On Wed, Sep 19, 2012 at 9:20 PM, Pawel Jakub Dawidek <p...@freebsd.org> wrote: > On Wed, Sep 19, 2012 at 08:59:15PM +0100, Ben Laurie wrote: >> On Wed, Sep 19, 2012 at 8:29 PM, Pawel Jakub Dawidek <p...@freebsd.org> >> 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. >> > >> > Rights. 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). >> >> 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. > > I 'discard' ten bits from the estimation. I don't discard them by > zeroing them out. If the number is a 26 bit value then I feed entire > number, but pass estimation of 16 bits.
Sorry, should've read the code first! This is great. I also like your friend's analysis. _______________________________________________ freebsd-security@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-security To unsubscribe, send any mail to "freebsd-security-unsubscr...@freebsd.org"
