Re: Problem with random.c and PPC

From: Oliver Xymoron (
Date: Sat Aug 17 2002 - 02:23:10 EST

On Sat, Aug 17, 2002 at 12:05:07AM -0600, Andreas Dilger wrote:
> On Aug 16, 2002 19:45 -0500, Oliver Xymoron wrote:
> > Realistically, the hashing done by /dev/urandom is probably strong
> > enough for most purposes. It's as cryptographically strong as whatever
> > block cipher you're likely to use with it. /dev/random goes one step
> > further and tries to offer something that's theoretically
> > unbreakable. Useful for generating things like large public keys, less
> > useful for generating the session keys used by SSL and the
> > like. They're easier to break by direct attack.
> One of the problems, I believe, is that reading from /dev/urandom will
> also deplete the entropy pool, just like reading from /dev/random.
> The only difference is that when the entropy is gone /dev/random will
> stop and /dev/urandom will continue to provide data.

Yep, this is a longstanding problem. Will look into it and a couple
other things once I get the my current batch of patches running
against -current.
BTW, did ttyso ever ACK your last set of random changes or is it safe
to assume it's unmaintained?

> If you are in there fixing things, it might make sense to have
> /dev/urandom extract entropy from the random pool far less often than
> /dev/random. This way people who use /dev/urandom for a source of
> less-strong randomness (e.g. TCP sequence numbers or whatever), will
> not be shooting themselves in the foot for when they need a 2048-byte
> PGP key, if they are low on entropy sources.

Not sure this is an ideal fix. We might instead have an entropy
low-water mark (say 1/2 pool size), below which /dev/urandom will not
deplete the pool. This way when we have ample entropy, both devices
will behave like TRNGs, with /dev/urandom falling back to PRNG when a
shortage is threatened.

 "Love the dolphins," she advised him. "Write by W.A.S.T.E.." 
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