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The Bouncy Castle Java APIs for CMS, PKCS, EAC, TSP, CMP, CRMF, OCSP, and certificate generation. This jar contains APIs for JDK 1.5 and up. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs.
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package org.bouncycastle.jcajce.provider.asymmetric.util;

import java.io.ByteArrayOutputStream;
import java.security.AlgorithmParameters;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.KeyFactory;
import java.security.NoSuchAlgorithmException;
import java.security.NoSuchProviderException;
import java.security.PrivateKey;
import java.security.spec.InvalidKeySpecException;
import java.security.spec.PKCS8EncodedKeySpec;
import java.security.spec.X509EncodedKeySpec;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.CipherSpi;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.NoSuchPaddingException;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.PBEParameterSpec;
import javax.crypto.spec.RC2ParameterSpec;
import javax.crypto.spec.RC5ParameterSpec;
import javax.crypto.spec.SecretKeySpec;

import org.bouncycastle.asn1.pkcs.PrivateKeyInfo;
import org.bouncycastle.crypto.InvalidCipherTextException;
import org.bouncycastle.crypto.Wrapper;
import org.bouncycastle.jcajce.util.BCJcaJceHelper;
import org.bouncycastle.jcajce.util.JcaJceHelper;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.util.Arrays;

public abstract class BaseCipherSpi
    extends CipherSpi
{
    //
    // specs we can handle.
    //
    private Class[]                 availableSpecs =
                                    {
                                        IvParameterSpec.class,
                                        PBEParameterSpec.class,
                                        RC2ParameterSpec.class,
                                        RC5ParameterSpec.class
                                    };

    private final JcaJceHelper helper = new BCJcaJceHelper();

    protected AlgorithmParameters     engineParams = null;

    protected Wrapper                 wrapEngine = null;

    private int                       ivSize;
    private byte[]                    iv;

    protected BaseCipherSpi()
    {
    }

    protected int engineGetBlockSize()
    {
        return 0;
    }

    protected byte[] engineGetIV()
    {
        return null;
    }

    protected int engineGetKeySize(
        Key     key)
    {
        return key.getEncoded().length;
    }

    protected int engineGetOutputSize(
        int     inputLen)
    {
        return -1;
    }

    protected AlgorithmParameters engineGetParameters()
    {
        return null;
    }

    protected final AlgorithmParameters createParametersInstance(String algorithm)
        throws NoSuchAlgorithmException, NoSuchProviderException
    {
        return helper.createAlgorithmParameters(algorithm);
    }

    protected void engineSetMode(
        String  mode)
        throws NoSuchAlgorithmException
    {
        throw new NoSuchAlgorithmException("can't support mode " + mode);
    }

    protected void engineSetPadding(
        String  padding)
    throws NoSuchPaddingException
    {
        throw new NoSuchPaddingException("Padding " + padding + " unknown.");
    }

    protected byte[] engineWrap(
        Key     key)
    throws IllegalBlockSizeException, InvalidKeyException
    {
        byte[] encoded = key.getEncoded();
        if (encoded == null)
        {
            throw new InvalidKeyException("Cannot wrap key, null encoding.");
        }

        try
        {
            if (wrapEngine == null)
            {
                return engineDoFinal(encoded, 0, encoded.length);
            }
            else
            {
                return wrapEngine.wrap(encoded, 0, encoded.length);
            }
        }
        catch (BadPaddingException e)
        {
            throw new IllegalBlockSizeException(e.getMessage());
        }
    }

    protected Key engineUnwrap(
        byte[]  wrappedKey,
        String  wrappedKeyAlgorithm,
        int     wrappedKeyType)
    throws InvalidKeyException
    {
        byte[] encoded;
        try
        {
            if (wrapEngine == null)
            {
                encoded = engineDoFinal(wrappedKey, 0, wrappedKey.length);
            }
            else
            {
                encoded = wrapEngine.unwrap(wrappedKey, 0, wrappedKey.length);
            }
        }
        catch (InvalidCipherTextException e)
        {
            throw new InvalidKeyException(e.getMessage());
        }
        catch (final BadPaddingException e)
        {
            throw new InvalidKeyException("unable to unwrap")
            {
                public synchronized Throwable getCause()
                {
                    return e;
                }
            };
        }
        catch (IllegalBlockSizeException e2)
        {
            throw new InvalidKeyException(e2.getMessage());
        }

        if (wrappedKeyType == Cipher.SECRET_KEY)
        {
            return new SecretKeySpec(encoded, wrappedKeyAlgorithm);
        }
        else if (wrappedKeyAlgorithm.equals("") && wrappedKeyType == Cipher.PRIVATE_KEY)
        {
            /*
                 * The caller doesn't know the algorithm as it is part of
                 * the encrypted data.
                 */
            try
            {
                PrivateKeyInfo       in = PrivateKeyInfo.getInstance(encoded);

                PrivateKey privKey = BouncyCastleProvider.getPrivateKey(in);

                if (privKey != null)
                {
                    return privKey;
                }
                else
                {
                    throw new InvalidKeyException("algorithm " + in.getPrivateKeyAlgorithm().getAlgorithm() + " not supported");
                }
            }
            catch (Exception e)
            {
                throw new InvalidKeyException("Invalid key encoding.");
            }
        }
        else
        {
            try
            {
                KeyFactory kf = helper.createKeyFactory(wrappedKeyAlgorithm);

                if (wrappedKeyType == Cipher.PUBLIC_KEY)
                {
                    return kf.generatePublic(new X509EncodedKeySpec(encoded));
                }
                else if (wrappedKeyType == Cipher.PRIVATE_KEY)
                {
                    return kf.generatePrivate(new PKCS8EncodedKeySpec(encoded));
                }
            }
            catch (NoSuchAlgorithmException e)
            {
                throw new InvalidKeyException("Unknown key type " + e.getMessage());
            }
            catch (InvalidKeySpecException e)
            {
                throw new InvalidKeyException("Unknown key type " + e.getMessage());
            }
            catch (NoSuchProviderException e)
            {
                throw new InvalidKeyException("Unknown key type " + e.getMessage());
            }

            throw new InvalidKeyException("Unknown key type " + wrappedKeyType);
        }
    }

    protected static final class ErasableOutputStream
        extends ByteArrayOutputStream
    {
        public ErasableOutputStream()
        {
        }

        public byte[] getBuf()
        {
            return buf;
        }

        public void erase()
        {
            Arrays.fill(this.buf, (byte)0);
            reset();
        }
    }
}