random numbers !
Wed, 18 Dec 2002 16:14:20 -0500
> So, I expect, that each time, I'll run the programm, I'll get some
> new numbers.. because they are chosen randomly.. but it's not the
> case, and I always get the same.
>That's how things are supposed to work. All computer random
>number generators work like that--i.e., they are psuedo random.
>To change the sequence of psuedo random numbers, you need to
>seed the generators. Please see the GMP manual for details
>on how to do that.
If you start from default seed, you'll get the same results over and over.
In fact, computer can't create really 'random' numbers. That's why we call
those functios 'pseudo-random'. It look like series of random numbers but
in fact, the way it work, is that at each call, the computer take the
preceeding results and compute a new number from it. Then, it put the
result into the seed for use in the next function call.
If you want a new series of numbers to be generated on each run of your
program, you need to feed it with a different seed each time. The easiest
way to do this is to take the system clock (possibly down to the
milliseconds) and feed this to the seed once at the beginning of your
program (see gmp_randseed in section 9).
The useness of this function depend mostly on what you want to do. If you
want to generate pseudo-random numbers for anything that it isn't security
related, that's fine. If you want to use this for cryptographic pourpous,
then this may not be secure enough. For that, you would probably need to
take more that the system clock.
I saw somewhere a program that displayed a box on the screen over which you
pass the mouse cursor for a few seconds. The algorithm took the actual
points of the box over which you mouse cursor pass to generate the seed.
This is more secure as there no *probable* way of plotting the same pattern
For strong cryptographic porpous, the best way would be to use an hardware
random number generator. Those are built with a diode as a noise source. 0
and 1 are derived from that noise. Seed is feed from the result.
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