From: Randall Farmer <rfarmer@HiWAAY.net>
Date: Wed, 15 Oct 1997 09:13:14 +0800
To: Cypherpunks List <cypherpunks@toad.com>
Subject: Re: Encryption Program
Message-ID: <Pine.OSF.3.96.971014193558.16804C-100000@fly.HiWAAY.net>
MIME-Version: 1.0
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Although I don't know the PRNG in Visual Basic (the one 3dmx uses) well enough 
to do much real analysis, the description of an attempt to correct a problem in
it shows a weakness in the "enhanced" version...here's a slightly
abridged/clarified copy of the message I sent the author about it.

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...
>     While it is true that PRNG's are not very good,

Well, there isn't any big problem with PRNGs as a class -- RC4 is based on a
PRNG, and it's okay to use as long as you know its limitations (i.e., just so 
you don't try using the same key twice or anything similarly silly).

...  
> I believe I found a way around that problem...I used a syst[e]m of cubic 
> arrays. The program first creates sixteen cubic arrays, and fills them one
> space at a time with random characters. When the stream of characters to be
> XORed with the plaintext is generated, it picks a random cube and a random
> location with[in] that cube. 

I can't do much real analysis since I don't know how Visual Basic's PRNG works,
but with a truly secure PRNG like one you would see in a good stream cipher,
you can't predict x bits of the PRNG's output with more than 1/2^x probability
of bring right without doing exhaustive search of the keyspace.

However, your arrays almost surely won't be filled with the *exact* same
quantity of each character, so, even if the bytes in your PRNG's output are
selected randomly from the arrays, some bytes are more likely to be a byte in
your "enhanced" PRNG's output than others. Therefore, given a bunch of the
stream, one can guess the next 8 bits of the PRNG's output with more than 1/256
probability of being right, meaning your PRNG doesn't fit the bill.

To sum it up, no matter what Visual Basic's PRNG does, that method *can't* be
100% secure. 

...

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Randall Farmer
    rfarmer@hiwaay.net
    http://hiwaay.net/~rfarmer