Fix PRNG seeding to resist guessing

[empserver] / src / lib / gen / chance.c

diff --git a/src/lib/gen/chance.c b/src/lib/gen/chance.c
index 1892e2e20866dc4e025109898a740531f6e93d6a..4c38b4ae75db6d542c57dcace4c13548105d6022 100644 (file)
--- a/src/lib/gen/chance.c
+++ b/src/lib/gen/chance.c
@@ -1,11 +1,11 @@
 /*
  *  Empire - A multi-player, client/server Internet based war game.
- *  Copyright (C) 1986-2008, Dave Pare, Jeff Bailey, Thomas Ruschak,
- *                           Ken Stevens, Steve McClure
+ *  Copyright (C) 1986-2013, Dave Pare, Jeff Bailey, Thomas Ruschak,
+ *                Ken Stevens, Steve McClure, Markus Armbruster
  *
- *  This program is free software; you can redistribute it and/or modify
+ *  Empire is free software: you can redistribute it and/or modify
  *  it under the terms of the GNU General Public License as published by
- *  the Free Software Foundation; either version 2 of the License, or
+ *  the Free Software Foundation, either version 3 of the License, or
  *  (at your option) any later version.
  *
  *  This program is distributed in the hope that it will be useful,
@@ -14,8 +14,7 @@
  *  GNU General Public License for more details.
  *
  *  You should have received a copy of the GNU General Public License
- *  along with this program; if not, write to the Free Software
- *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  *  ---
  *
@@ -25,32 +24,83 @@
  *
  *  ---
  *
- *  chance.c: return 1 if "roll" is under the chance.
+ *  chance.c: Roll dice
  *
  *  Known contributors to this file:
- *
+ *     Markus Armbruster, 2006-2012
  */
 
 #include <config.h>
 
+#include <fcntl.h>
 #include <math.h>
-#include "prototypes.h"
+#include <stdint.h>
+#include <stdlib.h>
+#include <sys/time.h>
+#include <unistd.h>
+#include "chance.h"
+#include "mt19937ar.h"
 
+/*
+ * Return non-zero with probability D.
+ */
 int
 chance(double d)
 {
-    return d > (random() % 32768) / 32768.0;
+    return d > genrand_real2();
+}
+
+/*
+ * Return non-zero with probability PCT%.
+ */
+int
+pct_chance(int pct)
+{
+    return roll(100) <= pct;
+}
+
+static unsigned
+round_up_to_pow2(unsigned val)
+{
+    val--;
+    val |= val >> 1;
+    val |= val >> 2;
+    val |= val >> 4;
+    val |= val >> 8;
+    val |= val >> 16;
+    val++;
+    return val;
 }
 
+/*
+ * Return a random number in [0..N-1].
+ * N must be in [1..2^31-1].
+ */
+int
+roll0(int n)
+{
+    unsigned pow2 = round_up_to_pow2(n);
+    int r;
+
+    do
+       r = genrand_int32() & (pow2 - 1);
+    while (r >= n);
+    return r;
+}
+
+/*
+ * Return a random number in [1..N].
+ * N must be in [0..2^31-1].
+ */
 int
 roll(int n)
 {
-    return 1 + random() % n;
+    return 1 + roll0(n);
 }
 
 /*
- * round value to nearest int (on the average). E.g. rounds up
- * with a chance proportional to the size of the fractional part.
+ * Round VAL to nearest integer (on the average).
+ * VAL's fractional part is chance to round up.
  */
 int
 roundavg(double val)
@@ -58,3 +108,67 @@ roundavg(double val)
     double flr = floor(val);
     return (int)(flr + chance(val - flr));
 }
+
+/*
+ * Seed the pseudo-random number generator with SEED.
+ * The sequence of pseudo-random numbers is repeatable by seeding it
+ * with the same value.
+ */
+void
+seed_prng(unsigned seed)
+{
+    init_genrand(seed);
+}
+
+static uint32_t
+djb_hash(uint32_t hash, void *buf, size_t sz)
+{
+    unsigned char *bp;
+
+    for (bp = buf; bp < (unsigned char *)buf + sz; bp++)
+       hash = hash * 33 ^ *bp;
+
+    return hash;
+}
+
+/*
+ * Pick a reasonably random seed for the pseudo-random number generator.
+ */
+unsigned
+pick_seed(void)
+{
+    int fd;
+    uint32_t seed;
+    int got_seed = 0;
+    struct timeval tv;
+    pid_t pid;
+
+    /*
+     * Modern systems provide random number devices, but the details
+     * vary.  On many systems, /dev/random blocks when the kernel
+     * entropy pool has been depleted, while /dev/urandom doesn't.
+     * The former should only be used for generating long-lived
+     * cryptographic keys.  On other systems, both devices behave
+     * exactly the same, or only /dev/random exists.
+     *
+     * Try /dev/urandom first, and if it can't be opened, blindly try
+     * /dev/random.
+     */
+    fd = open("/dev/urandom", O_RDONLY | O_NONBLOCK);
+    if (fd < 0)
+       fd = open("/dev/random", O_RDONLY | O_NONBLOCK);
+    if (fd >= 0) {
+       got_seed = read(fd, &seed, sizeof(seed)) == sizeof(seed);
+       close(fd);
+    }
+
+    if (!got_seed) {
+       /* Kernel didn't provide, fall back to hashing time and PID */
+       gettimeofday(&tv, NULL);
+       seed = djb_hash(5381, &tv, sizeof(tv));
+       pid = getpid();
+       seed = djb_hash(seed, &pid, sizeof(pid));
+    }
+
+    return seed;
+}