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
28mpi_limb_t
29mpihelp_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}
51