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List:       best-of-security
Subject:    BoS: rand-test.c
From:       Julian Assange <proff () suburbia ! net>
Date:       1995-11-22 23:40:57
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/*
 * test-rand.c - test the strength of rand()
 * Time-stamp: <95/11/02 16:21:31 gildea>
 *
 * Determines how many unique MIT-MAGIC-COOKIE-1 keys are
 * generated by rand() on this system.
 *
 * Written by Chris Hall, distributed by gildea@x.org
 * This program is provided as is, without warranty of any kind.
 */

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

#ifdef RAND48
#define rand lrand48
#define srand srand48
#endif

#ifdef RANDOM
#define rand random
#define srand srandom
#endif

#define MASK 0xff

static int highest_bit = 8*sizeof(int) - 1;

int main()
{
   int    i, first, second, lb, ub;

   /*
   **  find the lowest bit that matters
   */

   for (i = 0; i <= highest_bit; i++)  {
      srand (0);
      first = rand ();
      srand (1 << i);
      second = rand ();

      if ((first & MASK) != (second & MASK))  {
         lb = i;
         printf ("lower bit = %d\n", i);
         break;
      }
   }

   /*
   **  find the highest bit which matters
   */

   for (i = highest_bit; i >= 0; i--)  {
      srand (0);
      first = rand ();
      srand (1 << i);
      second = rand ();

      if ((first & MASK) != (second & MASK))  {
         ub = i;
         printf ("upper bit = %d\n", i);
         break;
      }
   }

   printf ("There are %u possible cookies (%d bits).\n",
           1 << (ub - lb + 1),
           ub - lb + 1);
   if (ub-lb+1 >= 24) {
       printf("You have a relatively strong rand() function.\n");
       /* but note that any rand() function is only as good as its inputs. */
   }
   else if (ub-lb+1 >= 16)
       printf("You have a medium strength rand() function.\n");
   else
       printf("You have a weak rand() function.\n");

   return 0;
}

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