From fa3f9b888fc924e77bc05323c5aaf4ba64b98d1c Mon Sep 17 00:00:00 2001
From: Salvatore Ingala <6681844+bigspider@users.noreply.github.com>
Date: Thu, 14 Nov 2024 09:35:38 +0000
Subject: [PATCH] Make cx_math_is_zero run in constant time

---
 lib_cxng/include/lcx_math.h | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/lib_cxng/include/lcx_math.h b/lib_cxng/include/lcx_math.h
index 40b9a90d3..d04b68fc6 100644
--- a/lib_cxng/include/lcx_math.h
+++ b/lib_cxng/include/lcx_math.h
@@ -568,13 +568,11 @@ DEPRECATED static inline void cx_math_next_prime(uint8_t *r, uint32_t len)
  */
 static inline bool cx_math_is_zero(const uint8_t *a, size_t len)
 {
-    uint32_t i;
-    for (i = 0; i < len; i++) {
-        if (a[i] != 0) {
-            return 0;
-        }
+    uint8_t acc = 0;  // accumulate all the bytes in order to run in constant time
+    for (size_t i = 0; i < len; i++) {
+        acc |= a[i];
     }
-    return 1;
+    return acc == 0;
 }
 
 #endif  // HAVE_MATH