improve ltc_math_descriptor a bit

* introduce LTC_MILLER_RABIN_REPS which is used as default value
  in the isprime() implementations

src/math/gmp_desc.c

@@ -446,7 +446,7 @@ static int isprime(void *a, int b, int *c)
    LTC_ARGCHK(a != NULL);
    LTC_ARGCHK(c != NULL);
    if (b == 0) {
-       b = 8;
+       b = LTC_MILLER_RABIN_REPS;
    } /* if */
    *c = mpz_probab_prime_p(a, b) > 0 ? LTC_MP_YES : LTC_MP_NO;
    return CRYPT_OK;

src/math/ltm_desc.c

@@ -404,7 +404,7 @@ static int isprime(void *a, int b, int *c)
    LTC_ARGCHK(a != NULL);
    LTC_ARGCHK(c != NULL);
    if (b == 0) {
-       b = 8;
+       b = LTC_MILLER_RABIN_REPS;
    } /* if */
    err = mpi_to_ltc_error(mp_prime_is_prime(a, b, c));
    *c = (*c == MP_YES) ? LTC_MP_YES : LTC_MP_NO;

src/math/rand_prime.c

@@ -66,7 +66,7 @@ int rand_prime(void *N, long len, prng_state *prng, int wprng)
       }
 
       /* test */
-      if ((err = mp_prime_is_prime(N, 8, &res)) != CRYPT_OK) {
+      if ((err = mp_prime_is_prime(N, LTC_MILLER_RABIN_REPS, &res)) != CRYPT_OK) {
          XFREE(buf);
          return err;
       }

src/math/tfm_desc.c

@@ -415,8 +415,10 @@ static int isprime(void *a, int b, int *c)
 {
    LTC_ARGCHK(a != NULL);
    LTC_ARGCHK(c != NULL);
-   (void)b;
-   *c = (fp_isprime(a) == FP_YES) ? LTC_MP_YES : LTC_MP_NO;
+   if (b == 0) {
+       b = LTC_MILLER_RABIN_REPS;
+   } /* if */
+   *c = (fp_isprime_ex(a, b) == FP_YES) ? LTC_MP_YES : LTC_MP_NO;
    return CRYPT_OK;
 }