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