package org.bouncycastle.crypto.agreement;

import java.math.BigInteger;

import org.bouncycastle.crypto.BasicAgreement;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.params.ECDomainParameters;
import org.bouncycastle.crypto.params.ECPrivateKeyParameters;
import org.bouncycastle.crypto.params.ECPublicKeyParameters;
import org.bouncycastle.math.ec.ECAlgorithms;
import org.bouncycastle.math.ec.ECConstants;
import org.bouncycastle.math.ec.ECPoint;

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).
/**
 * 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 int getFieldSize()
    {
        return (key.getParameters().getCurve().getFieldSize() + 7) / 8;
    }

    public BigInteger calculateAgreement(
        CipherParameters pubKey)
    {
        ECPublicKeyParameters pub = (ECPublicKeyParameters)pubKey;
        ECDomainParameters params = key.getParameters();
        if (!params.equals(pub.getParameters()))
        {
            throw new IllegalStateException("ECDH public key has wrong domain parameters");
        }

        BigInteger d = key.getD();

        // Always perform calculations on the exact curve specified by our private key's parameters
        ECPoint Q = ECAlgorithms.cleanPoint(params.getCurve(), pub.getQ());
        if (Q.isInfinity())
        {
            throw new IllegalStateException("Infinity is not a valid public key for ECDH");
        }

        BigInteger h = params.getH();
        if (!h.equals(ECConstants.ONE))
        {
            d = params.getHInv().multiply(d).mod(params.getN());
            Q = ECAlgorithms.referenceMultiply(Q, h);
        }

        ECPoint P = Q.multiply(d).normalize();
        if (P.isInfinity())
        {
            throw new IllegalStateException("Infinity is not a valid agreement value for ECDH");
        }

        return P.getAffineXCoord().toBigInteger();
    }
}