linux-kernel - Re: Why does reading from /dev/urandom deplete entropy so much?

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Message-ID:  <1197183090.12636.15.camel@perihelion>
Date:	Sun, 09 Dec 2007 01:52:05 -0500
From:	Jon Masters <jonathan@...masters.org>
To:	linux-kernel@...r.kernel.org
Subject:  Re: Why does reading from /dev/urandom deplete entropy so much?

On Sun, 2007-12-09 at 06:21 +0100, Willy Tarreau wrote:

> Wouldn't it be possible to mix the data with the pid+uid of the reading
> process so that even if another one tries to collect data from urandom,
> he cannot predict what another process will get ? BTW, I think that the
> tuple (pid,uid,timestamp of open) is unpredictable and uncontrollable
> enough to provide one or even a few bits of entropy by itself.

Timestamp perhaps, but pid/uid are trivially guessable in automated
environments, such as LiveCDs. And if you're also running on an embedded
system without a RTC (common, folks like to save a few cents) then it's
all pretty much "trivially" guessable on some level.

Jon.

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