Date: Tue, 23 Sep 2008 00:51:02 -0700 From: "Ted Mittelstaedt" <tedm@toybox.placo.com> To: "Robert Huff" <roberthuff@rcn.com>, <questions@freebsd.org> Subject: RE: using /dev/random Message-ID: <BMEDLGAENEKCJFGODFOCAEOKCFAA.tedm@toybox.placo.com> In-Reply-To: <18648.30321.369520.631459@jerusalem.litteratus.org>
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> -----Original Message-----
> From: owner-freebsd-questions@freebsd.org
> [mailto:owner-freebsd-questions@freebsd.org]On Behalf Of Robert Huff
> Sent: Monday, September 22, 2008 9:54 PM
> To: questions@freebsd.org
> Subject: using /dev/random
>
>
>
> What is the canonical way to get data from /dev/random?
> Specifically: having opened the file, how do I read the stream?
> I'm currently using
>
>
> union {
> float f;
> char c[4];
> } foo;
>
> foo.f = 0.0;
>
> fscanf(rand_fp,"%4c",foo.c);
>
>
> which doesn't seem to produce anywhere near "random bytes" as
> promised by the man page.
>
>
> Robert Huff
>
The canonical way is to use the functions random(), or srandom()
or srandomdev() or arc4random() depending on what
you need the random data for. /dev/random is really only
useful for seeding these functions (some of them pull data
from /dev/random internally)
The thrust behind the FreeBSD /dev/random device is that
we know that getting lots of real random data from /dev/random is
difficult, however getting non-repeating seeds from
/dev/random is easy. The device has thus been optimized
for seed generation to feed these other functions.
If you really want to roll-your-own and not use these functions
then you could read blocks from /dev/random and run
a Chi-square and Monte Carlo test on each
block and discard the ones that don't pass.
I've done my experimenting with the ENT program:
http://www.fourmilab.ch/random/
ie:
dd if=/dev/urandom bs=3000 count=100 of=random-sample
ent random-sample
Successive runs of that with different data sets and blocksizes
clearly illustrates the generator can't pass Chi-square quite
a lot of times.
Ted
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