src/posix/rand.c

   1 /*****************************************************************************
   2  * rand.c : non-predictible random bytes generator
   3  *****************************************************************************
   4  * Copyright © 2007 Rémi Denis-Courmont
   5  * $Id$
   6  *
   7  * This program is free software; you can redistribute it and/or modify it
   8  * under the terms of the GNU Lesser General Public License as published by
   9  * the Free Software Foundation; either version 2.1 of the License, or
  10  * (at your option) any later version.
  11  *
  12  * This program is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
  15  * GNU Lesser General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU Lesser General Public License
  18  * along with this program; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin Street, Fifth Floor, Boston MA 02110-1301, USA.
  20  *****************************************************************************/
  21
  22 #ifdef HAVE_CONFIG_H
  23 # include "config.h"
  24 #endif
  25
  26 #include <vlc_common.h>
  27 #include <vlc_rand.h>
  28
  29 #include <stdint.h>
  30 #include <string.h>
  31 #include <stdlib.h>
  32
  33 #include <sys/types.h>
  34 #include <fcntl.h>
  35 #include <unistd.h>
  36 #include <pthread.h>
  37 #include <vlc_fs.h>
  38
  39 #include <vlc_md5.h>
  40
  41 /*
  42  * Pseudo-random number generator using a HMAC-MD5 in counter mode.
  43  * Probably not very secure (expert patches welcome) but definitely
  44  * better than rand() which is defined to be reproducible...
  45  */
  46 #define BLOCK_SIZE 64
  47
  48 static uint8_t okey[BLOCK_SIZE], ikey[BLOCK_SIZE];
  49
  50 static void vlc_rand_init (void)
  51 {
  52     uint8_t key[BLOCK_SIZE];
  53
  54     /* Get non-predictible value as key for HMAC */
  55     int fd = vlc_open ("/dev/urandom", O_RDONLY);
  56     if (fd == -1)
  57         return; /* Uho! */
  58
  59     for (size_t i = 0; i < sizeof (key);)
  60     {
  61          ssize_t val = read (fd, key + i, sizeof (key) - i);
  62          if (val > 0)
  63              i += val;
  64     }
  65
  66     /* Precompute outer and inner keys for HMAC */
  67     for (size_t i = 0; i < sizeof (key); i++)
  68     {
  69         okey[i] = key[i] ^ 0x5c;
  70         ikey[i] = key[i] ^ 0x36;
  71     }
  72
  73     vlc_close (fd);
  74 }
  75
  76
  77 void vlc_rand_bytes (void *buf, size_t len)
  78 {
  79     static pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
  80     static uint64_t counter = 0;
  81
  82     uint64_t stamp = NTPtime64 ();
  83
  84     while (len > 0)
  85     {
  86         uint64_t val;
  87         struct md5_s mdi, mdo;
  88
  89         InitMD5 (&mdi);
  90         InitMD5 (&mdo);
  91
  92         pthread_mutex_lock (&lock);
  93         if (counter == 0)
  94             vlc_rand_init ();
  95         val = counter++;
  96
  97         AddMD5 (&mdi, ikey, sizeof (ikey));
  98         AddMD5 (&mdo, okey, sizeof (okey));
  99         pthread_mutex_unlock (&lock);
 100
 101         AddMD5 (&mdi, &stamp, sizeof (stamp));
 102         AddMD5 (&mdi, &val, sizeof (val));
 103         EndMD5 (&mdi);
 104         AddMD5 (&mdo, mdi.buf, 16);
 105         EndMD5 (&mdo);
 106
 107         if (len < 16)
 108         {
 109             memcpy (buf, mdo.buf, len);
 110             break;
 111         }
 112
 113         memcpy (buf, mdo.buf, 16);
 114         len -= 16;
 115         buf = ((uint8_t *)buf) + 16;
 116     }
 117 }