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
mpihelp_rshift(mpi_ptr_t wp,mpi_ptr_t up,mpi_size_t usize,unsigned cnt)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 }
64