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 } 51