The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/libkern/arc4random.c

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    1 /*-
    2  * THE BEER-WARE LICENSE
    3  *
    4  * <dan@FreeBSD.ORG> wrote this file.  As long as you retain this notice you
    5  * can do whatever you want with this stuff.  If we meet some day, and you
    6  * think this stuff is worth it, you can buy me a beer in return.
    7  *
    8  * Dan Moschuk
    9  */
   10 
   11 #include <sys/cdefs.h>
   12 __FBSDID("$FreeBSD: releng/8.3/sys/libkern/arc4random.c 199583 2009-11-20 15:27:52Z jhb $");
   13 
   14 #include <sys/types.h>
   15 #include <sys/param.h>
   16 #include <sys/kernel.h>
   17 #include <sys/random.h>
   18 #include <sys/libkern.h>
   19 #include <sys/lock.h>
   20 #include <sys/mutex.h>
   21 #include <sys/time.h>
   22 
   23 #define ARC4_RESEED_BYTES 65536
   24 #define ARC4_RESEED_SECONDS 300
   25 #define ARC4_KEYBYTES (256 / 8)
   26 
   27 static u_int8_t arc4_i, arc4_j;
   28 static int arc4_numruns = 0;
   29 static u_int8_t arc4_sbox[256];
   30 static time_t arc4_t_reseed;
   31 static struct mtx arc4_mtx;
   32 
   33 static u_int8_t arc4_randbyte(void);
   34 
   35 static __inline void
   36 arc4_swap(u_int8_t *a, u_int8_t *b)
   37 {
   38         u_int8_t c;
   39 
   40         c = *a;
   41         *a = *b;
   42         *b = c;
   43 }       
   44 
   45 /*
   46  * Stir our S-box.
   47  */
   48 static void
   49 arc4_randomstir (void)
   50 {
   51         u_int8_t key[256];
   52         int r, n;
   53         struct timeval tv_now;
   54 
   55         /*
   56          * XXX read_random() returns unsafe numbers if the entropy
   57          * device is not loaded -- MarkM.
   58          */
   59         r = read_random(key, ARC4_KEYBYTES);
   60         getmicrouptime(&tv_now);
   61         mtx_lock(&arc4_mtx);
   62         /* If r == 0 || -1, just use what was on the stack. */
   63         if (r > 0) {
   64                 for (n = r; n < sizeof(key); n++)
   65                         key[n] = key[n % r];
   66         }
   67 
   68         for (n = 0; n < 256; n++) {
   69                 arc4_j = (arc4_j + arc4_sbox[n] + key[n]) % 256;
   70                 arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
   71         }
   72         arc4_i = arc4_j = 0;
   73 
   74         /* Reset for next reseed cycle. */
   75         arc4_t_reseed = tv_now.tv_sec + ARC4_RESEED_SECONDS;
   76         arc4_numruns = 0;
   77 
   78         /*
   79          * Throw away the first N words of output, as suggested in the
   80          * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
   81          * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
   82          */
   83         for (n = 0; n < 256*4; n++)
   84                 arc4_randbyte();
   85         mtx_unlock(&arc4_mtx);
   86 }
   87 
   88 /*
   89  * Initialize our S-box to its beginning defaults.
   90  */
   91 static void
   92 arc4_init(void)
   93 {
   94         int n;
   95 
   96         mtx_init(&arc4_mtx, "arc4_mtx", NULL, MTX_DEF);
   97         arc4_i = arc4_j = 0;
   98         for (n = 0; n < 256; n++)
   99                 arc4_sbox[n] = (u_int8_t) n;
  100 
  101         arc4_t_reseed = 0;
  102 }
  103 
  104 SYSINIT(arc4_init, SI_SUB_LOCK, SI_ORDER_ANY, arc4_init, NULL);
  105 
  106 /*
  107  * Generate a random byte.
  108  */
  109 static u_int8_t
  110 arc4_randbyte(void)
  111 {
  112         u_int8_t arc4_t;
  113 
  114         arc4_i = (arc4_i + 1) % 256;
  115         arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
  116 
  117         arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
  118 
  119         arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
  120         return arc4_sbox[arc4_t];
  121 }
  122 
  123 /*
  124  * MPSAFE
  125  */
  126 void
  127 arc4rand(void *ptr, u_int len, int reseed)
  128 {
  129         u_char *p;
  130         struct timeval tv;
  131 
  132         getmicrouptime(&tv);
  133         if (reseed || 
  134            (arc4_numruns > ARC4_RESEED_BYTES) ||
  135            (tv.tv_sec > arc4_t_reseed))
  136                 arc4_randomstir();
  137 
  138         mtx_lock(&arc4_mtx);
  139         arc4_numruns += len;
  140         p = ptr;
  141         while (len--)
  142                 *p++ = arc4_randbyte();
  143         mtx_unlock(&arc4_mtx);
  144 }
  145 
  146 uint32_t
  147 arc4random(void)
  148 {
  149         uint32_t ret;
  150 
  151         arc4rand(&ret, sizeof ret, 0);
  152         return ret;
  153 }

Cache object: a907dd707ef87e702e91a3cbbae393ea


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