using SRAM state as a source of randomness

[Steven M. Bellovin]

On Mon, 17 Sep 2007 11:20:32 -0700
Netsecurity <netsecurity at sound-by-design.com> wrote:

> Back in the late 60's I was playing with audio and a magazine I
> subscribed to had a circut for creating warble tones for standing
> wave and room resonance testing.
> 
> The relevance of this is that they were using a "random" noise
> generating chip that they acknowledged was not random enough for good
> measurements. The fix suggested was to parallel a number, six as I
> recall, to improve the randomness by mixing the signals to achieve
> better randomness. I don't recall the math but the approach improved
> the randomness by more than an order of magnitude. 
> 
> I have also seen the same effect on reverse biased zener diodes used
> as random noise generators and that seemed - no real hard
> measurements that I can recall - to work quite well. Mind you these
> were not zeners all fabricated on a single chip, but rather
> individuals soldered together so the charateristics of each were more
> random because of the semi-randomness of the manufacturing process.
> 
This is an old technique.  We could even go back to von Neumann's
scheme: look at two successive bits.  If they're equal, discard them.
Otherwise, map <0,1> to 0 and <1,0> to 1.

See the section on "Software whitening" in
http://en.wikipedia.org/wiki/Hardware_random_number_generator (which
was correct as of when I looked at it, a few minutes before the
timestamp on this email; check the Wiki history to be sure....).


		--Steve Bellovin, http://www.cs.columbia.edu/~smb

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