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package org.bouncycastle.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 width = getWidthForCombSize(size);
FixedPointPreCompInfo info = FixedPointUtil.precompute(p, width);
ECPoint[] lookupTable = info.getPreComp();
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;
}
}
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