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Spongy Castle is a package-rename (org.bouncycastle.* to org.spongycastle.*) of Bouncy Castle intended for Android. Android ships with a stripped-down version of Bouncy Castle - this causes classloader collisions if you try to add an alternative (updated/complete) Bouncy Castle jar. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5.

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package org.spongycastle.crypto.agreement.kdf;

import org.spongycastle.asn1.ASN1EncodableVector;
import org.spongycastle.asn1.DERNull;
import org.spongycastle.asn1.DERObjectIdentifier;
import org.spongycastle.asn1.DEROctetString;
import org.spongycastle.asn1.DERSequence;
import org.spongycastle.asn1.DERTaggedObject;
import org.spongycastle.asn1.x509.AlgorithmIdentifier;
import org.spongycastle.crypto.DataLengthException;
import org.spongycastle.crypto.DerivationFunction;
import org.spongycastle.crypto.DerivationParameters;
import org.spongycastle.crypto.Digest;
import org.spongycastle.crypto.generators.KDF2BytesGenerator;
import org.spongycastle.crypto.params.KDFParameters;

/**
 * X9.63 based key derivation function for ECDH CMS.
 */
public class ECDHKEKGenerator
    implements DerivationFunction
{
    private DerivationFunction kdf;

    private DERObjectIdentifier algorithm;
    private int                 keySize;
    private byte[]              z;

    public ECDHKEKGenerator(
        Digest digest)
    {
        this.kdf = new KDF2BytesGenerator(digest);
    }

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

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

    public Digest getDigest()
    {
        return kdf.getDigest();
    }

    public int generateBytes(byte[] out, int outOff, int len)
        throws DataLengthException, IllegalArgumentException
    {
        // TODO Create an ASN.1 class for this (RFC3278)
        // ECC-CMS-SharedInfo
        ASN1EncodableVector v = new ASN1EncodableVector();

        v.add(new AlgorithmIdentifier(algorithm, new DERNull()));
        v.add(new DERTaggedObject(true, 2, new DEROctetString(integerToBytes(keySize))));

        kdf.init(new KDFParameters(z, new DERSequence(v).getDEREncoded()));

        return kdf.generateBytes(out, outOff, len);
    }

    private byte[] integerToBytes(int keySize)
    {
        byte[] val = new byte[4];

        val[0] = (byte)(keySize >> 24);
        val[1] = (byte)(keySize >> 16);
        val[2] = (byte)(keySize >> 8);
        val[3] = (byte)keySize;

        return val;
    }
}