On Tue, May 17, 2022 at 1:10 PM Steffen Nurpmeso <stef...@sdaoden.eu> wrote:
> Joerg Sonnenberger wrote in
> <yoqhj3yy9qrax...@bec.de>:
> |Am Fri, May 13, 2022 at 09:43:26AM -0500 schrieb Luke Small:
> |> I made a couple new versions of a new kind of arc4random_uniform-like
> ...
> |If your main use case is limiting the amount of cryptography when using
> |small bounds, there is a much simpler approach to be taken here. For
> |boundaries below 256, use arc4random_buf to extract one byte if bound is
> |a power of two, otherwise two. This gives most of the performance
> |benefit without complicating the algorithm. Extracting two bytes ensures
> |that the propability of success is > 99% and the double extracting
> |doesn't eat up the benefits.
>
> You can use (really implemented) _buf() if you need a 8-bit or
> 16-bit etc number.
>
> I find that _uniform() often makes no difference to a simple
> modulo because like the comment in _uniform() says "p > 0.5 (worst
case, usually far better", and usually RNGs sprinkle bits nicely,
>
What does that statement mean? You seem to be saying "module is uniform,
except when it isn't, which could be almost half the time for some cases,
but when it's uniform it's uniform, so why bother making it actually
correct and dependable".
I mean, what does that _mean_??? It's as if I said "my text handling
program handles all characters uniformly, except those with accents, but
that's less than 10% of the characters I type, so it handles all characters
uniformly." WTF, NO!
> 0 bytes "do not occur", so a 32-bit RNG value "is" >=0x01FFFFFF in
> most cases for "my RNG" (of 100000 803/759/793 NOT; 776/805/793
> NOT for Linux getrandom(2)), which is a pretty high cut off.
> Using _uniform() just because of its name seems strange thus.
>
Where do these ideas come from, that "0 bytes 'do not occur'"?? If your
rand generator doesn't provide zero bytes at the expected frequency, you
know, 1 in 256, then you're using a garbage random number generator.
Please stop making such suggestions here because THEY ARE NOT TRUE ABOUT
OPENBSD. Do ya'll not bother to test the claims that you make?
: bleys; cat f.c
#include <stdio.h>
#include <stdlib.h>
int
main(void)
{
uint32_t u;
long count;
count = 0;
while ((u = arc4random()) > 0x1ffffff)
count++;
printf("%08x\t%ld\n", u, count);
count = 0;
for (;;) {
u = arc4random();
if ((u & 0xff000000) == 0 ||
(u & 0x00ff0000) == 0 ||
(u & 0x0000ff00) == 0 ||
(u & 0x000000ff) == 0)
break;
count++;
}
printf("%08x\t%ld\n", u, count);
return 0;
}
: bleys; cc f.c
: bleys; ./a.out
00b82e5c 58
ab478800 36
: bleys;
Philip Guenther
