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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.8 and up. Note: this package includes the NTRU encryption algorithms.

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package org.bouncycastle.math.ec;

import java.math.BigInteger;

import org.bouncycastle.math.ec.endo.EndoUtil;
import org.bouncycastle.math.ec.endo.GLVEndomorphism;

public class GLVMultiplier extends AbstractECMultiplier
{
    protected final ECCurve curve;
    protected final GLVEndomorphism glvEndomorphism;

    public GLVMultiplier(ECCurve curve, GLVEndomorphism glvEndomorphism)
    {
        if (curve == null || curve.getOrder() == null)
        {
            throw new IllegalArgumentException("Need curve with known group order");
        }

        this.curve = curve;
        this.glvEndomorphism = glvEndomorphism;
    }

    protected ECPoint multiplyPositive(ECPoint p, BigInteger k)
    {
        if (!curve.equals(p.getCurve()))
        {
            throw new IllegalStateException();
        }

        BigInteger n = p.getCurve().getOrder();
        BigInteger[] ab = glvEndomorphism.decomposeScalar(k.mod(n));
        BigInteger a = ab[0], b = ab[1];

        if (glvEndomorphism.hasEfficientPointMap())
        {
            return ECAlgorithms.implShamirsTrickWNaf(glvEndomorphism, p, a, b);
        }

        ECPoint q = EndoUtil.mapPoint(glvEndomorphism, p);

        return ECAlgorithms.implShamirsTrickWNaf(p, a, q, b);
    }
}