LXR linux/lib/mpi/generic

   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
  27mpi_limb_t
  28mpihelp_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}
  51