linux/lib/mpi/generic_mpih-rshift.c
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   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
  41mpi_limb_t
  42mpihelp_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}
  64
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