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SubjectRe: Using Yarrow in /dev/random
   Date: 	Mon, 11 Sep 2000 13:08:59 +0000
From: Pravir Chandra <pchandra@rstcorp.com>

I've been working to change the implementation of /dev/random over to the
Yarrow-160a algorithm created by Bruce Schneier and John Kelsey. We've been
working on parallel development for Linux and NT so that the algorithms are
matching. The Yarrow 160A algorithm is a variant of Yarrow-160 that has come
about from discussions with John Kelsey. We've been in contact with him
throughout our development effort.

I'm not a big fan of Yarrow, since it (in my opinion) places too much
faith in the crypto algorithms. It uses a pathetically small entropy
pool, and assumes that hash function will do the rest. Which is fine,
but that makes it a pseudo-RNG, or a crypto-RNG, and not really an
entropy collector. That's not necessarily a bad thing, but IMO that
should live outside the kernel. There's no real point putting something
like Yarrow in kernel space. Better that it collects entropy from a
true entropy collector that's in-kernel (like the current /dev/random),
and that it does the rest of the processing in user space. After all,
the main rason why /dev/random is in the kernel is precisely because
that's the most efficient place to collect as much entropy as possible.

If you look at the current 2.4 /dev/random code, you'll see that there
already are two entropy pools in use, which is designed to be able to do
something like the "catastrpohic reseed" features which Bruce Schneier
is so fond of. What's currently there isn't perfect, in that there's
still a bit too much leakage between the two pools, but I think that
approach is much better than the pure Yarrow approach.

- Ted
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