1 // SPDX-License-Identifier: GPL-2.0-or-later 2 /* mpihelp-mul_3.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_submul_1(mpi_ptr_t res_ptr, mpi_ptr_t s1_ptr, 21 mpi_size_t s1_size, mpi_limb_t s2_limb) 22 { 23 mpi_limb_t cy_limb; 24 mpi_size_t j; 25 mpi_limb_t prod_high, prod_low; 26 mpi_limb_t x; 27 28 /* The loop counter and index J goes from -SIZE to -1. This way 29 * the loop becomes faster. */ 30 j = -s1_size; 31 res_ptr -= j; 32 s1_ptr -= j; 33 34 cy_limb = 0; 35 do { 36 umul_ppmm(prod_high, prod_low, s1_ptr[j], s2_limb); 37 38 prod_low += cy_limb; 39 cy_limb = (prod_low < cy_limb ? 1 : 0) + prod_high; 40 41 x = res_ptr[j]; 42 prod_low = x - prod_low; 43 cy_limb += prod_low > x ? 1 : 0; 44 res_ptr[j] = prod_low; 45 } while (++j); 46 47 return cy_limb; 48 } 49