lib/mpi/generic_mpih-rshift.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /* mpih-rshift.c  -  MPI helper functions
   3  * Copyright (C) 1994, 1996, 1998, 1999,
   4  *               2000, 2001 Free Software Foundation, Inc.
   5  *
   6  * This file is part of GNUPG
   7  *
   8  * Note: This code is heavily based on the GNU MP Library.
   9  *       Actually it's the same code with only minor changes in the
  10  *       way the data is stored; this is to support the abstraction
  11  *       of an optional secure memory allocation which may be used
  12  *       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 right
  20  * 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_LIMB
  25  * 2. If the result is to be written over the input, WP must be <= UP.
  26  */
  27
  28 mpi_limb_t
  29 mpihelp_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 }