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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.cert.crmf.jcajce;

import java.io.InputStream;
import java.security.Key;
import java.security.PrivateKey;
import java.security.Provider;

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

import org.bouncycastle.asn1.x509.AlgorithmIdentifier;
import org.bouncycastle.cert.crmf.CRMFException;
import org.bouncycastle.cert.crmf.ValueDecryptorGenerator;
import org.bouncycastle.jcajce.io.CipherInputStream;
import org.bouncycastle.jcajce.util.DefaultJcaJceHelper;
import org.bouncycastle.jcajce.util.NamedJcaJceHelper;
import org.bouncycastle.jcajce.util.ProviderJcaJceHelper;
import org.bouncycastle.operator.InputDecryptor;
import org.bouncycastle.operator.OperatorException;
import org.bouncycastle.operator.jcajce.JceAsymmetricKeyUnwrapper;

public class JceAsymmetricValueDecryptorGenerator
    implements ValueDecryptorGenerator
{
    private PrivateKey recipientKey;
    private CRMFHelper helper = new CRMFHelper(new DefaultJcaJceHelper());
    private Provider provider = null;
    private String providerName = null;

    public JceAsymmetricValueDecryptorGenerator(PrivateKey recipientKey)
    {
        this.recipientKey = recipientKey;
    }

    public JceAsymmetricValueDecryptorGenerator setProvider(Provider provider)
    {
        this.helper = new CRMFHelper(new ProviderJcaJceHelper(provider));
        this.provider = provider;
        this.providerName = null;

        return this;
    }

    public JceAsymmetricValueDecryptorGenerator setProvider(String providerName)
    {
        this.helper = new CRMFHelper(new NamedJcaJceHelper(providerName));
        this.provider = null;
        this.providerName = providerName;

        return this;
    }

    private Key extractSecretKey(AlgorithmIdentifier keyEncryptionAlgorithm, AlgorithmIdentifier contentEncryptionAlgorithm, byte[] encryptedContentEncryptionKey)
        throws CRMFException
    {
        try
        {
            JceAsymmetricKeyUnwrapper unwrapper = new JceAsymmetricKeyUnwrapper(keyEncryptionAlgorithm, recipientKey);
            if (provider != null)
            {
                unwrapper.setProvider(provider);
            }
            if (providerName != null)
            {
                unwrapper.setProvider(providerName);
            }

            return new SecretKeySpec((byte[])unwrapper.generateUnwrappedKey(contentEncryptionAlgorithm, encryptedContentEncryptionKey).getRepresentation(), contentEncryptionAlgorithm.getAlgorithm().getId());
        }
        catch (OperatorException e)
        {
            throw new CRMFException("key invalid in message: " + e.getMessage(), e);
        }
    }

    public InputDecryptor getValueDecryptor(AlgorithmIdentifier keyEncryptionAlgorithm, final AlgorithmIdentifier contentEncryptionAlgorithm, byte[] encryptedContentEncryptionKey)
        throws CRMFException
    {
        Key secretKey = extractSecretKey(keyEncryptionAlgorithm, contentEncryptionAlgorithm, encryptedContentEncryptionKey);

        final Cipher dataCipher = helper.createContentCipher(secretKey, contentEncryptionAlgorithm);

        return new InputDecryptor()
        {
            public AlgorithmIdentifier getAlgorithmIdentifier()
            {
                return contentEncryptionAlgorithm;
            }

            public InputStream getInputStream(InputStream dataIn)
            {
                return new CipherInputStream(dataIn, dataCipher);
            }
        };
    }
}