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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 1.5 and up.

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package org.bouncycastle.crypto.agreement;

import java.math.BigInteger;

import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.params.ECDomainParameters;
import org.bouncycastle.crypto.params.ECPrivateKeyParameters;
import org.bouncycastle.crypto.params.ECPublicKeyParameters;
import org.bouncycastle.crypto.params.ParametersWithUKM;
import org.bouncycastle.math.ec.ECAlgorithms;
import org.bouncycastle.math.ec.ECPoint;
import org.bouncycastle.util.BigIntegers;

/**
 * GOST VKO key agreement class - RFC 7836 Section 4.3
 */
public class ECVKOAgreement
{
    private final Digest digest;

    private ECPrivateKeyParameters key;
    private BigInteger ukm;

    public ECVKOAgreement(Digest digest)
    {
        this.digest = digest;
    }

    public void init(
        CipherParameters key)
    {
        ParametersWithUKM p = (ParametersWithUKM)key;

        this.key = (ECPrivateKeyParameters)p.getParameters();
        this.ukm = toInteger(p.getUKM());
    }

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

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

        BigInteger hd = params.getH().multiply(ukm).multiply(key.getD()).mod(params.getN());

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

        ECPoint P = pubPoint.multiply(hd).normalize();

        if (P.isInfinity())
        {
            throw new IllegalStateException("Infinity is not a valid agreement value for ECVKO");
        }

        return fromPoint(P);
    }

    private static BigInteger toInteger(byte[] ukm)
    {
        byte[] v = new byte[ukm.length];

        for (int i = 0; i != v.length; i++)
        {
            v[i] = ukm[ukm.length - i - 1];
        }

        return new BigInteger(1, v);
    }

    private byte[] fromPoint(ECPoint v)
    {
        BigInteger bX = v.getAffineXCoord().toBigInteger();
        BigInteger bY = v.getAffineYCoord().toBigInteger();

        int size;
        if (bX.toByteArray().length > 33)
        {
            size = 64;
        }
        else
        {
            size = 32;
        }

        byte[] bytes = new byte[2 * size];
        byte[] x = BigIntegers.asUnsignedByteArray(size, bX);
        byte[] y = BigIntegers.asUnsignedByteArray(size, bY);

        for (int i = 0; i != size; i++)
        {
            bytes[i] = x[size - i - 1];
        }
        for (int i = 0; i != size; i++)
        {
            bytes[size + i] = y[size - i - 1];
        }

        digest.update(bytes, 0, bytes.length);

        byte[] rv = new byte[digest.getDigestSize()];

        digest.doFinal(rv, 0);

        return rv;
    }
}