Provide a constant-time zero test

2 files changed, 18 insertions, 4 deletions

diff --git a/crypto/src/math/ec/rfc7748/X25519Field.cs b/crypto/src/math/ec/rfc7748/X25519Field.cs
index fd5599657..b5938e2e7 100644
--- a/crypto/src/math/ec/rfc7748/X25519Field.cs
+++ b/crypto/src/math/ec/rfc7748/X25519Field.cs
@@ -179,14 +179,21 @@ namespace Org.BouncyCastle.Math.EC.Rfc7748
             Mul(t, x2, z);
         }
 
-        public static bool IsZeroVar(int[] x)
+        public static int IsZero(int[] x)
         {
             int d = 0;
             for (int i = 0; i < Size; ++i)
             {
                 d |= x[i];
             }
-            return d == 0;
+            d |= d >> 16;
+            d &= 0xFFFF;
+            return (d - 1) >> 31;
+        }
+
+        public static bool IsZeroVar(int[] x)
+        {
+            return 0 != IsZero(x);
         }
 
         public static void Mul(int[] x, int y, int[] z)
diff --git a/crypto/src/math/ec/rfc7748/X448Field.cs b/crypto/src/math/ec/rfc7748/X448Field.cs
index 5a682714d..7cda6ebcc 100644
--- a/crypto/src/math/ec/rfc7748/X448Field.cs
+++ b/crypto/src/math/ec/rfc7748/X448Field.cs
@@ -195,14 +195,21 @@ namespace Org.BouncyCastle.Math.EC.Rfc7748
             Mul(t, x, z);
         }
 
-        public static bool IsZeroVar(uint[] x)
+        public static int IsZero(uint[] x)
         {
             uint d = 0;
             for (int i = 0; i < Size; ++i)
             {
                 d |= x[i];
             }
-            return d == 0U;
+            d |= d >> 16;
+            d &= 0xFFFF;
+            return ((int)d - 1) >> 31;
+        }
+
+        public static bool IsZeroVar(uint[] x)
+        {
+            return 0U != IsZero(x);
         }
 
         public static void Mul(uint[] x, uint y, uint[] z)