Re: OpenID/Debian PRNG/DNS Cache poisoning advisory

From: Dan Kaminsky <dan@doxpara.com>
To: Eric Rescorla <ekr@networkresonance.com>
Cc: Dave Korn <dave.korn@artimi.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:




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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