lib/mpi/generic_mpih-rshift.c

   1 /* mpih-rshift.c  -  MPI helper functions
   2  * Copyright (C) 1994, 1996, 1998, 1999,
   3  *               2000, 2001 Free Software Foundation, Inc.
   4  *
   5  * This file is part of GNUPG
   6  *
   7  * GNUPG is free software; you can redistribute it and/or modify
   8  * it under the terms of the GNU General Public License as published by
   9  * the Free Software Foundation; either version 2 of the License, or
  10  * (at your option) any later version.
  11  *
  12  * GNUPG is distributed in the hope that it will be useful,
  13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15  * GNU General Public License for more details.
  16  *
  17  * You should have received a copy of the GNU General Public License
  18  * along with this program; if not, write to the Free Software
  19  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  20  *
  21  * Note: This code is heavily based on the GNU MP Library.
  22  *       Actually it's the same code with only minor changes in the
  23  *       way the data is stored; this is to support the abstraction
  24  *       of an optional secure memory allocation which may be used
  25  *       to avoid revealing of sensitive data due to paging etc.
  26  *       The GNU MP Library itself is published under the LGPL;
  27  *       however I decided to publish this code under the plain GPL.
  28  */
  29
  30 #include "mpi-internal.h"
  31
  32 /* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right
  33  * and store the USIZE least significant limbs of the result at WP.
  34  * The bits shifted out to the right are returned.
  35  *
  36  * Argument constraints:
  37  * 1. 0 < CNT < BITS_PER_MP_LIMB
  38  * 2. If the result is to be written over the input, WP must be <= UP.
  39  */
  40
  41 mpi_limb_t
  42 mpihelp_rshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt)
  43 {
  44         mpi_limb_t high_limb, low_limb;
  45         unsigned sh_1, sh_2;
  46         mpi_size_t i;
  47         mpi_limb_t retval;
  48
  49         sh_1 = cnt;
  50         wp -= 1;
  51         sh_2 = BITS_PER_MPI_LIMB - sh_1;
  52         high_limb = up[0];
  53         retval = high_limb << sh_2;
  54         low_limb = high_limb;
  55         for (i = 1; i < usize; i++) {
  56                 high_limb = up[i];
  57                 wp[i] = (low_limb >> sh_1) | (high_limb << sh_2);
  58                 low_limb = high_limb;
  59         }
  60         wp[i] = low_limb >> sh_1;
  61
  62         return retval;
  63 }