1/* mpih-rshift.c - MPI helper functions 2 * Copyright (C) 1994, 1996, 1998, 1999, 3 * 2000, 2001, 2002 Free Software Foundation, Inc. 4 * 5 * This file is part of Libgcrypt. 6 * 7 * Libgcrypt is free software; you can redistribute it and/or modify 8 * it under the terms of the GNU Lesser General Public License as 9 * published by the Free Software Foundation; either version 2.1 of 10 * the License, or (at your option) any later version. 11 * 12 * Libgcrypt 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 Lesser General Public License for more details. 16 * 17 * You should have received a copy of the GNU Lesser General Public 18 * License 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 */ 27 28#include <config.h> 29#include <stdio.h> 30#include <stdlib.h> 31#include "mpi-internal.h" 32 33 34/* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right 35 * and store the USIZE least significant limbs of the result at WP. 36 * The bits shifted out to the right are returned. 37 * 38 * Argument constraints: 39 * 1. 0 < CNT < BITS_PER_MP_LIMB 40 * 2. If the result is to be written over the input, WP must be <= UP. 41 */ 42 43mpi_limb_t 44_gcry_mpih_rshift( mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt) 45{ 46 mpi_limb_t high_limb, low_limb; 47 unsigned sh_1, sh_2; 48 mpi_size_t i; 49 mpi_limb_t retval; 50 51 sh_1 = cnt; 52 wp -= 1; 53 sh_2 = BITS_PER_MPI_LIMB - sh_1; 54 high_limb = up[0]; 55 retval = high_limb << sh_2; 56 low_limb = high_limb; 57 for (i=1; i < usize; i++) 58 { 59 high_limb = up[i]; 60 wp[i] = (low_limb >> sh_1) | (high_limb << sh_2); 61 low_limb = high_limb; 62 } 63 wp[i] = low_limb >> sh_1; 64 65 return retval; 66} 67 68