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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.security.InvalidKeyException;
import java.security.Key;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.KeySpec;

import javax.crypto.interfaces.DHPrivateKey;
import javax.crypto.interfaces.DHPublicKey;
import javax.crypto.spec.DHPrivateKeySpec;
import javax.crypto.spec.DHPublicKeySpec;

import org.spongycastle.asn1.ASN1ObjectIdentifier;
import org.spongycastle.asn1.oiw.OIWObjectIdentifiers;
import org.spongycastle.asn1.pkcs.PKCSObjectIdentifiers;
import org.spongycastle.asn1.pkcs.PrivateKeyInfo;
import org.spongycastle.asn1.x509.SubjectPublicKeyInfo;
import org.spongycastle.asn1.x9.X9ObjectIdentifiers;
import org.spongycastle.jcajce.provider.asymmetric.util.BaseKeyFactorySpi;
import org.spongycastle.jce.interfaces.ElGamalPrivateKey;
import org.spongycastle.jce.interfaces.ElGamalPublicKey;
import org.spongycastle.jce.spec.ElGamalPrivateKeySpec;
import org.spongycastle.jce.spec.ElGamalPublicKeySpec;

public class KeyFactorySpi
    extends BaseKeyFactorySpi
{
    public KeyFactorySpi()
    {
    }

    protected PrivateKey engineGeneratePrivate(
        KeySpec keySpec)
        throws InvalidKeySpecException
    {
        if (keySpec instanceof ElGamalPrivateKeySpec)
        {
            return new BCElGamalPrivateKey((ElGamalPrivateKeySpec)keySpec);
        }
        else if (keySpec instanceof DHPrivateKeySpec)
        {
            return new BCElGamalPrivateKey((DHPrivateKeySpec)keySpec);
        }

        return super.engineGeneratePrivate(keySpec);
    }

    protected PublicKey engineGeneratePublic(
        KeySpec keySpec)
        throws InvalidKeySpecException
    {
        if (keySpec instanceof ElGamalPublicKeySpec)
        {
            return new BCElGamalPublicKey((ElGamalPublicKeySpec)keySpec);
        }
        else if (keySpec instanceof DHPublicKeySpec)
        {
            return new BCElGamalPublicKey((DHPublicKeySpec)keySpec);
        }
        return super.engineGeneratePublic(keySpec);
    }

    protected KeySpec engineGetKeySpec(
        Key key,
        Class spec)
        throws InvalidKeySpecException
    {
        if (spec.isAssignableFrom(DHPrivateKeySpec.class) && key instanceof DHPrivateKey)
        {
            DHPrivateKey k = (DHPrivateKey)key;

            return new DHPrivateKeySpec(k.getX(), k.getParams().getP(), k.getParams().getG());
        }
        else if (spec.isAssignableFrom(DHPublicKeySpec.class) && key instanceof DHPublicKey)
        {
            DHPublicKey k = (DHPublicKey)key;

            return new DHPublicKeySpec(k.getY(), k.getParams().getP(), k.getParams().getG());
        }

        return super.engineGetKeySpec(key, spec);
    }

    protected Key engineTranslateKey(
        Key key)
        throws InvalidKeyException
    {
        if (key instanceof DHPublicKey)
        {
            return new BCElGamalPublicKey((DHPublicKey)key);
        }
        else if (key instanceof DHPrivateKey)
        {
            return new BCElGamalPrivateKey((DHPrivateKey)key);
        }
        else if (key instanceof ElGamalPublicKey)
        {
            return new BCElGamalPublicKey((ElGamalPublicKey)key);
        }
        else if (key instanceof ElGamalPrivateKey)
        {
            return new BCElGamalPrivateKey((ElGamalPrivateKey)key);
        }

        throw new InvalidKeyException("key type unknown");
    }

    public PrivateKey generatePrivate(PrivateKeyInfo info)
        throws IOException
    {
        ASN1ObjectIdentifier algOid = info.getPrivateKeyAlgorithm().getAlgorithm();

        if (algOid.equals(PKCSObjectIdentifiers.dhKeyAgreement))
        {
            return new BCElGamalPrivateKey(info);
        }
        else if (algOid.equals(X9ObjectIdentifiers.dhpublicnumber))
        {
            return new BCElGamalPrivateKey(info);
        }
        else if (algOid.equals(OIWObjectIdentifiers.elGamalAlgorithm))
        {
            return new BCElGamalPrivateKey(info);
        }
        else
        {
            throw new IOException("algorithm identifier " + algOid + " in key not recognised");
        }
    }

    public PublicKey generatePublic(SubjectPublicKeyInfo info)
        throws IOException
    {
        ASN1ObjectIdentifier algOid = info.getAlgorithm().getAlgorithm();

        if (algOid.equals(PKCSObjectIdentifiers.dhKeyAgreement))
        {
            return new BCElGamalPublicKey(info);
        }
        else if (algOid.equals(X9ObjectIdentifiers.dhpublicnumber))
        {
            return new BCElGamalPublicKey(info);
        }
        else if (algOid.equals(OIWObjectIdentifiers.elGamalAlgorithm))
        {
            return new BCElGamalPublicKey(info);
        }
        else
        {
            throw new IOException("algorithm identifier " + algOid + " in key not recognised");
        }
    }
}