Special handling for zero-valued scalars

Some optimizations for NAF generation

1 files changed, 38 insertions, 20 deletions

diff --git a/crypto/src/math/ec/multiplier/WNafUtilities.cs b/crypto/src/math/ec/multiplier/WNafUtilities.cs

index b6e32382f..179d6d6eb 100644
--- a/crypto/src/math/ec/multiplier/WNafUtilities.cs
+++ b/crypto/src/math/ec/multiplier/WNafUtilities.cs
@@ -6,36 +6,42 @@ namespace Org.BouncyCastle.Math.EC.Multiplier
     {
         public static readonly string PRECOMP_NAME = "bc_wnaf";
 
-        private static int[] DEFAULT_WINDOW_SIZE_CUTOFFS = new int[]{ 13, 41, 121, 337, 897, 2305 };
+        private static readonly int[] DEFAULT_WINDOW_SIZE_CUTOFFS = new int[]{ 13, 41, 121, 337, 897, 2305 };
+
+        private static readonly byte[] EMPTY_BYTES = new byte[0];
+        private static readonly int[] EMPTY_INTS = new int[0];
 
         public static int[] GenerateCompactNaf(BigInteger k)
         {
             if ((k.BitLength >> 16) != 0)
                 throw new ArgumentException("must have bitlength < 2^16", "k");
+            if (k.SignValue == 0)
+                return EMPTY_INTS;
 
             BigInteger _3k = k.ShiftLeft(1).Add(k);
 
-            int digits = _3k.BitLength - 1;
-            int[] naf = new int[(digits + 1) >> 1];
+            int bits = _3k.BitLength;
+            int[] naf = new int[bits >> 1];
 
-            int length = 0, zeroes = 0;
-            for (int i = 1; i <= digits; ++i)
-            {
-                bool _3kBit = _3k.TestBit(i);
-                bool kBit = k.TestBit(i);
+            BigInteger diff = _3k.Xor(k);
 
-                if (_3kBit == kBit)
+            int highBit = bits - 1, length = 0, zeroes = 0;
+            for (int i = 1; i < highBit; ++i)
+            {
+                if (!diff.TestBit(i))
                 {
                     ++zeroes;
+                    continue;
                 }
-                else
-                {
-                    int digit  = kBit ? -1 : 1;
-                    naf[length++] = (digit << 16) | zeroes;
-                    zeroes = 0;
-                }
+
+                int digit = k.TestBit(i) ? -1 : 1;
+                naf[length++] = (digit << 16) | zeroes;
+                zeroes = 1;
+                ++i;
             }
 
+            naf[length++] = (1 << 16) | zeroes;
+
             if (naf.Length > length)
             {
                 naf = Trim(naf, length);
@@ -55,6 +61,8 @@ namespace Org.BouncyCastle.Math.EC.Multiplier
                 throw new ArgumentException("must be in the range [2, 16]", "width");
             if ((k.BitLength >> 16) != 0)
                 throw new ArgumentException("must have bitlength < 2^16", "k");
+            if (k.SignValue == 0)
+                return EMPTY_INTS;
 
             int[] wnaf = new int[k.BitLength / width + 1];
 
@@ -166,19 +174,27 @@ namespace Org.BouncyCastle.Math.EC.Multiplier
 
         public static byte[] GenerateNaf(BigInteger k)
         {
+            if (k.SignValue == 0)
+                return EMPTY_BYTES;
+
             BigInteger _3k = k.ShiftLeft(1).Add(k);
 
             int digits = _3k.BitLength - 1;
             byte[] naf = new byte[digits];
 
-            for (int i = 1; i <= digits; ++i)
-            {
-                bool _3kBit = _3k.TestBit(i);
-                bool kBit = k.TestBit(i);
+            BigInteger diff = _3k.Xor(k);
 
-                naf[i - 1] = (byte)(_3kBit == kBit ? 0 : kBit ? -1 : 1);
+            for (int i = 1; i < digits; ++i)
+            {
+                if (diff.TestBit(i))
+                {
+                    naf[i - 1] = (byte)(k.TestBit(i) ? -1 : 1);
+                    ++i;
+                }
             }
 
+            naf[digits - 1] = 1;
+
             return naf;
         }
 
@@ -203,6 +219,8 @@ namespace Org.BouncyCastle.Math.EC.Multiplier
 
             if (width < 2 || width > 8)
                 throw new ArgumentException("must be in the range [2, 8]", "width");
+            if (k.SignValue == 0)
+                return EMPTY_BYTES;
 
             byte[] wnaf = new byte[k.BitLength + 1];