random2.c

a very popular packet of cryptography tools,it encloses the most common used algorithm and protocols

/* mpn_random2 -- Generate random numbers with relatively long strings
   of ones and zeroes.  Suitable for border testing.

Copyright 1992, 1993, 1994, 1996, 2000, 2001, 2002 Free Software Foundation, Inc.

This file is part of the GNU MP Library.

The GNU MP Library is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or (at your
option) any later version.

The GNU MP Library is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
License for more details.

You should have received a copy of the GNU Lesser General Public License
along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
MA 02111-1307, USA. */

#include "gmp.h"
#include "gmp-impl.h"


/* It's a bit tricky to get this right, so please test the code well if you
   hack with it.  Some early versions of the function produced random numbers
   with the leading limb == 0, and some versions never made the most
   significant bit set.

   This code and mpz_rrandomb are almost identical, though the latter makes bit
   runs of 1 to 16, and doesn't force the first block to contain 1-bits.

   The RANDS random state currently produces 32 random bits per underlying lc
   invocation (BITS_PER_RANDCALL).  We therefore ask for that, presuming that
   limbs are at least 32 bits.  FIXME: Handle smaller limbs, such as 4-bit
   limbs useful for testing purposes, or limbs truncated by nailing.

   For efficiency, we make sure to use most bits returned from _gmp_rand, since
   the underlying random number generator is slow.  Keep returned bits in
   ranm/ran, and a count of how many bits remaining in ran_nbits.  */

#define LOGBITS_PER_BLOCK 4

/* Ask _gmp_rand for 32 bits per call unless that's more than a limb can hold.
   Thus, we get the same random number sequence in the common cases.
   FIXME: We should always generate the same random number sequence!  */
#if GMP_NUMB_BITS < 32
#define BITS_PER_RANDCALL GMP_NUMB_BITS
#else
#define BITS_PER_RANDCALL 32
#endif

void
mpn_random2 (mp_ptr rp, mp_size_t n)
{
  gmp_randstate_ptr rstate = RANDS;
  int nb;
  int bit_pos;			/* bit number of least significant bit where
				   next bit field to be inserted */
  mp_size_t ri;			/* index in rp */
  mp_limb_t ran, ranm;		/* buffer for random bits */
  mp_limb_t acc;		/* accumulate output random data here */
  int ran_nbits;		/* number of valid bits in ran */

  /* FIXME: Is n==0 supposed to be allowed? */
  ASSERT (n >= 0);
  ASSERT_ALWAYS (BITS_PER_MP_LIMB > LOGBITS_PER_BLOCK);

  _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
  ran = ranm;

  /* Start off at a random bit position in the most significant limb.  */
  bit_pos = ran % GMP_NUMB_BITS;
  ran >>= 6;				/* Ideally   log2(GMP_NUMB_BITS) */
  ran_nbits = BITS_PER_RANDCALL - 6;	/* Ideally - log2(GMP_NUMB_BITS) */

  /* Bit 0 of ran chooses string of ones/string of zeroes.
     Make most significant limb be non-zero by setting bit 0 of RAN.  */
  ran |= 1;

  ri = n - 1;

  acc = 0;
  while (ri >= 0)
    {
      if (ran_nbits < LOGBITS_PER_BLOCK + 1)
	{
	  _gmp_rand (&ranm, rstate, BITS_PER_RANDCALL);
	  ran = ranm;
	  ran_nbits = BITS_PER_RANDCALL;
	}

      nb = (ran >> 1) % (1 << LOGBITS_PER_BLOCK) + 1;
      if ((ran & 1) != 0)
	{
	  /* Generate a string of nb ones.  */
	  if (nb > bit_pos)
	    {
	      rp[ri--] = acc | (((mp_limb_t) 2 << bit_pos) - 1);
	      bit_pos += GMP_NUMB_BITS;
	      bit_pos -= nb;
	      acc = ((~(mp_limb_t) 1) << bit_pos) & GMP_NUMB_MASK;
	    }
	  else
	    {
	      bit_pos -= nb;
	      acc |= (((mp_limb_t) 2 << nb) - 2) << bit_pos;
	    }
	}
      else
	{
	  /* Generate a string of nb zeroes.  */
	  if (nb > bit_pos)
	    {
	      rp[ri--] = acc;
	      acc = 0;
	      bit_pos += GMP_NUMB_BITS;
	    }
	  bit_pos -= nb;
	}
      ran_nbits -= LOGBITS_PER_BLOCK + 1;
      ran >>= LOGBITS_PER_BLOCK + 1;
    }
}