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/10.2/sys/libkern/arc4random.c 249631 2013-04-19 00:30:52Z ache $");
   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 int arc4rand_iniseed_state = ARC4_ENTR_NONE;
   28 
   29 static u_int8_t arc4_i, arc4_j;
   30 static int arc4_numruns = 0;
   31 static u_int8_t arc4_sbox[256];
   32 static time_t arc4_t_reseed;
   33 static struct mtx arc4_mtx;
   34 
   35 static u_int8_t arc4_randbyte(void);
   36 
   37 static __inline void
   38 arc4_swap(u_int8_t *a, u_int8_t *b)
   39 {
   40         u_int8_t c;
   41 
   42         c = *a;
   43         *a = *b;
   44         *b = c;
   45 }       
   46 
   47 /*
   48  * Stir our S-box.
   49  */
   50 static void
   51 arc4_randomstir (void)
   52 {
   53         u_int8_t key[256];
   54         int r, n;
   55         struct timeval tv_now;
   56 
   57         /*
   58          * XXX read_random() returns unsafe numbers if the entropy
   59          * device is not loaded -- MarkM.
   60          */
   61         r = read_random(key, ARC4_KEYBYTES);
   62         getmicrouptime(&tv_now);
   63         mtx_lock(&arc4_mtx);
   64         /* If r == 0 || -1, just use what was on the stack. */
   65         if (r > 0) {
   66                 for (n = r; n < sizeof(key); n++)
   67                         key[n] = key[n % r];
   68         }
   69 
   70         for (n = 0; n < 256; n++) {
   71                 arc4_j = (arc4_j + arc4_sbox[n] + key[n]) % 256;
   72                 arc4_swap(&arc4_sbox[n], &arc4_sbox[arc4_j]);
   73         }
   74         arc4_i = arc4_j = 0;
   75 
   76         /* Reset for next reseed cycle. */
   77         arc4_t_reseed = tv_now.tv_sec + ARC4_RESEED_SECONDS;
   78         arc4_numruns = 0;
   79 
   80         /*
   81          * Throw away the first N words of output, as suggested in the
   82          * paper "Weaknesses in the Key Scheduling Algorithm of RC4"
   83          * by Fluher, Mantin, and Shamir.  (N = 256 in our case.)
   84          */
   85         for (n = 0; n < 256*4; n++)
   86                 arc4_randbyte();
   87         mtx_unlock(&arc4_mtx);
   88 }
   89 
   90 /*
   91  * Initialize our S-box to its beginning defaults.
   92  */
   93 static void
   94 arc4_init(void)
   95 {
   96         int n;
   97 
   98         mtx_init(&arc4_mtx, "arc4_mtx", NULL, MTX_DEF);
   99         arc4_i = arc4_j = 0;
  100         for (n = 0; n < 256; n++)
  101                 arc4_sbox[n] = (u_int8_t) n;
  102 
  103         arc4_t_reseed = 0;
  104 }
  105 
  106 SYSINIT(arc4_init, SI_SUB_LOCK, SI_ORDER_ANY, arc4_init, NULL);
  107 
  108 /*
  109  * Generate a random byte.
  110  */
  111 static u_int8_t
  112 arc4_randbyte(void)
  113 {
  114         u_int8_t arc4_t;
  115 
  116         arc4_i = (arc4_i + 1) % 256;
  117         arc4_j = (arc4_j + arc4_sbox[arc4_i]) % 256;
  118 
  119         arc4_swap(&arc4_sbox[arc4_i], &arc4_sbox[arc4_j]);
  120 
  121         arc4_t = (arc4_sbox[arc4_i] + arc4_sbox[arc4_j]) % 256;
  122         return arc4_sbox[arc4_t];
  123 }
  124 
  125 /*
  126  * MPSAFE
  127  */
  128 void
  129 arc4rand(void *ptr, u_int len, int reseed)
  130 {
  131         u_char *p;
  132         struct timeval tv;
  133 
  134         getmicrouptime(&tv);
  135         if (atomic_cmpset_int(&arc4rand_iniseed_state, ARC4_ENTR_HAVE,
  136             ARC4_ENTR_SEED) || reseed ||
  137            (arc4_numruns > ARC4_RESEED_BYTES) ||
  138            (tv.tv_sec > arc4_t_reseed))
  139                 arc4_randomstir();
  140 
  141         mtx_lock(&arc4_mtx);
  142         arc4_numruns += len;
  143         p = ptr;
  144         while (len--)
  145                 *p++ = arc4_randbyte();
  146         mtx_unlock(&arc4_mtx);
  147 }
  148 
  149 uint32_t
  150 arc4random(void)
  151 {
  152         uint32_t ret;
  153 
  154         arc4rand(&ret, sizeof ret, 0);
  155         return ret;
  156 }

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