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