pb_invmod.c

   1 /* LibTomPoly, Polynomial Basis Math -- Tom St Denis
   2  *
   3  * LibTomPoly is a public domain library that provides
   4  * polynomial basis arithmetic support.  It relies on
   5  * LibTomMath for large integer support.
   6  *
   7  * This library is free for all purposes without any
   8  * express guarantee that it works.
   9  *
  10  * Tom St Denis, tomstdenis@iahu.ca, http://poly.libtomcrypt.org
  11  */
  12 #include <tompoly.h>
  13
  14 int pb_invmod(pb_poly *a, pb_poly *b, pb_poly *c)
  15 {
  16    int err;
  17    pb_poly tmp;
  18
  19    if ((err = pb_init(&tmp, &(c->characteristic))) != MP_OKAY) {
  20       return err;
  21    }
  22
  23    if ((err = pb_exteuclid(a, b, c, NULL, &tmp)) != MP_OKAY)    { goto _ERR; }
  24
  25    /* if deg(tmp(x)) > 0 then there is no invmod */
  26    if (tmp.used > 1)                                            { err = MP_VAL; goto _ERR; }
  27
  28    err = MP_OKAY;
  29 _ERR: pb_clear(&tmp);
  30    return err;
  31 }
  32
  33