org.bouncycastle.math.ec.FixedPointCombMultiplier Maven / Gradle / Ivy
package org.bouncycastle.math.ec;
import java.math.BigInteger;
import org.bouncycastle.math.raw.Nat;
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");
}
FixedPointPreCompInfo info = FixedPointUtil.precompute(p);
ECLookupTable lookupTable = info.getLookupTable();
int width = info.getWidth();
int d = (size + width - 1) / width;
ECPoint R = c.getInfinity();
int fullComb = d * width;
int[] K = Nat.fromBigInteger(fullComb, k);
int top = fullComb - 1;
for (int i = 0; i < d; ++i)
{
int secretIndex = 0;
for (int j = top - i; j >= 0; j -= d)
{
int secretBit = K[j >>> 5] >>> (j & 0x1F);
secretIndex ^= secretBit >>> 1;
secretIndex <<= 1;
secretIndex ^= secretBit;
}
ECPoint add = lookupTable.lookup(secretIndex);
R = R.twicePlus(add);
}
return R.add(info.getOffset());
}
}