lib/mpi/generic_mpih-lshift.c

   1 // SPDX-License-Identifier: GPL-2.0-or-later
   2 /* mpihelp-lshift.c  -  MPI helper functions
   3  * Copyright (C) 1994, 1996, 1998, 2001 Free Software Foundation, Inc.
   4  *
   5  * This file is part of GnuPG.
   6  *
   7  * Note: This code is heavily based on the GNU MP Library.
   8  *       Actually it's the same code with only minor changes in the
   9  *       way the data is stored; this is to support the abstraction
  10  *       of an optional secure memory allocation which may be used
  11  *       to avoid revealing of sensitive data due to paging etc.
  12  *       The GNU MP Library itself is published under the LGPL;
  13  *       however I decided to publish this code under the plain GPL.
  14  */
  15
  16 #include "mpi-internal.h"
  17
  18 /* Shift U (pointed to by UP and USIZE digits long) CNT bits to the left
  19  * and store the USIZE least significant digits of the result at WP.
  20  * Return the bits shifted out from the most significant digit.
  21  *
  22  * Argument constraints:
  23  * 1. 0 < CNT < BITS_PER_MP_LIMB
  24  * 2. If the result is to be written over the input, WP must be >= UP.
  25  */
  26
  27 mpi_limb_t
  28 mpihelp_lshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned int cnt)
  29 {
  30         mpi_limb_t high_limb, low_limb;
  31         unsigned sh_1, sh_2;
  32         mpi_size_t i;
  33         mpi_limb_t retval;
  34
  35         sh_1 = cnt;
  36         wp += 1;
  37         sh_2 = BITS_PER_MPI_LIMB - sh_1;
  38         i = usize - 1;
  39         low_limb = up[i];
  40         retval = low_limb >> sh_2;
  41         high_limb = low_limb;
  42         while (--i >= 0) {
  43                 low_limb = up[i];
  44                 wp[i] = (high_limb << sh_1) | (low_limb >> sh_2);
  45                 high_limb = low_limb;
  46         }
  47         wp[i] = high_limb << sh_1;
  48
  49         return retval;
  50 }