1 // SPDX-License-Identifier: GPL-2.0-or-later 2 /* mpihelp-add_2.c - MPI helper functions 3 * Copyright (C) 1994, 1996, 1997, 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 #include "longlong.h" 18 19 mpi_limb_t 20 mpihelp_sub_n(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr, 21 mpi_ptr_t s2_ptr, mpi_size_t size) 22 { 23 mpi_limb_t x, y, cy; 24 mpi_size_t j; 25 26 /* The loop counter and index J goes from -SIZE to -1. This way 27 the loop becomes faster. */ 28 j = -size; 29 30 /* Offset the base pointers to compensate for the negative indices. */ 31 s1_ptr -= j; 32 s2_ptr -= j; 33 res_ptr -= j; 34 35 cy = 0; 36 do { 37 y = s2_ptr[j]; 38 x = s1_ptr[j]; 39 y += cy; /* add previous carry to subtrahend */ 40 cy = y < cy; /* get out carry from that addition */ 41 y = x - y; /* main subtract */ 42 cy += y > x; /* get out carry from the subtract, combine */ 43 res_ptr[j] = y; 44 } while (++j); 45 46 return cy; 47 } 48