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465ae836 | 1 | // SPDX-License-Identifier: GPL-2.0-or-later |
cdec9cb5 DK |
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 | * | |
cdec9cb5 DK |
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 | } |