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=20
math statistic knowledge please run some test to be sure that NSHUFF is=20
good enough for this simple PRNG (i.e. not 100% good, but good for average=
=20
quality PRNG we have). My simple test shown that 100 is enough, but I=20
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 */
=20
+#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 =3D seed;
+ for (i =3D 0; i < NSHUFF; i++)
+ (void)do_rand(&next);
}
=20
=20
@@ -137,7 +143,7 @@
unsigned long junk;
=20
gettimeofday(&tv, NULL);
- next =3D (getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk;
+ srand((getpid() << 16) ^ tv.tv_sec ^ tv.tv_usec ^ junk);
}
}
=20
--=20
Andrey A. Chernov
http://ache.pp.ru/
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