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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.operator.jcajce;

import java.security.AlgorithmParameters;
import java.security.GeneralSecurityException;
import java.security.InvalidKeyException;
import java.security.Key;
import java.security.PrivateKey;
import java.security.Provider;
import java.security.ProviderException;
import java.util.HashMap;
import java.util.Map;

import javax.crypto.BadPaddingException;
import javax.crypto.Cipher;
import javax.crypto.IllegalBlockSizeException;
import javax.crypto.spec.SecretKeySpec;

import org.bouncycastle.asn1.ASN1ObjectIdentifier;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.jcajce.util.DefaultJcaJceHelper;
import org.bouncycastle.jcajce.util.NamedJcaJceHelper;
import org.bouncycastle.jcajce.util.ProviderJcaJceHelper;
import org.bouncycastle.operator.AsymmetricKeyUnwrapper;
import org.bouncycastle.operator.GenericKey;
import org.bouncycastle.operator.OperatorException;

public class JceAsymmetricKeyUnwrapper
    extends AsymmetricKeyUnwrapper
{
    private OperatorHelper helper = new OperatorHelper(new DefaultJcaJceHelper());
    private Map extraMappings = new HashMap();
    private PrivateKey privKey;
    private boolean unwrappedKeyMustBeEncodable;

    public JceAsymmetricKeyUnwrapper(AlgorithmIdentifier algorithmIdentifier, PrivateKey privKey)
    {
        super(algorithmIdentifier);

        this.privKey = privKey;
    }

    public JceAsymmetricKeyUnwrapper setProvider(Provider provider)
    {
        this.helper = new OperatorHelper(new ProviderJcaJceHelper(provider));

        return this;
    }

    public JceAsymmetricKeyUnwrapper setProvider(String providerName)
    {
        this.helper = new OperatorHelper(new NamedJcaJceHelper(providerName));

        return this;
    }

    /**
     * Flag that unwrapping must produce a key that will return a meaningful value from a call to Key.getEncoded().
     * This is important if you are using a HSM for unwrapping and using a software based provider for
     * with the unwrapped key. Default value: false.
     *
     * @param unwrappedKeyMustBeEncodable true if getEncoded() should return key bytes, false if not necessary.
     * @return this recipient.
     */
    public JceAsymmetricKeyUnwrapper setMustProduceEncodableUnwrappedKey(boolean unwrappedKeyMustBeEncodable)
    {
        this.unwrappedKeyMustBeEncodable = unwrappedKeyMustBeEncodable;

        return this;
    }

    /**
     * Internally algorithm ids are converted into cipher names using a lookup table. For some providers
     * the standard lookup table won't work. Use this method to establish a specific mapping from an
     * algorithm identifier to a specific algorithm.
     * 
     *     For example:
     * 
     *     unwrapper.setAlgorithmMapping(PKCSObjectIdentifiers.rsaEncryption, "RSA");
     * 
     * @param algorithm  OID of algorithm in recipient.
     * @param algorithmName JCE algorithm name to use.
     * @return  the current Unwrapper.
     */
    public JceAsymmetricKeyUnwrapper setAlgorithmMapping(ASN1ObjectIdentifier algorithm, String algorithmName)
    {
        extraMappings.put(algorithm, algorithmName);

        return this;
    }

    public GenericKey generateUnwrappedKey(AlgorithmIdentifier encryptedKeyAlgorithm, byte[] encryptedKey)
        throws OperatorException
    {
        try
        {
            Key sKey = null;

            Cipher keyCipher = helper.createAsymmetricWrapper(this.getAlgorithmIdentifier().getAlgorithm(), extraMappings);
            AlgorithmParameters algParams = helper.createAlgorithmParameters(this.getAlgorithmIdentifier());

            try
            {
                if (algParams != null)
                {
                    keyCipher.init(Cipher.UNWRAP_MODE, privKey, algParams);
                }
                else
                {
                    keyCipher.init(Cipher.UNWRAP_MODE, privKey);
                }

                sKey = keyCipher.unwrap(encryptedKey, helper.getKeyAlgorithmName(encryptedKeyAlgorithm.getAlgorithm()), Cipher.SECRET_KEY);

                // check key will work with a software provider.
                if (unwrappedKeyMustBeEncodable)
                {
                    try
                    {
                        byte[] keyBytes = sKey.getEncoded();

                        if (keyBytes == null || keyBytes.length == 0)
                        {
                            sKey = null;
                        }
                    }
                    catch (Exception e)
                    {
                        sKey = null; // try doing a decrypt
                    }
                }
            }
            catch (GeneralSecurityException e)
            {   // try decrypt
            }
            catch (IllegalStateException e)
            {   // try decrypt
            }
            catch (UnsupportedOperationException e)
            {   // try decrypt
            }
            catch (ProviderException e)
            {    // try decrypt
            }

            // some providers do not support UNWRAP (this appears to be only for asymmetric algorithms)
            if (sKey == null)
            {
                keyCipher.init(Cipher.DECRYPT_MODE, privKey);
                sKey = new SecretKeySpec(keyCipher.doFinal(encryptedKey), encryptedKeyAlgorithm.getAlgorithm().getId());
            }

            return new JceGenericKey(encryptedKeyAlgorithm, sKey);
        }
        catch (InvalidKeyException e)
        {
            throw new OperatorException("key invalid: " + e.getMessage(), e);
        }
        catch (IllegalBlockSizeException e)
        {
            throw new OperatorException("illegal blocksize: " + e.getMessage(), e);
        }
        catch (BadPaddingException e)
        {
            throw new OperatorException("bad padding: " + e.getMessage(), e);
        }
    }
}