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 | } |
51 | |