“Well, that was unexpected…”: The Raspberry Pi’s Hardware Random Number Generator

Hey! This is a bit old! Things may have changed and I haven’t necessarily fixed them.

Most computers can’t create true random numbers. They use a formula which makes a very long stream of pseudo-random numbers, but real randomness comes from thermal noise in analogue components. The Raspberry Pi has such a circuit in its SoC, as it helps making the seed data for secure transactions. It was only recently that a driver for this circuit was supplied. To enable it (on Raspbian): I think the module is enabled by default now for the different versions of the SoC.

  1. Make sure your system is up to date with
    sudo apt-get update
    sudo apt -y upgrade
  2. Install the module:
    sudo modprobe bcm2708-rng
  3. To make sure it’s always loaded, add the following line to /etc/modules (editing as root):
    bcm2708-rng
  4. For some RNG-related stuff, install rng-tools:
    sudo apt-get install rng-tools

The /dev/hwrng device should now be available, but can only be read by the root user.

Nico pointed out that you also need to:

  1. Edit /etc/default/rng-tools, and remove the # at the start of the line
    HRNGDEVICE=/dev/hwrng
  2. Restart rng-tools with
    sudo service rng-tools restart

What random looks like

random20130606210642random20130606210630

Random data look pretty dull. Here are random RGB values made with:

sudo cat /dev/hwrng  | rawtoppm -rgb 256 256 | pnmtopng > random$(date +%Y%m%d%H%M%S).png

(you’ll need to install the netpbm toolkit to do this.)

What random sounds like

Two short WAV samples of, well, noise:

Yup, sounds like static. It was made with the rndsound.sh script. You’ll need to install sox to run it.

This is not random

If it sounds like static, and even if it sometimes looks like static, it may not actually be true random noise. An infamous case of a pseudo random number generator being not very random at all was RANDU, which at first glance appeared to produce nearly random results, but close study showed it to be very predictable.

I wrote (what I think to be) a C implementation of RANDU: randu.c. While it produces appropriately random-sounding audio data (randu17.wav), if you output it as an image:

randu17_rgbThose stripes are a giveaway; there should be no order in the output. (Then again, I have no idea if I’ve implemented RANDU correctly.) Testing random data is hard, then — you really need a barrage of tests, and even some of them might fail even for truly random output. Thankfully, when you installed rngtools, it included rngtest, a simple checker for random data:

sudo cat /dev/hwrng | rngtest -c 1000
rngtest 2-unofficial-mt.14
Copyright (c) 2004 by Henrique de Moraes Holschuh
This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

rngtest: starting FIPS tests…
rngtest: bits received from input: 20000032
rngtest: FIPS 140-2 successes: 1000
rngtest: FIPS 140-2 failures: 0
rngtest: FIPS 140-2(2001-10-10) Monobit: 0
rngtest: FIPS 140-2(2001-10-10) Poker: 0
rngtest: FIPS 140-2(2001-10-10) Runs: 0
rngtest: FIPS 140-2(2001-10-10) Long run: 0
rngtest: FIPS 140-2(2001-10-10) Continuous run: 0
rngtest: input channel speed: (min=67.969; avg=921.967; max=1953125.000)Kibits/s
rngtest: FIPS tests speed: (min=842.881; avg=3208.336; max=6407.890)Kibits/s
rngtest: Program run time: 27658884 microseconds

We were lucky that none of the tests failed for that run; sometimes there are a few failures. RANDU, on the other hand fares very badly:

./randu 17  | rngtest -c 1000
rngtest 2-unofficial-mt.14
Copyright (c) 2004 by Henrique de Moraes Holschuh
This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

rngtest: starting FIPS tests…
rngtest: bits received from input: 20000032
rngtest: FIPS 140-2 successes: 0
rngtest: FIPS 140-2 failures: 1000
rngtest: FIPS 140-2(2001-10-10) Monobit: 730
rngtest: FIPS 140-2(2001-10-10) Poker: 1000
rngtest: FIPS 140-2(2001-10-10) Runs: 289
rngtest: FIPS 140-2(2001-10-10) Long run: 0
rngtest: FIPS 140-2(2001-10-10) Continuous run: 0
rngtest: input channel speed: (min=45.630; avg=14255.221; max=19073.486)Mibits/s
rngtest: FIPS tests speed: (min=23.694; avg=154.238; max=176.606)Mibits/s
rngtest: Program run time: 141071 microseconds

See? Lots of failures there. It’s hardly random at all. If you really want to get out testing randomness, there are the dieharder tests. They takes ages to run, though.

(Note: newish Intel machines also have a real hardware RNG in the shape of Rdrand.)

pgmrnoise – a more random (or less repeatable) pgmnoise

Update: If you have a recent NetPBM, this is fixed.
I’d previously alluded that netpbm’s pgmnoise wasn’t as random as it could be if you called it several times in quick succession. Nerdy discussion after the break, but here’s a (perhaps slightly linux-centric) alternative:

#!/bin/sh
# pgmrnoise - a more random pgmnoise; limited to 8-bit images
# created by scruss on Sun Oct 12 19:36:37 EDT 2008

echo P5
echo $1 $2
echo 255
dd if=/dev/urandom bs=$1 count=$2 2> /dev/null

I just pasted the shell text in there; you’ll need to save it as a file. It works the same way as pgmnoise:

 pgmrnoise width height > noise.pgm

It is limited as written to 8 bit-per-pixel output, but is a fairly trivial edit to make it 16 or more bits.

Continue reading “pgmrnoise – a more random (or less repeatable) pgmnoise”