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The Long Term Stable (LTS) Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains the JCA/JCE provider and low-level API for the BC LTS version 2.73.7 for Java 8 and later.

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package org.bouncycastle.crypto.agreement.kdf;

import java.io.IOException;

import org.bouncycastle.asn1.ASN1EncodableVector;
import org.bouncycastle.asn1.ASN1Encoding;
import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.DEROctetString;
import org.bouncycastle.asn1.DERSequence;
import org.bouncycastle.asn1.DERTaggedObject;
import org.bouncycastle.crypto.DataLengthException;
import org.bouncycastle.crypto.DerivationFunction;
import org.bouncycastle.crypto.DerivationParameters;
import org.bouncycastle.crypto.Digest;
import org.bouncycastle.crypto.OutputLengthException;
import org.bouncycastle.util.Pack;

/**
 * RFC 2631 Diffie-hellman KEK derivation function.
 */
public class DHKEKGenerator
    implements DerivationFunction
{
    private final Digest digest;

    private ASN1ObjectIdentifier algorithm;
    private int                 keySize;
    private byte[]              z;
    private byte[]              partyAInfo;

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

    public void init(DerivationParameters param)
    {
        DHKDFParameters params = (DHKDFParameters)param;

        this.algorithm = params.getAlgorithm();
        this.keySize = params.getKeySize();
        this.z = params.getZ();
        this.partyAInfo = params.getExtraInfo();
    }

    public Digest getDigest()
    {
        return digest;
    }

    public int generateBytes(byte[] out, int outOff, int len)
        throws DataLengthException, IllegalArgumentException
    {
        if ((out.length - len) < outOff)
        {
            throw new OutputLengthException("output buffer too small");
        }

        long    oBytes = len;
        int     outLen = digest.getDigestSize();

        //
        // this is at odds with the standard implementation, the
        // maximum value should be hBits * (2^32 - 1) where hBits
        // is the digest output size in bits. We can't have an
        // array with a long index at the moment...
        //
        if (oBytes > ((2L << 32) - 1))
        {
            throw new IllegalArgumentException("Output length too large");
        }

        int cThreshold = (int)((oBytes + outLen - 1) / outLen);

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

        int counter = 1;

        for (int i = 0; i < cThreshold; i++)
        {
            digest.update(z, 0, z.length);

            // OtherInfo
            ASN1EncodableVector v1 = new ASN1EncodableVector();
            // KeySpecificInfo
            ASN1EncodableVector v2 = new ASN1EncodableVector();

            v2.add(algorithm);
            v2.add(new DEROctetString(Pack.intToBigEndian(counter)));

            v1.add(new DERSequence(v2));

            if (partyAInfo != null)
            {
                v1.add(new DERTaggedObject(true, 0, new DEROctetString(partyAInfo)));
            }

            v1.add(new DERTaggedObject(true, 2, new DEROctetString(Pack.intToBigEndian(keySize))));

            try
            {
                byte[] other = new DERSequence(v1).getEncoded(ASN1Encoding.DER);

                digest.update(other, 0, other.length);
            }
            catch (IOException e)
            {
                throw new IllegalArgumentException("unable to encode parameter info: " + e.getMessage());
            }

            digest.doFinal(dig, 0);

            if (len > outLen)
            {
                System.arraycopy(dig, 0, out, outOff, outLen);
                outOff += outLen;
                len -= outLen;
            }
            else
            {
                System.arraycopy(dig, 0, out, outOff, len);
            }

            counter++;
        }

        digest.reset();

        return (int)oBytes;
    }
}