full-disclosure-uk August 2008 archive
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full-disclosure-uk: Re: [Full-disclosure] OpenID/Debian PRNG/DNS

Re: [Full-disclosure] OpenID/Debian PRNG/DNS Cache poisoning advisory

From: Dan Kaminsky <dan_at_nospam>
Date: Fri Aug 08 2008 - 17:43:53 GMT
To: Eric Rescorla <ekr@networkresonance.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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