Messages in this thread | | | Date | Sat, 16 Apr 2005 12:22:35 -0700 | From | Matt Mackall <> | Subject | Re: Fortuna |
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On Sat, Apr 16, 2005 at 05:16:22PM -0000, linux@horizon.com wrote: > > "How does the entropy estimator measure entropy of the event?" becomes a > > crucial concern here. What if, by your leading example, there is 1/2 bit > > of entropy in each event? Will the estimator even account for 1/2 bits? > > Or will it see each event as 3 bits of entropy? How much of a margin > > of error can we tolerate? > > H'm... the old code *used* to handle fractional bits, but the new code > seems to round down to the nearest bit. May have to get fixed to > handle low-rate inputs.
I don't believe that was ever true, though it can fairly trivially be added. JLC, please note that entropy estimation is much more conservative now than it was a month ago.
> As for margin of error, any persistent entropy overestimate is Bad. > a 6-fold overestimate is disastrous. > > What we can do is refuse to drain the main pool below, say, 128 bits of > entropy. Then we're safe against any *occasional* overestimates > as long as they don't add up to 128 bits.
I've been moving in that direction already, most of the infrastructure is already in place.
> > /dev/random will output once it has at least 160 bits of entropy > > (iirc), 1/2 bit turning into 3 bits would mean that 160bits of output > > it effectively only 27 bits worth of true entropy (again, assuming the > > catastrophic reseeder and output function don't waste entropy). > > > > It's a lot of "ifs" for my taste. > > /dev/random will output once it has as many bits of entropy as you're > asking for. If you do a 20-byte read, it'll output once it has 160 > bits. If you do a 1-byte read, it'll output once it has 8 bits.
That's not quite right. It needs 8 bits in the relevant output pool. Failing that, it needs 64 bits in the input pool to reseed the output pool. In the case of /dev/urandom, it needs 128 bits in the input pool, it always leaves enough for /dev/random to reseed.
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