Avoid infinity appearing in lookup table for FixedPointCombMultiplier

timzhang926 · May 8, 2017 · 05b83be · 05b83be
1 parent ccc3d43
commit 05b83be
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Showing 3 changed files with 22 additions and 6 deletions.
diff --git a/core/src/main/java/org/bouncycastle/math/ec/FixedPointCombMultiplier.java b/core/src/main/java/org/bouncycastle/math/ec/FixedPointCombMultiplier.java
@@ -47,7 +47,7 @@ protected ECPoint multiplyPositive(ECPoint p, BigInteger k)
             R = R.twicePlus(lookupTable[index]);
         }
 
-        return R;
+        return R.add(info.getOffset());
     }
 
     protected int getWidthForCombSize(int combSize)

diff --git a/core/src/main/java/org/bouncycastle/math/ec/FixedPointPreCompInfo.java b/core/src/main/java/org/bouncycastle/math/ec/FixedPointPreCompInfo.java
@@ -5,6 +5,8 @@
  */
 public class FixedPointPreCompInfo implements PreCompInfo
 {
+    protected ECPoint offset = null;
+
     /**
      * Array holding the precomputed <code>ECPoint</code>s used for a fixed
      * point multiplication.
@@ -18,6 +20,16 @@ public class FixedPointPreCompInfo implements PreCompInfo
      */
     protected int width = -1;
 
+    public ECPoint getOffset()
+    {
+        return offset;
+    }
+
+    public void setOffset(ECPoint offset)
+    {
+        this.offset = offset;
+    }
+
     public ECPoint[] getPreComp()
     {
         return preComp;

diff --git a/core/src/main/java/org/bouncycastle/math/ec/FixedPointUtil.java b/core/src/main/java/org/bouncycastle/math/ec/FixedPointUtil.java
@@ -35,18 +35,21 @@ public static FixedPointPreCompInfo precompute(ECPoint p, int minWidth)
             int bits = getCombSize(c);
             int d = (bits + minWidth - 1) / minWidth;
 
-            ECPoint[] pow2Table = new ECPoint[minWidth];
+            ECPoint[] pow2Table = new ECPoint[minWidth + 1];
             pow2Table[0] = p;
             for (int i = 1; i < minWidth; ++i)
             {
                 pow2Table[i] = pow2Table[i - 1].timesPow2(d);
             }
-
+
+            // This will be the 'offset' value 
+            pow2Table[minWidth] = pow2Table[0].subtract(pow2Table[1]);
+
             c.normalizeAll(pow2Table);
-    
+
             lookupTable = new ECPoint[n];
-            lookupTable[0] = c.getInfinity();
-    
+            lookupTable[0] = pow2Table[0];
+
             for (int bit = minWidth - 1; bit >= 0; --bit)
             {
                 ECPoint pow2 = pow2Table[bit];
@@ -60,6 +63,7 @@ public static FixedPointPreCompInfo precompute(ECPoint p, int minWidth)
 
             c.normalizeAll(lookupTable);
 
+            info.setOffset(pow2Table[minWidth]);
             info.setPreComp(lookupTable);
             info.setWidth(minWidth);