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
27 mpi_limb_t
28 mpihelp_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 }