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1 | /* mpihelp-lshift.c - MPI helper functions | |
2 | * Copyright (C) 1994, 1996, 1998, 2001 Free Software Foundation, Inc. | |
3 | * | |
4 | * This file is part of GnuPG. | |
5 | * | |
6 | * GnuPG is free software; you can redistribute it and/or modify | |
7 | * it under the terms of the GNU General Public License as published by | |
8 | * the Free Software Foundation; either version 2 of the License, or | |
9 | * (at your option) any later version. | |
10 | * | |
11 | * GnuPG is distributed in the hope that it will be useful, | |
12 | * but WITHOUT ANY WARRANTY; without even the implied warranty of | |
13 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the | |
14 | * GNU General Public License for more details. | |
15 | * | |
16 | * You should have received a copy of the GNU General Public License | |
17 | * along with this program; if not, write to the Free Software | |
18 | * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA | |
19 | * | |
20 | * Note: This code is heavily based on the GNU MP Library. | |
21 | * Actually it's the same code with only minor changes in the | |
22 | * way the data is stored; this is to support the abstraction | |
23 | * of an optional secure memory allocation which may be used | |
24 | * to avoid revealing of sensitive data due to paging etc. | |
25 | * The GNU MP Library itself is published under the LGPL; | |
26 | * however I decided to publish this code under the plain GPL. | |
27 | */ | |
28 | ||
29 | #include "mpi-internal.h" | |
30 | ||
31 | /* Shift U (pointed to by UP and USIZE digits long) CNT bits to the left | |
32 | * and store the USIZE least significant digits of the result at WP. | |
33 | * Return the bits shifted out from the most significant digit. | |
34 | * | |
35 | * Argument constraints: | |
36 | * 1. 0 < CNT < BITS_PER_MP_LIMB | |
37 | * 2. If the result is to be written over the input, WP must be >= UP. | |
38 | */ | |
39 | ||
40 | mpi_limb_t | |
41 | mpihelp_lshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned int cnt) | |
42 | { | |
43 | mpi_limb_t high_limb, low_limb; | |
44 | unsigned sh_1, sh_2; | |
45 | mpi_size_t i; | |
46 | mpi_limb_t retval; | |
47 | ||
48 | sh_1 = cnt; | |
49 | wp += 1; | |
50 | sh_2 = BITS_PER_MPI_LIMB - sh_1; | |
51 | i = usize - 1; | |
52 | low_limb = up[i]; | |
53 | retval = low_limb >> sh_2; | |
54 | high_limb = low_limb; | |
55 | while (--i >= 0) { | |
56 | low_limb = up[i]; | |
57 | wp[i] = (high_limb << sh_1) | (low_limb >> sh_2); | |
58 | high_limb = low_limb; | |
59 | } | |
60 | wp[i] = high_limb << sh_1; | |
61 | ||
62 | return retval; | |
63 | } |