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68 lines
2.2 KiB
C
68 lines
2.2 KiB
C
/* mpih-rshift.c - MPI helper functions
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* Copyright (C) 1994, 1996, 1998 Free Software Foundation, Inc.
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*
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* This file is part of GNUPG
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*
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* GNUPG is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* GNUPG is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
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*
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* Note: This code is heavily based on the GNU MP Library.
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* Actually it's the same code with only minor changes in the
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* way the data is stored; this is to support the abstraction
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* of an optional secure memory allocation which may be used
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* to avoid revealing of sensitive data due to paging etc.
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* The GNU MP Library itself is published under the LGPL;
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* however I decided to publish this code under the plain GPL.
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*/
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#include <config.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include "mpi-internal.h"
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/* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right
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* and store the USIZE least significant limbs of the result at WP.
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* The bits shifted out to the right are returned.
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*
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* Argument constraints:
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* 1. 0 < CNT < BITS_PER_MP_LIMB
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* 2. If the result is to be written over the input, WP must be <= UP.
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*/
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mpi_limb_t
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mpihelp_rshift( mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt)
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{
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mpi_limb_t high_limb, low_limb;
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unsigned sh_1, sh_2;
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mpi_size_t i;
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mpi_limb_t retval;
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sh_1 = cnt;
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wp -= 1;
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sh_2 = BITS_PER_MPI_LIMB - sh_1;
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high_limb = up[0];
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retval = high_limb << sh_2;
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low_limb = high_limb;
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for( i=1; i < usize; i++) {
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high_limb = up[i];
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wp[i] = (low_limb >> sh_1) | (high_limb << sh_2);
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low_limb = high_limb;
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}
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wp[i] = low_limb >> sh_1;
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return retval;
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}
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