package org.bouncycastle.crypto.agreement;import java.math.BigInteger;import org.bouncycastle.math.ec.ECPoint;import org.bouncycastle.crypto.BasicAgreement;import org.bouncycastle.crypto.CipherParameters;import org.bouncycastle.crypto.params.ECPublicKeyParameters;import org.bouncycastle.crypto.params.ECPrivateKeyParameters;/** * P1363 7.2.1 ECSVDP-DH * * ECSVDP-DH is Elliptic Curve Secret Value Derivation Primitive, * Diffie-Hellman version. It is based on the work of [DH76], [Mil86], * and [Kob87]. This primitive derives a shared secret value from one * party's private key and another party's public key, where both have * the same set of EC domain parameters. If two parties correctly * execute this primitive, they will produce the same output. This * primitive can be invoked by a scheme to derive a shared secret key; * specifically, it may be used with the schemes ECKAS-DH1 and * DL/ECKAS-DH2. It assumes that the input keys are valid (see also * Section 7.2.2). */public class ECDHBasicAgreement    implements BasicAgreement{    private ECPrivateKeyParameters key;    public void init(        CipherParameters key)    {        this.key = (ECPrivateKeyParameters)key;    }    public BigInteger calculateAgreement(        CipherParameters pubKey)    {        ECPublicKeyParameters pub = (ECPublicKeyParameters)pubKey;        ECPoint P = pub.getQ().multiply(key.getD());        // if (p.isInfinity()) throw new RuntimeException("d*Q == infinity");        return P.getX().toBigInteger();    }}