Date: Mon, 3 Feb 2003 14:46:29 +0300 From: "Andrey A. Chernov" <ache@nagual.pp.ru> To: Kris Kennaway <kris@obsecurity.org> Cc: current@FreeBSD.ORG Subject: Final fix for correlation problem (was Re: rand() is broken) Message-ID: <20030203114629.GA77557@nagual.pp.ru> In-Reply-To: <20030202090422.GA59750@nagual.pp.ru> References: <20030202070644.GA9987@rot13.obsecurity.org> <20030202090422.GA59750@nagual.pp.ru>
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This is final fix for 1st value correlation problem. Somebody with
math statistic knowledge please run some test to be sure that NSHUFF is
good enough for this simple PRNG (i.e. not 100% good, but good for average
quality PRNG we have). My simple test shown that 100 is enough, but I
am not PRNG math expert. Maybe increasing NSHUFF up too 2000 required.
Similar patch is needed for random(3) TYPE_0 too, I'll produce it when we
find good NSHUFF.
--- stdlib/rand.c.bak Mon Feb 3 13:22:12 2003
+++ stdlib/rand.c Mon Feb 3 14:03:58 2003
@@ -51,6 +51,8 @@
#include <stdio.h>
#endif /* TEST */
+#define NSHUFF 100 /* to drop seed -> 1st value correlation */
+
static int
do_rand(unsigned long *ctx)
{
@@ -108,7 +110,11 @@
srand(seed)
u_int seed;
{
+ int i;
+
next = seed;
+ for (i = 0; i < NSHUFF; i++)
+ (void)do_rand(&next);
}
@@ -137,7 +143,7 @@
unsigned long junk;
gettimeofday(&tv, NULL);
- next = (getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk;
+ srand((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk);
}
}
--
Andrey A. Chernov
http://ache.pp.ru/
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