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Re: PRNGs and testers.



At 09:00 AM 10/23/98 +0200, Mok-Kong Shen wrote:
>David Honig wrote:
>
>> Recall that Ueli's Universal Statistical Test
>> is valid only for real sources of entropy.
>> PRNGs have zero entropy asymptotically ;-)
>
>Sorry that I haven't yet understood. Could you explain a bit more
>from the entropy point of view? In my opinion, a statistical
>test does not and should not take into account how a sequence
>being tested is obtained. Given is simply a sequence and no other
>information and the test should give an answer.
>
>M. K. Shen

A *sequence* isn't random; a process is.   A sequence may be the result of
some process, but its always a sample of a process.

When you ask, "Is some process random", you *must* define "random"
operationally, thus, finitely.



A test for structure, such as Marsaglia's "Diehard" suite, 
measures various statistics (structure) of a sample and compares
them to the measurements you'd get for (abstractly modelled, "real")
randomness.  If the particular tests you've chosen 
don't see some structure that's present in the sample, you'll mistakenly 
accept the process as "random".  Diehard is nice because it includes
complex stats and some Monte Carlo evaluations.

Marsaglia's test was designed to find problems in PRNGs, which he was
working on.

Maurer suggests a way to compute the entropy of the data, taken as N-bit
blocks, and computes the value you'd compute for random data.  His test
consumes a lot more data, and is 

Now, a PRNG has a finite amount of (unknown) initial internal state.
This state is expanded into the output stream, which goes on forever,
thus the finite initial entropy is spread infinitestimally thin.

A true RNG, even an imperfect one, generates a constant amount of entropy
per symbol ("bit per baud" averaged over any sufficiently large window).

-----------------
An experiment

Run a block cipher in a feedback mode, generating a large data file.
Any good cipher will pass Diehard's tests for structure.  So will
a true random file.  

(I've posted directions for producing decent true randomness from
a detuned FM radio, soundcard, and 8->1 parity-reduction filtering.)

Now run Maurer's test; I've posted a version for blocksize = 16.  
The cipher-PRNG output will not have the entropy expected for randomness.
The physical-random file will.


-----------------------------

D. Honig

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