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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 1.4.
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package org.bouncycastle.jcajce.provider.asymmetric.elgamal;

import java.security.AlgorithmParameters;
import java.security.InvalidAlgorithmParameterException;
import java.security.InvalidKeyException;
import java.security.InvalidParameterException;
import java.security.Key;
import java.security.NoSuchAlgorithmException;
import java.security.PrivateKey;
import java.security.PublicKey;
import java.security.SecureRandom;
import java.security.spec.AlgorithmParameterSpec;

import javax.crypto.BadPaddingException;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.interfaces.DHKey;

import org.bouncycastle.crypto.AsymmetricBlockCipher;
import org.bouncycastle.crypto.BufferedAsymmetricBlockCipher;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.InvalidCipherTextException;
import org.bouncycastle.crypto.encodings.ISO9796d1Encoding;
import org.bouncycastle.crypto.encodings.OAEPEncoding;
import org.bouncycastle.crypto.encodings.PKCS1Encoding;
import org.bouncycastle.crypto.engines.ElGamalEngine;
import org.bouncycastle.crypto.params.ParametersWithRandom;
import org.bouncycastle.jcajce.provider.asymmetric.util.BaseCipherSpi;
import org.bouncycastle.jce.interfaces.ElGamalKey;
import org.bouncycastle.jce.interfaces.ElGamalPrivateKey;
import org.bouncycastle.jce.interfaces.ElGamalPublicKey;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.util.Strings;
import org.bouncycastle.jcajce.util.BCJcaJceHelper;
import org.bouncycastle.jcajce.util.JcaJceHelper;

public class CipherSpi
    extends BaseCipherSpi
{
    private final JcaJceHelper     helper = new BCJcaJceHelper();

    private BufferedAsymmetricBlockCipher   cipher;
    private AlgorithmParameterSpec          paramSpec;
    private AlgorithmParameters             engineParams;

    public CipherSpi(
        AsymmetricBlockCipher engine)
    {
        cipher = new BufferedAsymmetricBlockCipher(engine);
    }
   
    protected int engineGetBlockSize() 
    {
        return cipher.getInputBlockSize();
    }

    protected int engineGetKeySize(
        Key     key) 
    {
        if (key instanceof ElGamalKey)
        {
            ElGamalKey   k = (ElGamalKey)key;

            return k.getParameters().getP().bitLength();
        }
        else if (key instanceof DHKey)
        {
            DHKey   k = (DHKey)key;

            return k.getParams().getP().bitLength();
        }

        throw new IllegalArgumentException("not an ElGamal key!");
    }

    protected int engineGetOutputSize(
        int     inputLen) 
    {
        return cipher.getOutputBlockSize();
    }

    protected AlgorithmParameters engineGetParameters() 
    {
        if (engineParams == null)
        {
            if (paramSpec != null)
            {
                try
                {
                    engineParams = helper.createAlgorithmParameters("OAEP");
                    engineParams.init(paramSpec);
                }
                catch (Exception e)
                {
                    throw new RuntimeException(e.toString());
                }
            }
        }

        return engineParams;
    }

    protected void engineSetMode(
        String  mode)
        throws NoSuchAlgorithmException
    {
        String md = Strings.toUpperCase(mode);
        
        if (md.equals("NONE") || md.equals("ECB"))
        {
            return;
        }
        
        throw new NoSuchAlgorithmException("can't support mode " + mode);
    }

    protected void engineSetPadding(
        String  padding) 
        throws NoSuchPaddingException
    {
        String pad = Strings.toUpperCase(padding);

        if (pad.equals("NOPADDING"))
        {
            cipher = new BufferedAsymmetricBlockCipher(new ElGamalEngine());
        }
        else if (pad.equals("PKCS1PADDING"))
        {
            cipher = new BufferedAsymmetricBlockCipher(new PKCS1Encoding(new ElGamalEngine()));
        }
        else if (pad.equals("ISO9796-1PADDING"))
        {
            cipher = new BufferedAsymmetricBlockCipher(new ISO9796d1Encoding(new ElGamalEngine()));
        }
        else if (pad.equals("OAEPPADDING"))
        {
            cipher = new BufferedAsymmetricBlockCipher(new OAEPEncoding(new ElGamalEngine()));
        }
        else if (pad.equals("OAEPWITHSHA1ANDMGF1PADDING"))
        {
            cipher = new BufferedAsymmetricBlockCipher(new OAEPEncoding(new ElGamalEngine()));
        }
        else
        {
            throw new NoSuchPaddingException(padding + " unavailable with ElGamal.");
        }
    }

    protected void engineInit(
        int                     opmode,
        Key                     key,
        AlgorithmParameterSpec  params,
        SecureRandom            random) 
    throws InvalidKeyException
    {
        CipherParameters        param;

        if (params == null)
        {
            if (key instanceof ElGamalPublicKey)
            {
                param = ElGamalUtil.generatePublicKeyParameter((PublicKey)key);
            }
            else if (key instanceof ElGamalPrivateKey)
            {
                param = ElGamalUtil.generatePrivateKeyParameter((PrivateKey)key);
            }
            else
            {
                throw new InvalidKeyException("unknown key type passed to ElGamal");
            }
        }
        else
        {
            throw new IllegalArgumentException("unknown parameter type.");
        }

        if (random != null)
        {
            param = new ParametersWithRandom(param, random);
        }

        switch (opmode)
        {
        case javax.crypto.Cipher.ENCRYPT_MODE:
        case javax.crypto.Cipher.WRAP_MODE:
            cipher.init(true, param);
            break;
        case javax.crypto.Cipher.DECRYPT_MODE:
        case javax.crypto.Cipher.UNWRAP_MODE:
            cipher.init(false, param);
            break;
        default:
            throw new InvalidParameterException("unknown opmode " + opmode + " passed to ElGamal");
        }
    }

    protected void engineInit(
        int                 opmode,
        Key                 key,
        AlgorithmParameters params,
        SecureRandom        random) 
    throws InvalidKeyException, InvalidAlgorithmParameterException
    {
        throw new InvalidAlgorithmParameterException("can't handle parameters in ElGamal");
    }

    protected void engineInit(
        int                 opmode,
        Key                 key,
        SecureRandom        random) 
    throws InvalidKeyException
    {
        engineInit(opmode, key, (AlgorithmParameterSpec)null, random);
    }

    protected byte[] engineUpdate(
        byte[]  input,
        int     inputOffset,
        int     inputLen) 
    {
        cipher.processBytes(input, inputOffset, inputLen);
        return null;
    }

    protected int engineUpdate(
        byte[]  input,
        int     inputOffset,
        int     inputLen,
        byte[]  output,
        int     outputOffset) 
    {
        cipher.processBytes(input, inputOffset, inputLen);
        return 0;
    }

    protected byte[] engineDoFinal(
        byte[]  input,
        int     inputOffset,
        int     inputLen) 
        throws IllegalBlockSizeException, BadPaddingException
    {
        cipher.processBytes(input, inputOffset, inputLen);

        return getOutput();
    }

    protected int engineDoFinal(
        byte[]  input,
        int     inputOffset,
        int     inputLen,
        byte[]  output,
        int     outputOffset)
        throws IllegalBlockSizeException, BadPaddingException
    {
        cipher.processBytes(input, inputOffset, inputLen);

        byte[]  out = getOutput();

        for (int i = 0; i != out.length; i++)
        {
            output[outputOffset + i] = out[i];
        }

        return out.length;
    }

    private byte[] getOutput()
        throws BadPaddingException
    {
        try
        {
            return cipher.doFinal();
        }
        catch (final InvalidCipherTextException e)
        {
            throw new BadPaddingException("unable to decrypt block")
            {
                public synchronized Throwable getCause()
                {
                    return e;
                }
            };
        }
    }

    /**
     * classes that inherit from us.
     */
    static public class NoPadding
        extends CipherSpi
    {
        public NoPadding()
        {
            super(new ElGamalEngine());
        }
    }
    
    static public class PKCS1v1_5Padding
        extends CipherSpi
    {
        public PKCS1v1_5Padding()
        {
            super(new PKCS1Encoding(new ElGamalEngine()));
        }
    }
}