Collecting entropy from device_attach() times.

[Pawel Jakub Dawidek]

On Wed, Sep 19, 2012 at 09:29:23PM +0200, Pawel Jakub Dawidek wrote:
> On Wed, Sep 19, 2012 at 07:30:52PM +0100, Jonathan Anderson wrote:
> > > If all the times are more or less equally probable in this range […]
> > 
> > They're very unlikely to be equally probable. It would make sense to do some characterization of these times and their statistics: a highly non-uniform distribution would mean that we don't actually get many bits per attach.
> 
> I have times for ~2000 device_attach() calls when loading sound card
> driver on totally idle system. If someone could take those and analyse
> the distribution that would be great.
> 
> > > […] we have more
> > > than 19 bits of entropy from this one call, but I reduced if to four
> > > bits only, because there are devices that are much faster to attach.
> > >  
> > 
> > Another reason for doing the above characterization is that, if a particular device_attach() really does provide 12 bits of uncertainty, it's a shame to drop eight of them on the floor.
> 
> Right. That's why I've prepared another patch:
> 
> 	http://people.freebsd.org/~pjd/patches/harvest_device_attach.2.patch
> 
> which effectively discards top ten bits, which means we expect 0.1% of
> the attach time to be unpredictable (the attach time in most cases vary
> by few percent, not sure yet how much of this variation is really
> unpredictable).

Here's how the distribution looks like for device_attach() times of my
sound card. The times were 26bit numbers, so this is after discarding
top ten bits, which leave us with 16 lower bits of pure entropy:)

	http://people.freebsd.org/~pjd/misc/harvest_device_attach.png

Kudos to my friend Mariusz (CCed) who is mathematician and who helped me
with visualization and also promissed to prepare formal proof:)

-- 
Pawel Jakub Dawidek                       http://www.wheelsystems.com
FreeBSD committer                         http://www.FreeBSD.org
Am I Evil? Yes, I Am!                     http://tupytaj.pl
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