From aa14607a6f2444f1a4ee8cc5a1a5f542d897e410 Mon Sep 17 00:00:00 2001
From: Sean Parkinson <sean@wolfssl.com>
Date: Mon, 21 Mar 2022 10:43:06 +1000
Subject: [PATCH] TFM fp_div_2_ct: rework to avoid overflow

Don't set the overflow word. Instead integrate the div by 2 into the
function so that the overflow word doesn't need to be stored.
---
 wolfcrypt/src/tfm.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/wolfcrypt/src/tfm.c b/wolfcrypt/src/tfm.c
index 5d282caa4..05c397d84 100644
--- a/wolfcrypt/src/tfm.c
+++ b/wolfcrypt/src/tfm.c
@@ -894,11 +894,13 @@ int fp_div_2_mod_ct(fp_int *a, fp_int *b, fp_int *c)
       c->dp[i]   = (fp_digit)w;
       w        >>= DIGIT_BIT;
   }
-  c->dp[i] = (fp_digit)w;
+  for (i = 0; i < b->used-1; i++) {
+      c->dp[i] = (c->dp[i] >> 1) | (c->dp[i+1] << (DIGIT_BIT - 1));
+  }
+  c->dp[i] = (c->dp[i] >> 1) | ((fp_digit)w << (DIGIT_BIT - 1));
   c->used = i + 1;
   c->sign = FP_ZPOS;
   fp_clamp(c);
-  fp_div_2(c, c);
 
   return FP_OKAY;
 }