1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /* mpih-rshift.c - MPI helper functions
3 * Copyright (C) 1994, 1996, 1998, 1999,
4 * 2000, 2001 Free Software Foundation, Inc.
5 *
6 * This file is part of GNUPG
7 *
8 * Note: This code is heavily based on the GNU MP Library.
9 * Actually it's the same code with only minor changes in the
10 * way the data is stored; this is to support the abstraction
11 * of an optional secure memory allocation which may be used
12 * to avoid revealing of sensitive data due to paging etc.
13 * The GNU MP Library itself is published under the LGPL;
14 * however I decided to publish this code under the plain GPL.
15 */
16
17 #include "mpi-internal.h"
18
19 /* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right
20 * and store the USIZE least significant limbs of the result at WP.
21 * The bits shifted out to the right are returned.
22 *
23 * Argument constraints:
24 * 1. 0 < CNT < BITS_PER_MP_LIMB
25 * 2. If the result is to be written over the input, WP must be <= UP.
26 */
27
28 mpi_limb_t
29 mpihelp_rshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt)
30 {
31 mpi_limb_t high_limb, low_limb;
32 unsigned sh_1, sh_2;
33 mpi_size_t i;
34 mpi_limb_t retval;
35
36 sh_1 = cnt;
37 wp -= 1;
38 sh_2 = BITS_PER_MPI_LIMB - sh_1;
39 high_limb = up[0];
40 retval = high_limb << sh_2;
41 low_limb = high_limb;
42 for (i = 1; i < usize; i++) {
43 high_limb = up[i];
44 wp[i] = (low_limb >> sh_1) | (high_limb << sh_2);
45 low_limb = high_limb;
46 }
47 wp[i] = low_limb >> sh_1;
48
49 return retval;
50 }