Refactor reduction methods and change scope of PExt fields

1 files changed, 36 insertions, 30 deletions

diff --git a/crypto/src/math/ec/custom/sec/SecP256R1Field.cs b/crypto/src/math/ec/custom/sec/SecP256R1Field.cs
index a01cb5840..4d98b5508 100644
--- a/crypto/src/math/ec/custom/sec/SecP256R1Field.cs
+++ b/crypto/src/math/ec/custom/sec/SecP256R1Field.cs
@@ -8,12 +8,12 @@ namespace Org.BouncyCastle.Math.EC.Custom.Sec
         // 2^256 - 2^224 + 2^192 + 2^96 - 1
         internal static readonly uint[] P = new uint[]{ 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0x00000000, 0x00000000, 0x00000000,
             0x00000001, 0xFFFFFFFF };
+        internal static readonly uint[] PExt = new uint[]{ 0x00000001, 0x00000000, 0x00000000, 0xFFFFFFFE, 0xFFFFFFFF,
+            0xFFFFFFFF, 0xFFFFFFFE, 0x00000001, 0xFFFFFFFE, 0x00000001, 0xFFFFFFFE, 0x00000001, 0x00000001, 0xFFFFFFFE,
+            0x00000002, 0xFFFFFFFE };
         private static readonly uint[] _2P = new uint[]{ 0xFFFFFFFE, 0xFFFFFFFF, 0xFFFFFFFF, 0x00000001, 0x00000000, 0x00000000,
             0x00000002, 0xFFFFFFFE, 0x00000001 };
         private const uint P7 = 0xFFFFFFFF;
-        private static readonly uint[] PExt = new uint[]{ 0x00000001, 0x00000000, 0x00000000, 0xFFFFFFFE, 0xFFFFFFFF,
-            0xFFFFFFFF, 0xFFFFFFFE, 0x00000001, 0xFFFFFFFE, 0x00000001, 0xFFFFFFFE, 0x00000001, 0x00000001, 0xFFFFFFFE,
-            0x00000002, 0xFFFFFFFE };
 
         public static void Add(uint[] x, uint[] y, uint[] z)
         {
@@ -125,7 +125,7 @@ namespace Org.BouncyCastle.Math.EC.Custom.Sec
             cc >>= 32;
 
             int c = (int)cc;
-            if (c > 0)
+            if (c >= 0)
             {
                 Reduce32((uint)c, z);
             }
@@ -144,33 +144,39 @@ namespace Org.BouncyCastle.Math.EC.Custom.Sec
 
         public static void Reduce32(uint x, uint[] z)
         {
-            long xx08 = x;
-
             long cc = 0;
-            cc += (long)z[0] + xx08;
-            z[0] = (uint)cc;
-            cc >>= 32;
-            cc += (long)z[1];
-            z[1] = (uint)cc;
-            cc >>= 32;
-            cc += (long)z[2];
-            z[2] = (uint)cc;
-            cc >>= 32;
-            cc += (long)z[3] - xx08;
-            z[3] = (uint)cc;
-            cc >>= 32;
-            cc += (long)z[4];
-            z[4] = (uint)cc;
-            cc >>= 32;
-            cc += (long)z[5];
-            z[5] = (uint)cc;
-            cc >>= 32;
-            cc += (long)z[6] - xx08;
-            z[6] = (uint)cc;
-            cc >>= 32;
-            cc += (long)z[7] + xx08;
-            z[7] = (uint)cc;
-            cc >>= 32;
+
+            if (x != 0)
+            {
+                long xx08 = x;
+
+                cc += (long)z[0] + xx08;
+                z[0] = (uint)cc;
+                cc >>= 32;
+                cc += (long)z[1];
+                z[1] = (uint)cc;
+                cc >>= 32;
+                cc += (long)z[2];
+                z[2] = (uint)cc;
+                cc >>= 32;
+                cc += (long)z[3] - xx08;
+                z[3] = (uint)cc;
+                cc >>= 32;
+                cc += (long)z[4];
+                z[4] = (uint)cc;
+                cc >>= 32;
+                cc += (long)z[5];
+                z[5] = (uint)cc;
+                cc >>= 32;
+                cc += (long)z[6] - xx08;
+                z[6] = (uint)cc;
+                cc >>= 32;
+                cc += (long)z[7] + xx08;
+                z[7] = (uint)cc;
+                cc >>= 32;
+
+                Debug.Assert(cc == 0 || cc == 1);
+            }
 
             if (cc != 0 || (z[7] == P7 && Nat256.Gte(z, P)))
             {