michael@0: /*
michael@0:  *  invmod.c
michael@0:  *
michael@0:  *  Compute modular inverses
michael@0:  *
michael@0:  * This Source Code Form is subject to the terms of the Mozilla Public
michael@0:  * License, v. 2.0. If a copy of the MPL was not distributed with this
michael@0:  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
michael@0: 
michael@0: #include <stdio.h>
michael@0: #include <stdlib.h>
michael@0: 
michael@0: #include "mpi.h"
michael@0: 
michael@0: int main(int argc, char *argv[])
michael@0: {
michael@0:   mp_int    a, m;
michael@0:   mp_err    res;
michael@0:   char     *buf;
michael@0:   int       len, out = 0;
michael@0: 
michael@0:   if(argc < 3) {
michael@0:     fprintf(stderr, "Usage: %s <a> <m>\n", argv[0]);
michael@0:     return 1;
michael@0:   }
michael@0: 
michael@0:   mp_init(&a); mp_init(&m);
michael@0:   mp_read_radix(&a, argv[1], 10);
michael@0:   mp_read_radix(&m, argv[2], 10);
michael@0: 
michael@0:   if(mp_cmp(&a, &m) > 0)
michael@0:     mp_mod(&a, &m, &a);
michael@0: 
michael@0:   switch((res = mp_invmod(&a, &m, &a))) {
michael@0:   case MP_OKAY:
michael@0:     len = mp_radix_size(&a, 10);
michael@0:     buf = malloc(len);
michael@0: 
michael@0:     mp_toradix(&a, buf, 10);
michael@0:     printf("%s\n", buf);
michael@0:     free(buf);
michael@0:     break;
michael@0: 
michael@0:   case MP_UNDEF:
michael@0:     printf("No inverse\n");
michael@0:     out = 1;
michael@0:     break;
michael@0: 
michael@0:   default:
michael@0:     printf("error: %s (%d)\n", mp_strerror(res), res);
michael@0:     out = 2;
michael@0:     break;
michael@0:   }
michael@0: 
michael@0:   mp_clear(&a);
michael@0:   mp_clear(&m);
michael@0: 
michael@0:   return out;
michael@0: }