there are two kinds of random numbers:

1) Statistically random numbers
2) unpredictable numbers

The first are important for things like simulations and white noise and
such - over time the numbers provided meet mathematical definitions of
"random" such that algorithms that count on that will work right.  shift
registers with feedback generate fine random numbers of this sort.

The second type is more often used in assorted cryptography-related
schemes.  Mathematically correct randomness is less important than
being unpredictable and unrepeating.   shift registers are NOT good
at providing this sort of random number.  See:

 RFC1750 Randomness Recommendations for Security. D. Eastlake, 3rd, S.
         Crocker, J. Schiller. December 1994. (Format: TXT=73842 bytes)
         (Status: INFORMATIONAL)

for reasonably understandable explanations and some suggestions...

true random numbers (generated from radioactive decay or somesuch) are both
unpredictable AND statistically random, but they're not so easy to get.

BillW