[Freedombox-discuss] Fwd: Fwd: Entropy from the soundcard

Michael Rogers m-- at gmx.com
Fri Sep 16 13:11:31 UTC 2011

Thanks for forwarding this, and please pass on my thanks to John Denker
for the explanation. I'm tempted to dig out a voltmeter and start
calibrating my soundcard...


On 16/09/11 03:24, Sandy Harris wrote:
> An earlier message had some concerns about Turibid. I forwarded them
> to John Denker, the Turbid author. Now, with permission, I am
> forwarding his reply here.
> ---------- Forwarded message ----------
> From: John Denker <jsd at av8n.com>
> Date: Fri, Sep 16, 2011 at 9:23 AM
> Subject: Re: Fwd: [Freedombox-discuss] Entropy from the soundcard
> To: Sandy Harris <sandyinchina at gmail.com>
> Hi --
> On 09/15/2011 06:09 PM, you quoted Michael Rogers:
> Concerning turbid:
> >  As far as I can tell, it works by
> > sampling noise from a soundcard with nothing connected, calculating a
> > theoretical lower bound on the entropy of the noise, and using that
> > lower bound to determine how much noise to feed into a hash function
> > (SHA-1) to ensure 160 bits of entropy in the hash function's 160-bit output.
> OK.
> > It's a clever approach,
> :-)
> >  but one thing makes me nervous: the lower bound
> > is based on the assumption that the noise from the soundcard is white
> > noise. Possible sources of non-white noise (eg interference from the
> > power supply, which would produce a periodic "hum") are mentioned in
> > section 6.4 of the paper, but it's assumed that they can only add to the
> > entropy of the sampled noise.
> >
> > In a general sense that's true: adding a signal to white noise, even a
> > totally predictable signal, doesn't produce a combined signal that's any
> > more predictable than the white noise alone.
> That's very true, and that's the key to why turbid
> works correctly.
> > But in a more specific
> > sense it's problematic, because you can't determine the volume of the
> > white noise from the volume of the combined signal - and I *think*
> > that's what Turbid's calibration process tries to do.
> No, the calibration process doesn't do that.
> We agree that if it did that, it would be bad.
> Turbid *never* attempts to measure the noise.  It measures
> the input resistance and the bandwidth, not the noise.  The
> Johnson noise can then be calculated, and it *must* be there,
> as required by the second law of thermodynamics.
> We agree that there may be other noise on top of that, but
> it can't hurt, and turbid does need to account for it.
> > If I'm right, the presence of non-white noise might cause Turbid to
> > overestimate the amount of white noise available from the soundcard, in
> > which case it wouldn't feed enough noise into the hash function to
> > ensure a high-entropy output.
> Absolutely not.  Turbid does not estimate the noise.
> Therefor it cannot possibly overestimate the noise.
> > I say "might" because I don't really understand how Turbid's calibration
> > process works. For each model of soundcard you have to calculate some
> > parameters by soldering cables, playing test tones and taking
> > measurements with a voltmeter. How that produces an estimate of the
> > amount of white noise, as opposed to noise-of-all-colours, I can't tell.
> > Maybe someone who understands the maths better than I do can explain?
> It's not math.  It's physics, in particular thermodynamics.  Some
> relevant references are provided in the turbid principles-of-operation
> paper.  Or you could look for yourself:
>  http://www.google.com/search?q=johnson+nyquist+noise
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