Date: Tue, 04 Feb 2003 14:00:27 +0100 From: Dag-Erling Smorgrav <des@ofug.org> To: David Schultz <dschultz@uclink.Berkeley.EDU> Cc: "Andrey A. Chernov" <ache@nagual.pp.ru>, Kris Kennaway <kris@obsecurity.org>, current@FreeBSD.ORG Subject: Re: rand() is broken Message-ID: <xzpfzr4b3pw.fsf@flood.ping.uio.no> In-Reply-To: <20030204115237.GA6483@HAL9000.homeunix.com> (David Schultz's message of "Tue, 4 Feb 2003 03:52:37 -0800") References: <20030202070644.GA9987@rot13.obsecurity.org> <20030202090422.GA59750@nagual.pp.ru> <20030203002639.GB44914@HAL9000.homeunix.com> <20030203100002.GA73386@nagual.pp.ru> <20030204054020.GA2447@HAL9000.homeunix.com> <20030204094659.GA87303@nagual.pp.ru> <20030204115237.GA6483@HAL9000.homeunix.com>
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[-- Attachment #1 --]
The attached patch, based on one of the m/k pairs suggested on that
page, results in the following:
des@des ~/src% ./rndtest | head
0 b 5 f 8 5 2 c 8 5 1 8 4 9 7 b d 9 e a d 3 8 8 5 9 2 3 e a c f
7 a d b f 0 1 3 6 7 4 f 0 6 0 1 2 3 1 c b a 6 1 e 0 9 c f 7 c 6
6 1 a a d b 3 e b a a 0 e d a f 8 6 3 f f 9 3 7 6 6 d 7 f 6 8 d
b d 6 2 b f 2 2 b 1 7 4 4 3 5 5 2 9 6 4 e 1 9 4 7 b f 6 f 7 8 c
1 5 7 e 3 5 d b 8 c c 4 d 1 3 6 e 0 3 3 3 2 e 5 3 b 5 e 7 b c a
0 a 3 2 d 5 f c 9 f c 3 9 a 1 f 3 f 2 c 5 9 4 6 2 d 2 3 e 2 7 5
a 8 8 1 5 7 0 5 4 e c f 6 3 b 0 8 4 d 2 a 8 0 5 7 b 2 1 4 e e 9
7 1 5 5 6 f f e 6 f a 4 e 5 a f 8 d f 7 b 3 8 d e 3 f 5 4 0 2 e
5 c a f 5 a e 5 0 9 4 6 0 0 9 f d 9 6 4 b f 8 2 3 5 e 4 5 9 9 a
d 3 4 5 5 0 f 9 9 1 8 6 7 a 5 0 2 0 5 8 b 6 1 6 7 f a 9 a 6 6 d
Note that my patch assumes that RAND_MAX fits in 32 bits, which should
be acceptable for FreeBSD since it always uses 32-bit ints (all our
32-bit platforms are ILP32, and I believe all our 64-bit platforms are
I32LP64) and RAND_MAX is uniformly defined to 0x7fffffff.
DES
--
Dag-Erling Smorgrav - des@ofug.org
[-- Attachment #2 --]
Index: lib/libc/stdlib/rand.c
===================================================================
RCS file: /home/ncvs/src/lib/libc/stdlib/rand.c,v
retrieving revision 1.11
diff -u -r1.11 rand.c
--- lib/libc/stdlib/rand.c 2 Feb 2003 14:27:51 -0000 1.11
+++ lib/libc/stdlib/rand.c 4 Feb 2003 13:37:03 -0000
@@ -52,7 +52,7 @@
#endif /* TEST */
static int
-do_rand(unsigned long *ctx)
+do_rand(uint32_t *ctx)
{
#ifdef USE_WEAK_SEEDING
/*
@@ -63,21 +63,23 @@
return ((*ctx = *ctx * 1103515245 + 12345) % ((u_long)RAND_MAX + 1));
#else /* !USE_WEAK_SEEDING */
/*
- * Compute x = (7^5 * x) mod (2^31 - 1)
- * wihout overflowing 31 bits:
- * (2^31 - 1) = 127773 * (7^5) + 2836
- * From "Random number generators: good ones are hard to find",
- * Park and Miller, Communications of the ACM, vol. 31, no. 10,
- * October 1988, p. 1195.
+ * New algorithm derived from
+ * The Laws of Cryptography: Pseudo-random Number Generation
+ * by Neal R. Wagner
+ * http://www.cs.utsa.edu/~wagner/laws/rng.html
+ * which itself is derived from work by Donald E. Knuth.
+ *
+ * This is a linear congruence generator using the equation
+ *
+ * x(n+1) = (k * x(n) + a) mod m
+ *
+ * where m is 2^31 - 1, k is 62089911 and a is 0.
*/
- long hi, lo, x;
+ uint64_t tmp;
- hi = *ctx / 127773;
- lo = *ctx % 127773;
- x = 16807 * lo - 2836 * hi;
- if (x <= 0)
- x += 0x7fffffff;
- return ((*ctx = x) % ((u_long)RAND_MAX + 1));
+ tmp = *ctx * 62089911;
+ *ctx = (uint32_t)(tmp % (uint64_t)RAND_MAX);
+ return (*ctx);
#endif /* !USE_WEAK_SEEDING */
}
@@ -85,7 +87,7 @@
int
rand_r(unsigned int *ctx)
{
- u_long val = (u_long) *ctx;
+ uint32_t val = (uint32_t) *ctx;
int r = do_rand(&val);
*ctx = (unsigned int) val;
@@ -93,7 +95,7 @@
}
-static u_long next = 1;
+static uint32_t next = 1;
int
rand()
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