From 8145ee6cef54c80feae280577e26136e227ff2e0 Mon Sep 17 00:00:00 2001
From: Sean Parkinson <sean@wolfssl.com>
Date: Thu, 16 Jun 2022 10:39:23 +1000
Subject: [PATCH] TFM: mp_exptmod_ex didn't handle exceptional cases

fp_exptmod_ex() changed to match execptional case handling in
fp_exptmod().
---
 wolfcrypt/src/tfm.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/wolfcrypt/src/tfm.c b/wolfcrypt/src/tfm.c
index f9115f1c4..cc2f9554c 100644
--- a/wolfcrypt/src/tfm.c
+++ b/wolfcrypt/src/tfm.c
@@ -2895,14 +2895,21 @@ int fp_exptmod_ex(fp_int * G, fp_int * X, int digits, fp_int * P, fp_int * Y)
    int x = fp_count_bits (X);
 #endif
 
-   if (fp_iszero(G)) {
+   /* handle modulus of zero and prevent overflows */
+   if (fp_iszero(P) || (P->used > (FP_SIZE/2))) {
+      return FP_VAL;
+   }
+   if (fp_isone(P)) {
       fp_set(Y, 0);
       return FP_OKAY;
    }
-
-   /* prevent overflows */
-   if (P->used > (FP_SIZE/2)) {
-      return FP_VAL;
+   if (fp_iszero(X)) {
+      fp_set(Y, 1);
+      return FP_OKAY;
+   }
+   if (fp_iszero(G)) {
+      fp_set(Y, 0);
+      return FP_OKAY;
    }
 
 #if defined(WOLFSSL_ESP32WROOM32_CRYPT_RSA_PRI) && \