generic_mpih-rshift.c 1.5 KB

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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. #include "mpi-internal.h"
  17. /* Shift U (pointed to by UP and USIZE limbs long) CNT bits to the right
  18. * and store the USIZE least significant limbs of the result at WP.
  19. * The bits shifted out to the right are returned.
  20. *
  21. * Argument constraints:
  22. * 1. 0 < CNT < BITS_PER_MP_LIMB
  23. * 2. If the result is to be written over the input, WP must be <= UP.
  24. */
  25. mpi_limb_t
  26. mpihelp_rshift(mpi_ptr_t wp, mpi_ptr_t up, mpi_size_t usize, unsigned cnt)
  27. {
  28. mpi_limb_t high_limb, low_limb;
  29. unsigned sh_1, sh_2;
  30. mpi_size_t i;
  31. mpi_limb_t retval;
  32. sh_1 = cnt;
  33. wp -= 1;
  34. sh_2 = BITS_PER_MPI_LIMB - sh_1;
  35. high_limb = up[0];
  36. retval = high_limb << sh_2;
  37. low_limb = high_limb;
  38. for (i = 1; i < usize; i++) {
  39. high_limb = up[i];
  40. wp[i] = (low_limb >> sh_1) | (high_limb << sh_2);
  41. low_limb = high_limb;
  42. }
  43. wp[i] = low_limb >> sh_1;
  44. return retval;
  45. }