On Wed, Sep 19, 2012 at 8:29 PM, Pawel Jakub Dawidek <[email protected]> wrot= e: > 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 [=85] >> >> 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-uni= form distribution would mean that we don't actually get many bits per attac= h. > > 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. > >> > [=85] 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 partic= ular device_attach() really does provide 12 bits of uncertainty, it's a sha= me to drop eight of them on the floor. > > Rights. That's why I've prepared another patch: > > http://people.freebsd.org/~pjd/patches/harvest_device_attach.2.pa= tch > > 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). This is the wrong thing to do! There's no reason to discard bits on input (modulo the device throwing away inputs, that is) - just reduce your entropy estimate. "Extra" bits do no harm. _______________________________________________ [email protected] mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-security To unsubscribe, send any mail to "[email protected]"