[Cryptography] A TRNG review per day: Turbid

Clemens Ladisch clemens at ladisch.de
Wed Oct 29 18:16:10 EDT 2014

Bill Cox wrote:
> On Wed, Oct 29, 2014 at 8:55 AM, Clemens Ladisch <clemens at ladisch.de> wrote:
>> Bill Cox wrote:
>>> There will still be significant correlation between samples.  There is
>>> thermal noise in a band from 9X to 10X below the sample rate
>>> which will turn into a significant short-term correlation between
>>> samples 10 away from each other.
>> The noise in all the other bands will cancel out these correlations.
> Not in my experience, but that is somewhat limited.  A simple test would be
> seeing if the zero crossings are correlated between adjacent samples.  My
> guess is they are highly correlated, as in I have a 70% chance of guessing
> if your next sample is greater or less than zero if you tell me the full
> value of the previous sample.

And as it turns out, there are different decimation filters for different
sample rates (typically, higher rates have smaller passbands).

I made a quick test (using only the sign of adjacent samples), and there
are indeed lots of correlations (at any sample rate).

In any case, higher rates capture more useful noise.
(And Turbid uses the highest rate by default.)

> If you send me some typical Turbid maximum sample rate sound samples, I'd
> be happy to do that test.

Knock yourself out:

(The last one is an extreme case where Turbid would refuse to work.
And it doesn't have much entropy even under optimistic assumptions.)


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