Re: Another problem with IPG algorithm

[ichudov@algebra.com (Igor Chudov @ home)]

wichita@cyberstation.net wrote:
> On Sun, 1 Dec 1996, Igor Chudov @ home wrote:
> > Don and others,
> > 
> > At the heart of IPG algorithm there is a pseudo-random number generator
> > which generates values of A(JV). (see http://www.netprivacy.com/algo.html)
> > 
> >        DO
> >          JV=JV+1
> >          IF JV=53 THEN JV=0
> >          A(JV)=(A(JV)+B(JV)) MOD C(JV)
> >        UNTIL A(JV)<16384
> > 
> > Note that if B(JV) and C(JV) in a triplet (A(JV), B(JV), C(JV)) are not
> > mutually prime, they will generate very few numbers and not a whole set
> > 0-16383. For example, if C(JV) is 20000, and B(JV) is 10000, and initial
> > A is (for example) 57, the only two numbers that this triplet will
> > generate will be 57 and 10057.
> > 
> > This refutes Don Wood's claim that the distribution of results
> > approaches even. Even if only ONE triplet is such as I described (and it
> > is VERY likely to happen statistically), the distribution will be
> > skewed.
> > 
> > Don, what do you think about it?
> > 
> > igor
> > 
> Igor, 
> 
> Also included in the more detailed explanation is the set of As, Bs, and
> Cs that are used. Either the B or a C is prime in all instances,
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^ see below
> I would agree with you if that was not the case. All 16384 numbers, 0
> through 16383, are always generated. In addition of course, the numbers
> between 16384 and each of the Cs are also generated but not used. 

If only B is required to be a prime, that is incorrect. See below.


> Thus each of the C(JV) values are operating at different speeds and
> produce a staccato, meaning in this case, something that is discontinious
> or disjointed. However, the sum of series approaches an even distribution
> in the almost precisely same manner as a true random number generator
> approaches an even distribution. Thus over short frames variance is great
> but over long frames, the frequency of occurrence tends to approach a
> perfectly even distribution.
> 
> Also, keep in mind, that the values are never used directly - they are
> only used as a variable for a three dimensional table lookup, where all 
> three of the 4096 element tables are constantly changing depending upon
> the the sum of the A(JV) variables combined with the initial settings,
> which were initally determined determined by the user generated key, and 
> other related information.   
> 
> Also, remember that the order and the actual As, Bs, and Cs used are also
> randomly arrived at by using the user defined key, 256 bytes or 8192
> bytes.
> 
> There is a lot more revealed at the web site. If you have further
> interest, I would provide you, Igor, with source code for the 
> algorithm, subject to a binding NDA. I think you will be suprised if
> you examined it in detail.

Thanks for your response, Don. I appreciate that we are able to talk 
about cryptography because it is interesting.

Re: triplets. It seems that even if only one number in each triplet (B
or C) must be prime, as you said, we are still not guaranteed that the
PRNG will be "good".  For example, suppose that B is a prime around,
say, 10000, and C is 2*B.  In this case, the problem would still be the
same.

I agree that since lookup tables are used, the output for the XOR
engine would be more obscure than if the output of the PRNG was used
directly. This obscurity, however, does not imply that the resulting
XOR key data would be free of biases that I described.

There is a possibility that the biases could be exploited.

Take the extreme case: suppose that by an extreme stroke of bad luck
the output of your PRNG is only 11252 and 1. This may happen if B=11251
and C=11251*2 (I am too lazy to check if 11251 is a prime, but you get 
the idea) for all triplets, and all A=1. Would the three tables DIFF, etc.
be able to "randomise" the output? I doubt that, although can't say for
sure right now.

As for the code, I can sign the NDA *if* it is reasonable, but I can
only read your code if it is written and commented well. The problem is,
suppose I read your code and find a weakness. What do I do next since my
NDA forbids me from quotinbg relevant parts of the code?

Again, I am far from a professional cryptographer and would not be
able to do even a half-decent review. The comments that I make are
my attempt to find weaknesses in your algorithm.

I suggest that you hire a professional cryptographer with an academic
degree and ask him/her/it to produce an independent evaluation. It will
be worth more than my comments.

	- Igor.