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       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                 :          0 : mpihelp_rshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt)
      30                 :            : {
      31                 :          0 :         mpi_limb_t high_limb, low_limb;
      32                 :          0 :         unsigned sh_1, sh_2;
      33                 :          0 :         mpi_size_t i;
      34                 :          0 :         mpi_limb_t retval;
      35                 :            : 
      36                 :          0 :         sh_1 = cnt;
      37                 :          0 :         wp -= 1;
      38                 :          0 :         sh_2 = BITS_PER_MPI_LIMB - sh_1;
      39                 :          0 :         high_limb = up[0];
      40                 :          0 :         retval = high_limb << sh_2;
      41                 :          0 :         low_limb = high_limb;
      42         [ #  # ]:          0 :         for (i = 1; i < usize; i++) {
      43                 :          0 :                 high_limb = up[i];
      44                 :          0 :                 wp[i] = (low_limb >> sh_1) | (high_limb << sh_2);
      45                 :          0 :                 low_limb = high_limb;
      46                 :            :         }
      47                 :          0 :         wp[i] = low_limb >> sh_1;
      48                 :            : 
      49                 :          0 :         return retval;
      50                 :            : }