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1// SPDX-License-Identifier: GPL-2.0-or-later2/* mpih-rshift.c - MPI helper functions3 * Copyright (C) 1994, 1996, 1998, 1999,4 * 2000, 2001 Free Software Foundation, Inc.5 *6 * This file is part of GNUPG7 *8 * Note: This code is heavily based on the GNU MP Library.9 * Actually it's the same code with only minor changes in the10 * way the data is stored; this is to support the abstraction11 * of an optional secure memory allocation which may be used12 * 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 right20 * 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_LIMB25 * 2. If the result is to be written over the input, WP must be <= UP.26 */27 28mpi_limb_t29mpihelp_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}51