🚀 go-pugleaf

By far not being an expert on cryptology, I would appreciate some 
comments on the crypto scheme below. 

Where does it stand in terms of security when compared to transposition 
cyphers, DES etc.

My initial thoughts were that other then using a brute force approach,
e.g. trying all 2^31 seeds (or so), there is no easy way to deal 
with it - but I am probably wrong :-)

I presume that due to its simplicity this sort of crypto approach must
be quite well known, its just that in my rather limited explorations of 
the field I have not come accros any.

Algorithm:
- Create a sequence of random numbers that starts at 'seed' and is known 
  to be deterministic and to have a large period. e.g. linear congruential:
  X[i+1] = (a*X[i] + c) mod m, choose coefficients as follows 
  a = 16807, m = 2,147,483,647 (2^31 - 1), c = 0
  #define RANDOM ((unsigned char) (seed = (16807 * seed) % 2147483647))
  supposedly has a period of 2^31.

- use the 'key' to choose the first seed, e.g. add up all the bytes or
  some such, so that a key >= 4 byte could cover 0-2^31, roughly at least.

- take the the data stream and for each byte 'xor' the LSB of the 'seed'.
  A stronger method might be to use 4 byte chunks (assuming seed is 4 bytes)?
  The advantage of using xor is that there is that the algorithm is self
  reversing. Perhaps it entails some pitfall(s) however?

If anybody could point me to some literature that explains why the above
is (un?)reasonably (in?)secure I would be most grateful. Please note that
I make no claim to have carefully studied and researched this topic. 
I was hoping to avoid all that :-)

--
Thomas J. Essebier              Stallion Technologies Pty. Ltd.
Aarnet: tom@stallion.oz.au      56 Sylvan Rd., Toowong 4066, Australia
...!uunet!stallion.oz.au!tom    Fx: +61 7 371 8881, Ph: +61 7 870 4999