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From: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Newsgroups: comp.os.linux.development.system,comp.os.linux.development.apps
Subject: Re: shred or scrub
Date: Sat, 19 Apr 2014 23:05:10 +0100
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"Bill Cunningham" <nospam@nspam.invalid> writes:
> crankypuss wrote:
>
>> There exist pseudo-devices 'zero', 'random', and 'urandom':
>> http://en.wikipedia.org/wiki/Device_file#Pseudo-devices
>
>     What's the difference in urandom and random devices?

The kernel is supposed to collect 'bits with unpredictable values' from
'suitable sources' and the random-device will only return such bits,
blocking a reader in case the already collected ones have all been
handed out. /dev/urandom is a hash-based PRNG which generates bits as
required by performing a set of calculations on the existing
'unpredictable value bit pool'. In theory, someone who knows what the
PRNG returned in the past could be able to predict what it will return
in future. In practice no "method for doing so" has been published and
this is considered to be 'a very difficult problem which might be
impossible to solve'.