1 /* mpz_invert (inv, x, n). Find multiplicative inverse of X in Z(N). 2 If X has an inverse, return non-zero and store inverse in INVERSE, 3 otherwise, return 0 and put garbage in INVERSE. 4 5 Copyright 1996, 1997, 1998, 1999, 2000, 2001, 2005, 2012 Free Software 6 Foundation, Inc. 7 8 This file is part of the GNU MP Library. 9 10 The GNU MP Library is free software; you can redistribute it and/or modify 11 it under the terms of the GNU Lesser General Public License as published by 12 the Free Software Foundation; either version 3 of the License, or (at your 13 option) any later version. 14 15 The GNU MP Library is distributed in the hope that it will be useful, but 16 WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 17 or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public 18 License for more details. 19 20 You should have received a copy of the GNU Lesser General Public License 21 along with the GNU MP Library. If not, see http://www.gnu.org/licenses/. */ 22 23 #include "gmp.h" 24 #include "gmp-impl.h" 25 26 int 27 mpz_invert (mpz_ptr inverse, mpz_srcptr x, mpz_srcptr n) 28 { 29 mpz_t gcd, tmp; 30 mp_size_t xsize, nsize, size; 31 TMP_DECL; 32 33 xsize = ABSIZ (x); 34 nsize = ABSIZ (n); 35 36 /* No inverse exists if the leftside operand is 0. Likewise, no 37 inverse exists if the mod operand is 1. */ 38 if (xsize == 0 || (nsize == 1 && (PTR (n))[0] == 1)) 39 return 0; 40 41 size = MAX (xsize, nsize) + 1; 42 TMP_MARK; 43 44 MPZ_TMP_INIT (gcd, size); 45 MPZ_TMP_INIT (tmp, size); 46 mpz_gcdext (gcd, tmp, (mpz_ptr) 0, x, n); 47 48 /* If no inverse existed, return with an indication of that. */ 49 if (!MPZ_EQUAL_1_P (gcd)) 50 { 51 TMP_FREE; 52 return 0; 53 } 54 55 /* Make sure we return a positive inverse. */ 56 if (SIZ (tmp) < 0) 57 { 58 if (SIZ (n) < 0) 59 mpz_sub (inverse, tmp, n); 60 else 61 mpz_add (inverse, tmp, n); 62 } 63 else 64 mpz_set (inverse, tmp); 65 66 TMP_FREE; 67 return 1; 68 } 69