[clug-talk] Random numbers in Linux: an example

Gustin Johnson gustin at echostar.ca
Thu Sep 8 11:56:19 PDT 2005


A practical application is wiping a hard drive so that data recovery is not 
possible.  The following should be repeated 4 to 8 times depending on ones 
level of paranoia.

dd if=/dev/zero of=/dev/hdx
dd if=/dev/full of=/dev/hdx
dd if=/dev/urandom of=/dev/hdx

On Thursday 08 September 2005 10:33, Mark Carlson wrote:
> Hello everybody,
>
> This is my first email to the list, so forgive me if this is off-topic
> or formatted incorrectly.
>
> Last night at the meeting during the GPG demonstration someone asked
> why you should move your mouse around during the key generation.  I
> had mistakenly said that it was the standard C library that offers two
> methods of obtaining random numbers.  When I got home I looked for the
> functions and found out I was incorrect.  It is the kernel itself that
> offers these two methods.  And if you want to check it out, it's
> pretty cool (and it only takes a couple seconds to do.)
>
> At your shell, type in the command:
> od -t x1 /dev/random
>
> Notice that it only spits out a small amount of data (if nothing else
> on your system is doing any I/O).  Now move your mouse around (from
> corner to corner on your screen) or hit a bunch of keys on your
> keyboard.  Hopefully there will be more data generated as you do this.
>
> The other way of obtaining random numbers using the kernel is to get
> it from /dev/urandom.  To test this out, get ready to hit Ctrl+C to
> kill the program as it generates a lot of data very quickly.  Now type
> into your shell:
> od -t x1 /dev/urandom
>
> Anyways, I had fun at the first meeting I attended and I hope to see
> you all next month :-)
>
> -Mark Carlson
>
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