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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.7.
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package org.spongycastle.jcajce.provider.asymmetric.elgamal;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.math.BigInteger;

import javax.crypto.interfaces.DHPublicKey;
import javax.crypto.spec.DHParameterSpec;
import javax.crypto.spec.DHPublicKeySpec;

import org.spongycastle.asn1.ASN1Encoding;
import org.spongycastle.asn1.ASN1Sequence;
import org.spongycastle.asn1.DERInteger;
import org.spongycastle.asn1.oiw.ElGamalParameter;
import org.spongycastle.asn1.oiw.OIWObjectIdentifiers;
import org.spongycastle.asn1.x509.AlgorithmIdentifier;
import org.spongycastle.asn1.x509.SubjectPublicKeyInfo;
import org.spongycastle.crypto.params.ElGamalPublicKeyParameters;
import org.spongycastle.jce.interfaces.ElGamalPublicKey;
import org.spongycastle.jce.spec.ElGamalParameterSpec;
import org.spongycastle.jce.spec.ElGamalPublicKeySpec;

public class BCElGamalPublicKey
    implements ElGamalPublicKey, DHPublicKey
{
    static final long serialVersionUID = 8712728417091216948L;
        
    private BigInteger              y;
    private transient ElGamalParameterSpec    elSpec;

    BCElGamalPublicKey(
        ElGamalPublicKeySpec spec)
    {
        this.y = spec.getY();
        this.elSpec = new ElGamalParameterSpec(spec.getParams().getP(), spec.getParams().getG());
    }

    BCElGamalPublicKey(
        DHPublicKeySpec spec)
    {
        this.y = spec.getY();
        this.elSpec = new ElGamalParameterSpec(spec.getP(), spec.getG());
    }
    
    BCElGamalPublicKey(
        ElGamalPublicKey key)
    {
        this.y = key.getY();
        this.elSpec = key.getParameters();
    }

    BCElGamalPublicKey(
        DHPublicKey key)
    {
        this.y = key.getY();
        this.elSpec = new ElGamalParameterSpec(key.getParams().getP(), key.getParams().getG());
    }
    
    BCElGamalPublicKey(
        ElGamalPublicKeyParameters params)
    {
        this.y = params.getY();
        this.elSpec = new ElGamalParameterSpec(params.getParameters().getP(), params.getParameters().getG());
    }

    BCElGamalPublicKey(
        BigInteger y,
        ElGamalParameterSpec elSpec)
    {
        this.y = y;
        this.elSpec = elSpec;
    }

    BCElGamalPublicKey(
        SubjectPublicKeyInfo info)
    {
        ElGamalParameter        params = new ElGamalParameter((ASN1Sequence)info.getAlgorithmId().getParameters());
        DERInteger              derY = null;

        try
        {
            derY = (DERInteger)info.parsePublicKey();
        }
        catch (IOException e)
        {
            throw new IllegalArgumentException("invalid info structure in DSA public key");
        }

        this.y = derY.getValue();
        this.elSpec = new ElGamalParameterSpec(params.getP(), params.getG());
    }

    public String getAlgorithm()
    {
        return "ElGamal";
    }

    public String getFormat()
    {
        return "X.509";
    }

    public byte[] getEncoded()
    {
        try
        {
            SubjectPublicKeyInfo    info = new SubjectPublicKeyInfo(new AlgorithmIdentifier(OIWObjectIdentifiers.elGamalAlgorithm, new ElGamalParameter(elSpec.getP(), elSpec.getG())), new DERInteger(y));

            return info.getEncoded(ASN1Encoding.DER);
        }
        catch (IOException e)
        {
            return null;
        }
    }

    public ElGamalParameterSpec getParameters()
    {
        return elSpec;
    }
    
    public DHParameterSpec getParams()
    {
        return new DHParameterSpec(elSpec.getP(), elSpec.getG());
    }

    public BigInteger getY()
    {
        return y;
    }

    public int hashCode()
    {
        return this.getY().hashCode() ^ this.getParams().getG().hashCode()
                ^ this.getParams().getP().hashCode() ^ this.getParams().getL();
    }

    public boolean equals(
        Object o)
    {
        if (!(o instanceof DHPublicKey))
        {
            return false;
        }

        DHPublicKey other = (DHPublicKey)o;

        return this.getY().equals(other.getY())
            && this.getParams().getG().equals(other.getParams().getG())
            && this.getParams().getP().equals(other.getParams().getP())
            && this.getParams().getL() == other.getParams().getL();
    }

    private void readObject(
        ObjectInputStream   in)
        throws IOException, ClassNotFoundException
    {
        in.defaultReadObject();

        this.elSpec = new ElGamalParameterSpec((BigInteger)in.readObject(), (BigInteger)in.readObject());
    }

    private void writeObject(
        ObjectOutputStream  out)
        throws IOException
    {
        out.defaultWriteObject();

        out.writeObject(elSpec.getP());
        out.writeObject(elSpec.getG());
    }
}