Heap Math exptmod: fixes for valid modulus checks

mp_exptmod_base_2() uses Montogmery method and does not support even
modulus. Added check.
mp_exptmod_fast() uses Montogmery method when odd not when dr > 0.
Comment updated at call.
Reduce value of zero doesn't work for mp_reduece_2k. Changed setup of
use to check for zero and won't use it. Other methods won't work either.

Author: SparkiDev

wolfcrypt/src/integer.c

@@ -955,7 +955,7 @@ int wolfcrypt_mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y)
   }
 
 #ifdef BN_MP_EXPTMOD_BASE_2
-  if (G->used == 1 && G->dp[0] == 2) {
+  if (G->used == 1 && G->dp[0] == 2 && mp_isodd(P) == MP_YES) {
     return mp_exptmod_base_2(X, P, Y);
   }
 #endif
@@ -985,7 +985,7 @@ int wolfcrypt_mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y)
   }
 #endif
 
-  /* if the modulus is odd or dr != 0 use the montgomery method */
+  /* if the modulus is odd use the montgomery method, or use other known */
 #ifdef BN_MP_EXPTMOD_FAST_C
   if (mp_isodd (P) == MP_YES || dr !=  0) {
     return mp_exptmod_fast (G, X, P, Y, dr);
@@ -1985,7 +1985,6 @@ int mp_dr_is_modulus(mp_int *a)
    return 1;
 }
 
-
 /* computes Y == G**X mod P, HAC pp.616, Algorithm 14.85
  *
  * Uses a left-to-right k-ary sliding window to compute the modular
@@ -2113,7 +2112,10 @@ int mp_exptmod_fast (mp_int * G, mp_int * X, mp_int * P, mp_int * Y,
      if ((err = mp_reduce_2k_setup(P, &mp)) != MP_OKAY) {
         goto LBL_M;
      }
-     redux = mp_reduce_2k;
+     /* mp of zero is not usable */
+     if (mp != 0) {
+         redux = mp_reduce_2k;
+     }
 #endif
   }