Random Number Generator

A good Random Number Generator is not always provided in the libraries of small embedded systems. This one, from Makoto Matsumoto and Takuji Nishimura, seems to work pretty well and is not terribly consumptive of resources.

As an example of where a random number generator can be useful, cryptographic systems often use a nonce initialization vector and Cipher-block chaining to assure that data will vary randomly, even if the raw data is the same (i.e. a block of zeros for uninitialized data). This makes it that much harder to decode the encryption key.

Random.c

/*

   from:  http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/MT2002/emt19937ar.html

   A C-program for MT19937, with initialization improved 2002/1/26.
   Coded by Takuji Nishimura and Makoto Matsumoto.

   Before using, initialize the state by using init_genrand(seed)
   or init_by_array(init_key, key_length).

   Copyright (C) 1997 - 2002, Makoto Matsumoto and Takuji Nishimura,
   All rights reserved.

   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions
   are met:

   1. Redistributions of source code must retain the above copyright
      notice, this list of conditions and the following disclaimer.

   2. Redistributions in binary form must reproduce the above copyright
      notice, this list of conditions and the following disclaimer in the
      documentation and/or other materials provided with the distribution.

   3. The names of its contributors may not be used to endorse or promote
      products derived from this software without specific prior written
      permission.

   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
   A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
   CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
   EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
   PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

   Any feedback is very welcome.
   http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html
   email: m-mat @ math.sci.hiroshima-u.ac.jp (remove space)

*/

#include <STDIO.H>

/* Random Number Generator Data */

#define N            17U            /* Matrix Size                                     */
#define M            13U            /* Matrix Period                                   */

#define MATRIX_A     0x9908B0DFUL   /* constant vector a                               */
#define UPPER_MASK   0x80000000UL   /* most significant w-r bits                       */
#define LOWER_MASK   0x7FFFFFFFUL   /* least significant r bits                        */

static unsigned long mt [ N ];      /* the array for the state vector                  */
static int           mti = N + 1;   /* mti == N + 1 means mt [ N ] is not initialized  */

/* Initializes mt[N] with a seed */

void init_genrand ( unsigned long ulSeed )

   {

   mt [ 0 ]= ulSeed & 0xFFFFFFFFUL;

   for ( mti = 1; mti < N; mti++ )
      {

      /* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier.   */
      /* In the previous versions, MSBs of the seed affect     */
      /* only MSBs of the array mt[].                          */
      /* 2002/01/09 modified by Makoto Matsumoto               */

      mt [ mti ] = ( 1812433253UL * ( mt [ mti - 1 ] ^ ( mt [ mti - 1 ] >> 30 ) ) + mti );

      mt [ mti ] &= 0xFFFFFFFFUL;

      /* for >32 bit machines */
      }

   }  /* void init_genrand ( unsigned long ulSeed ) */

/* Generates a Random Number on [ 0, 0xFFFFFFFF ] - Interval */

unsigned long genrand_int32 ( void )

   {
   unsigned long              y;
   static const unsigned long mag01 [ 2 ] = { 0x0UL, MATRIX_A };  /* mag01 [ x ] = x * MATRIX_A  for x = 0, 1 */
   int                        kk;

   /* Generate N Words at One Time */

   if ( mti >= N )
      {

      if ( mti == N + 1 )           /* If init_genrand() has not been called, a default initial seed is used */
         init_genrand ( 5489UL );

      for ( kk = 0; kk < N - M; kk++ )
         {
         y = ( mt [ kk ] & UPPER_MASK ) | ( mt [ kk + 1 ] & LOWER_MASK );
         mt [ kk ] = mt [ kk + ( int ) M ] ^ ( int ) ( y >> 1 ) ^ mag01 [ ( int ) y & 1 ];
         }

      for (;kk < N - 1; kk++ )
         {
         y = ( mt [ kk ] & UPPER_MASK ) | ( mt [ kk + 1 ] & LOWER_MASK );
         mt [ kk ] = mt[ kk + ( int ) ( M - N ) ] ^ ( int ) ( y >> 1 ) ^ mag01 [ ( int ) y & 1 ];
         }

      y = ( mt [ N - 1 ] & UPPER_MASK ) | ( mt [ 0 ] & LOWER_MASK );
      mt [ N - 1 ] = mt [ M - 1 ] ^ ( int ) ( y >> 1 ) ^ mag01 [ ( int ) y & 1 ];

      mti = 0;
      }

   y = mt [ mti++ ];

   /* Tempering */

   y ^= ( y >> 11 );
   y ^= ( y <<  7 ) & 0x9D2C5680UL;
   y ^= ( y << 15 ) & 0xEFC60000UL;
   y ^= ( y >> 18 );

   return y;

   }  /* unsigned long genrand_int32 ( void ) */

int main ( void )

   {
   int i = 0;

   init_genrand ( 0x12345678LU );

   printf ( "1000 outputs of genrand_int32()\n" );

   for ( i = 0; i < 1000; i++ )
      {
      printf ( "%8.8lX ", genrand_int32() );
      if ( i % 5 == 4 ) printf ( "\n" );
      }

   return 0;

   }  /* int main ( void ) */