From 9a0a48e0939e39e81027d71a1d1b0f6facc62497 Mon Sep 17 00:00:00 2001
From: Jacob Barthelmeh <jacob@wolfssl.com>
Date: Fri, 22 Nov 2019 15:47:05 -0700
Subject: [PATCH] sanity check on "a" input to invmod

---
 wolfcrypt/src/tfm.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/wolfcrypt/src/tfm.c b/wolfcrypt/src/tfm.c
index a5c409f47..20f356de3 100644
--- a/wolfcrypt/src/tfm.c
+++ b/wolfcrypt/src/tfm.c
@@ -1023,6 +1023,11 @@ int fp_invmod(fp_int *a, fp_int *b, fp_int *c)
 #endif
   int     neg;
 
+  /* [modified] sanity check on "a" */
+  if (fp_iszero(a) == FP_YES) {
+    return FP_VAL; /* can not divide by 0 here */
+  }
+
   /* 2. [modified] b must be odd   */
   if (fp_iseven (b) == FP_YES) {
     return fp_invmod_slow(a,b,c);