lib/mpi/generic_mpih-lshift.c

   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
  40 mpi_limb_t
  41 mpihelp_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 }