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. ------------------------------------------------------------------------------- <headers snipped> ...
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. ... --------------------------------------------------------------------------- Randall Farmer rfarmer@hiwaay.net http://hiwaay.net/~rfarmer