LXR linux/lib/mpi/generic

   1/* mpihelp-lshift.c  -  MPI helper functions
   2 * Copyright (C) 1994, 1996, 1998, 2001 Free Software Foundation, Inc.
   3 *
   4 * This file is part of GnuPG.
   5 *
   6 * GnuPG is free software; you can redistribute it and/or modify
   7 * it under the terms of the GNU General Public License as published by
   8 * the Free Software Foundation; either version 2 of the License, or
   9 * (at your option) any later version.
  10 *
  11 * GnuPG is distributed in the hope that it will be useful,
  12 * but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 * GNU General Public License for more details.
  15 *
  16 * You should have received a copy of the GNU General Public License
  17 * along with this program; if not, write to the Free Software
  18 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
  19 *
  20 * Note: This code is heavily based on the GNU MP Library.
  21 *       Actually it's the same code with only minor changes in the
  22 *       way the data is stored; this is to support the abstraction
  23 *       of an optional secure memory allocation which may be used
  24 *       to avoid revealing of sensitive data due to paging etc.
  25 *       The GNU MP Library itself is published under the LGPL;
  26 *       however I decided to publish this code under the plain GPL.
  27 */
  28
  29#include "mpi-internal.h"
  30
  31/* Shift U (pointed to by UP and USIZE digits long) CNT bits to the left
  32 * and store the USIZE least significant digits of the result at WP.
  33 * Return the bits shifted out from the most significant digit.
  34 *
  35 * Argument constraints:
  36 * 1. 0 < CNT < BITS_PER_MP_LIMB
  37 * 2. If the result is to be written over the input, WP must be >= UP.
  38 */
  39
  40mpi_limb_t
  41mpihelp_lshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned int cnt)
  42{
  43        mpi_limb_t high_limb, low_limb;
  44        unsigned sh_1, sh_2;
  45        mpi_size_t i;
  46        mpi_limb_t retval;
  47
  48        sh_1 = cnt;
  49        wp += 1;
  50        sh_2 = BITS_PER_MPI_LIMB - sh_1;
  51        i = usize - 1;
  52        low_limb = up[i];
  53        retval = low_limb >> sh_2;
  54        high_limb = low_limb;
  55        while (--i >= 0) {
  56                low_limb = up[i];
  57                wp[i] = (high_limb << sh_1) | (low_limb >> sh_2);
  58                high_limb = low_limb;
  59        }
  60        wp[i] = high_limb << sh_1;
  61
  62        return retval;
  63}
  64