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 /*

  *  invmod.c

  *

  *  Compute modular inverses

  *

  * This Source Code Form is subject to the terms of the Mozilla Public

  * License, v. 2.0. If a copy of the MPL was not distributed with this

  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

 #include <stdio.h>

 #include <stdlib.h>

 #include "mpi.h"

 int main(int argc, char *argv[])

 {

   mp_int    a, m;

   mp_err    res;

   char     *buf;

   int       len, out = 0;

   if(argc < 3) {

     fprintf(stderr, "Usage: %s <a> <m>\n", argv[0]);

     return 1;

   }

   mp_init(&a); mp_init(&m);

   mp_read_radix(&a, argv[1], 10);

   mp_read_radix(&m, argv[2], 10);

   if(mp_cmp(&a, &m) > 0)

     mp_mod(&a, &m, &a);

   switch((res = mp_invmod(&a, &m, &a))) {

   case MP_OKAY:

     len = mp_radix_size(&a, 10);

     buf = malloc(len);

     mp_toradix(&a, buf, 10);

     printf("%s\n", buf);

     free(buf);

     break;

   case MP_UNDEF:

     printf("No inverse\n");

     out = 1;

     break;

   default:

     printf("error: %s (%d)\n", mp_strerror(res), res);

     out = 2;

     break;

   }

   mp_clear(&a);

   mp_clear(&m);

   return out;

 }