| Dmitry Kasatkin | cdec9cb | 2011-08-31 14:05:16 +0300 | [diff] [blame] | 1 | /* mpih-rshift.c - MPI helper functions |
| 2 | * Copyright (C) 1994, 1996, 1998, 1999, |
| 3 | * 2000, 2001 Free Software Foundation, Inc. |
| 4 | * |
| 5 | * This file is part of GNUPG |
| 6 | * |
| 7 | * GNUPG is free software; you can redistribute it and/or modify |
| 8 | * it under the terms of the GNU General Public License as published by |
| 9 | * the Free Software Foundation; either version 2 of the License, or |
| 10 | * (at your option) any later version. |
| 11 | * |
| 12 | * GNUPG is distributed in the hope that it will be useful, |
| 13 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 14 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 15 | * GNU General Public License for more details. |
| 16 | * |
| 17 | * You should have received a copy of the GNU General Public License |
| 18 | * along with this program; if not, write to the Free Software |
| 19 | * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA |
| 20 | * |
| 21 | * Note: This code is heavily based on the GNU MP Library. |
| 22 | * Actually it's the same code with only minor changes in the |
| 23 | * way the data is stored; this is to support the abstraction |
| 24 | * of an optional secure memory allocation which may be used |
| 25 | * to avoid revealing of sensitive data due to paging etc. |
| 26 | * The GNU MP Library itself is published under the LGPL; |
| 27 | * however I decided to publish this code under the plain GPL. |
| 28 | */ |
| 29 | |
| 30 | #include "mpi-internal.h" |
| 31 | |
| 32 | /* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right |
| 33 | * and store the USIZE least significant limbs of the result at WP. |
| 34 | * The bits shifted out to the right are returned. |
| 35 | * |
| 36 | * Argument constraints: |
| 37 | * 1. 0 < CNT < BITS_PER_MP_LIMB |
| 38 | * 2. If the result is to be written over the input, WP must be <= UP. |
| 39 | */ |
| 40 | |
| 41 | mpi_limb_t |
| 42 | mpihelp_rshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt) |
| 43 | { |
| 44 | mpi_limb_t high_limb, low_limb; |
| 45 | unsigned sh_1, sh_2; |
| 46 | mpi_size_t i; |
| 47 | mpi_limb_t retval; |
| 48 | |
| 49 | sh_1 = cnt; |
| 50 | wp -= 1; |
| 51 | sh_2 = BITS_PER_MPI_LIMB - sh_1; |
| 52 | high_limb = up[0]; |
| 53 | retval = high_limb << sh_2; |
| 54 | low_limb = high_limb; |
| 55 | for (i = 1; i < usize; i++) { |
| 56 | high_limb = up[i]; |
| 57 | wp[i] = (low_limb >> sh_1) | (high_limb << sh_2); |
| 58 | low_limb = high_limb; |
| 59 | } |
| 60 | wp[i] = low_limb >> sh_1; |
| 61 | |
| 62 | return retval; |
| 63 | } |