🚀 go-pugleaf

In article <tom.712397505@cluster> tom@stallion.oz.au (Thomas Essebier) writes:
>By far not being an expert on cryptology, I would appreciate some 
>comments on the crypto scheme below. 
> [ use a standard linear congruential random number generator (= RNG),
>   primed with the key, to derive a stream of bits, which is then xor-ed
>   with the data ]
>
>Where does it stand in terms of security when compared to transposition 
>cyphers, DES etc.

I'm sorry, but this is a very weak cipher -- it's very easy to break.
There are (at least) two main weaknesses:

(1)	As you described it, there are only 2^32 possible internal
	states of the RNG.  Computers are fast enough today that
	the brute-force attack of trying all of them is feasable
	for any mildly determined opponent.

(2)	The simple structure of a linear congruential RNG makes this
	scheme breakable using a much cheaper and more "elegant"
	attack.  The linear and self-inverse nature of xor
	(i.e. a xor b xor a = b) makes the RNG output stream
	recoverable from the ciphertext only with a bit of work,
	or trivially from a plaintext/ciphertext pair.  Given the
	RNG output stream, Knuth v.2 section 3.6 excercise 7
	describes how to recover the RNG parameters, including the
	starting value = crypto key.

In general, the fundamental lesson of crypto history is that good
cryptosystems are designed *only* by people who have both theoretical
knowledge of, *and* practical experience with, breaking existing
high-quality cryptosystems.  In other words, unless you have a lot
of crypto experience you're unlikely to come up with a system that
would withstand professional attack.

If you simply want a secure cryptosystem, use DES (appropriately,
i.e. in the right mode, with secure keys, and with proper attention
payed to defense against "practical cryptanalysis").  It's secure
against anyone short of a very big corporation or a medium sized
government.  And if Exxon or the FBI *really* want the contents
of your disk drive, they'll probably get it one way or the other...

- Jonathan Thornburg
  <jonathan@einstein.ph.utexas.edu> or <jonathan@hermes.chpc.utexas.edu>
  University of Texas at Austin / Physics Dept / Center for Relativity
  and (for a few more months) U of British Columbia / {Astronomy,Physics}