:52 +0200
From: "Stefan Kanthak" <stefan.kanthak@...go.de>
To: "Dan Kaminsky" <dan@...para.com>,
	"Eric Rescorla" <ekr@...workresonance.com>
Cc: "Dave Korn" <dave.korn@...imi.com>,
	"'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

Dan Kaminsky wrote:
> 
> 
> 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.

At least for the weak keys kudos to Debian's OpenSSL maintainer
there exists an extension to Firefox which checks the keys, see <http://codefromthe70s.org/sslblacklist.asp>, as well as c't's
SSLguardian for the Windows Crypto API, see
<http://www.heise-online.co.uk/security/features/111039/0> and
<http://www.heise-online.co.uk/security/features/111039/1>

Stefan