Re: Seeding /dev/random not working

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On 5/29/07, Theodore Tso <[email protected]> wrote:
On Tue, May 29, 2007 at 12:53:10PM +0100, M Macnair wrote:
Ok, so this is telling me a couple of things.  First of all, if you're
only getting three outputs, it means that you don't have any
peripherals feeding entropy into the system from the boot sequence.
Without any hard drives, keyboards or mice, and a NIC whose device
driver hasn't been configured to feed entropy, you're definitely
Yes.  However this isn't the issue I'm concerned with at the moment.

Secondly, and more importantly, your boot scripts aren't set up
Sorry, I only posted the startup bit - the same does indeed happen on
shutdown, however I wasn't interested in it for the purposes of the

The key point I was trying to put across was that I can't get the
seeding process to work.  No matter what I wrote to /dev/urandom on
startup, the output from reading /dev/urandom immediately afterwards
was the same (ignoring the occasional variation that I put down to
timing).  As I understand it (from man 4 random and the boot script
examples), writing to /dev/[u]random should increase the amount of
entropy, thereby altering the output from the PRNG.

Another thing which I noticed is that when Matt Mackall took over
maintainership of /dev/random, he apparently took out one of the
safeguards I had, which was that before, when entropy was extracted
from the pool the time stamp when it was extracted was mixed back into
the pool.  The theory was that an external attacker might not know
when a program might be calling /dev/random, so mixing in the time of
that entropy was extracted wouldn't hurt, and might help.  I'll submit
a patch to add that support back in, which will help you a little.
When did Matt take over?  From my experiments it would appear as
though the time is having an effect on the output, though as I say I'm
not totally sure.

Michael Macnair
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