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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for the Android platform. Android unfortunately ships with a stripped-down version of Bouncy Castle, which prevents easy upgrades - Spongy Castle overcomes this and provides a full, up-to-date version of the Bouncy Castle cryptographic libs.

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package org.spongycastle.math.ec;

import java.math.BigInteger;

/**
 * Class implementing the NAF (Non-Adjacent Form) multiplication algorithm (right-to-left) using
 * mixed coordinates.
 */
public class MixedNafR2LMultiplier extends AbstractECMultiplier
{
    protected int additionCoord, doublingCoord;

    /**
     * By default, addition will be done in Jacobian coordinates, and doubling will be done in
     * Modified Jacobian coordinates (independent of the original coordinate system of each point).
     */
    public MixedNafR2LMultiplier()
    {
        this(ECCurve.COORD_JACOBIAN, ECCurve.COORD_JACOBIAN_MODIFIED);
    }

    public MixedNafR2LMultiplier(int additionCoord, int doublingCoord)
    {
        this.additionCoord = additionCoord;
        this.doublingCoord = doublingCoord;
    }

    protected ECPoint multiplyPositive(ECPoint p, BigInteger k)
    {
        ECCurve curveOrig = p.getCurve();

        ECCurve curveAdd = configureCurve(curveOrig, additionCoord);
        ECCurve curveDouble = configureCurve(curveOrig, doublingCoord);

        int[] naf = WNafUtil.generateCompactNaf(k);

        ECPoint Ra = curveAdd.getInfinity();
        ECPoint Td = curveDouble.importPoint(p);

        int zeroes = 0;
        for (int i = 0; i < naf.length; ++i)
        {
            int ni = naf[i];
            int digit = ni >> 16;
            zeroes += ni & 0xFFFF;

            Td = Td.timesPow2(zeroes);

            ECPoint Tj = curveAdd.importPoint(Td);
            if (digit < 0)
            {
                Tj = Tj.negate();
            }

            Ra = Ra.add(Tj);

            zeroes = 1;
        }

        return curveOrig.importPoint(Ra);
    }

    protected ECCurve configureCurve(ECCurve c, int coord)
    {
        if (c.getCoordinateSystem() == coord)
        {
            return c;
        }

        if (!c.supportsCoordinateSystem(coord))
        {
            throw new IllegalArgumentException("Coordinate system " + coord + " not supported by this curve");
        }

        return c.configure().setCoordinateSystem(coord).create();
    }
}