Root/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
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

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