Re: OpenID/Debian PRNG/DNS Cache poisoning advisory

[Nicolas Williams <Nicolas.Williams () sun com>]

On Fri, Aug 08, 2008 at 11:20:15AM -0700, Eric Rescorla wrote:
> At Fri, 08 Aug 2008 10:43:53 -0700,
> Dan Kaminsky wrote:
> > 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.
>
> Why do you say a couple of megabytes? 99% of the value would be
> 1024-bit RSA keys. There are ~32,000 such keys. If you devote an
> 80-bit hash to each one (which is easily large enough to give you a
> vanishingly small false positive probability; you could probably get
> away with 64 bits), that's 320KB.  Given that the smallest Firefox
> [...]

You could store {<hash>, <seed>} and check matches for false positives
by generating a key with the corresponding seed and then checking for an
exact match -- slow, but rare.  This way you could choose your false
positive rate / table size comfort zone and vary the size of the hash
accordingly.

Nico
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