1 files changed, 57 insertions, 0 deletions
diff --git a/core/src/main/java/org/spongycastle/math/ec/FixedPointCombMultiplier.java b/core/src/main/java/org/spongycastle/math/ec/FixedPointCombMultiplier.java
new file mode 100644
index 00000000..42ba2cc8
--- /dev/null
+++ b/core/src/main/java/org/spongycastle/math/ec/FixedPointCombMultiplier.java
@@ -0,0 +1,57 @@
+package org.spongycastle.math.ec;
+
+import java.math.BigInteger;
+
+public class FixedPointCombMultiplier extends AbstractECMultiplier
+{
+    protected ECPoint multiplyPositive(ECPoint p, BigInteger k)
+    {
+        ECCurve c = p.getCurve();
+        int size = FixedPointUtil.getCombSize(c);
+
+        if (k.bitLength() > size)
+        {
+            /*
+             * TODO The comb works best when the scalars are less than the (possibly unknown) order.
+             * Still, if we want to handle larger scalars, we could allow customization of the comb
+             * size, or alternatively we could deal with the 'extra' bits either by running the comb
+             * multiple times as necessary, or by using an alternative multiplier as prelude.
+             */
+            throw new IllegalStateException("fixed-point comb doesn't support scalars larger than the curve order");
+        }
+
+        int minWidth = getWidthForCombSize(size);
+
+        FixedPointPreCompInfo info = FixedPointUtil.precompute(p, minWidth);
+        ECPoint[] lookupTable = info.getPreComp();
+        int width = info.getWidth();
+
+        int d = (size + width - 1) / width;
+
+        ECPoint R = c.getInfinity();
+
+        int top = d * width - 1; 
+        for (int i = 0; i < d; ++i)
+        {
+            int index = 0;
+
+            for (int j = top - i; j >= 0; j -= d)
+            {
+                index <<= 1;
+                if (k.testBit(j))
+                {
+                    index |= 1;
+                }
+            }
+
+            R = R.twicePlus(lookupTable[index]);
+        }
+
+        return R;
+    }
+
+    protected int getWidthForCombSize(int combSize)
+    {
+        return combSize > 257 ? 6 : 5;
+    }
+}