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The Bouncy Castle Java CMS and S/MIME APIs for handling the CMS and S/MIME protocols. This jar contains CMS and S/MIME APIs for JDK 1.5. The APIs can be used in conjunction with a JCE/JCA provider such as the one provided with the Bouncy Castle Cryptography APIs. If the S/MIME API is used, the JavaMail API and the Java activation framework will also be needed.
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package org.bouncycastle.cms;

import org.bouncycastle.asn1.ASN1Set;
import org.bouncycastle.asn1.DEREncodable;
import org.bouncycastle.asn1.cms.AttributeTable;
import org.bouncycastle.asn1.cms.ContentInfo;
import org.bouncycastle.asn1.cms.EncryptedContentInfo;
import org.bouncycastle.asn1.cms.EnvelopedData;
import org.bouncycastle.asn1.cms.KEKRecipientInfo;
import org.bouncycastle.asn1.cms.KeyAgreeRecipientInfo;
import org.bouncycastle.asn1.cms.KeyTransRecipientInfo;
import org.bouncycastle.asn1.cms.PasswordRecipientInfo;
import org.bouncycastle.asn1.cms.RecipientInfo;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.security.AlgorithmParameters;
import java.security.NoSuchProviderException;
import java.util.ArrayList;
import java.util.List;

/**
 * containing class for an CMS Enveloped Data object
 */
public class CMSEnvelopedData
{
    RecipientInformationStore   recipientInfoStore;
    ContentInfo                 contentInfo;
    
    private AlgorithmIdentifier    encAlg;
    private ASN1Set                unprotectedAttributes;

    public CMSEnvelopedData(
        byte[]    envelopedData) 
        throws CMSException
    {
        this(CMSUtils.readContentInfo(envelopedData));
    }

    public CMSEnvelopedData(
        InputStream    envelopedData) 
        throws CMSException
    {
        this(CMSUtils.readContentInfo(envelopedData));
    }

    public CMSEnvelopedData(
        ContentInfo contentInfo)
        throws CMSException
    {
        this.contentInfo = contentInfo;

        EnvelopedData  envData = EnvelopedData.getInstance(contentInfo.getContent());

        //
        // read the encrypted content info
        //
        EncryptedContentInfo encInfo = envData.getEncryptedContentInfo();

        this.encAlg = encInfo.getContentEncryptionAlgorithm();

        //
        // load the RecipientInfoStore
        //
        ASN1Set     s = envData.getRecipientInfos();
        List        infos = new ArrayList();
        byte[]      contentOctets = encInfo.getEncryptedContent().getOctets();

        for (int i = 0; i != s.size(); i++)
        {
            RecipientInfo   info = RecipientInfo.getInstance(s.getObjectAt(i));
            InputStream     contentStream = new ByteArrayInputStream(contentOctets);
            Object          type = info.getInfo();

            if (type instanceof KeyTransRecipientInfo)
            {
                infos.add(new KeyTransRecipientInformation(
                    (KeyTransRecipientInfo)type, encAlg, contentStream));
            }
            else if (type instanceof KEKRecipientInfo)
            {
                infos.add(new KEKRecipientInformation(
                    (KEKRecipientInfo)type, encAlg, contentStream));
            }
            else if (type instanceof KeyAgreeRecipientInfo)
            {
                infos.add(new KeyAgreeRecipientInformation(
                    (KeyAgreeRecipientInfo)type, encAlg, contentStream));
            }
            else if (type instanceof PasswordRecipientInfo)
            {
                infos.add(new PasswordRecipientInformation(
                    (PasswordRecipientInfo)type, encAlg, contentStream));
            }
        }

        this.recipientInfoStore = new RecipientInformationStore(infos);
        this.unprotectedAttributes = envData.getUnprotectedAttrs();
    }

    private byte[] encodeObj(
        DEREncodable    obj)
        throws IOException
    {
        if (obj != null)
        {
            return obj.getDERObject().getEncoded();
        }

        return null;
    }
    
    /**
     * return the object identifier for the content encryption algorithm.
     */
    public String getEncryptionAlgOID()
    {
        return encAlg.getObjectId().getId();
    }

    /**
     * return the ASN.1 encoded encryption algorithm parameters, or null if
     * there aren't any.
     */
    public byte[] getEncryptionAlgParams()
    {
        try
        {
            return encodeObj(encAlg.getParameters());
        }
        catch (Exception e)
        {
            throw new RuntimeException("exception getting encryption parameters " + e);
        }
    }
    
    /**
     * Return an AlgorithmParameters object giving the encryption parameters
     * used to encrypt the message content.
     * 
     * @param provider the provider to generate the parameters for.
     * @return the parameters object, null if there is not one.
     * @throws CMSException if the algorithm cannot be found, or the parameters can't be parsed.
     * @throws NoSuchProviderException if the provider cannot be found.
     */
    public AlgorithmParameters getEncryptionAlgorithmParameters(
        String  provider) 
    throws CMSException, NoSuchProviderException    
    {
        return CMSEnvelopedHelper.INSTANCE.getEncryptionAlgorithmParameters(getEncryptionAlgOID(), getEncryptionAlgParams(), provider);
    }
    
    /**
     * return a store of the intended recipients for this message
     */
    public RecipientInformationStore getRecipientInfos()
    {
        return recipientInfoStore;
    }

    /**
     * return the ContentInfo 
     */
    public ContentInfo getContentInfo()
    {
        return contentInfo;
    }

    /**
     * return a table of the unprotected attributes indexed by
     * the OID of the attribute.
     */
    public AttributeTable getUnprotectedAttributes()
    {
        if (unprotectedAttributes == null)
        {
            return null;
        }

        return new AttributeTable(unprotectedAttributes);
    }
    
    /**
     * return the ASN.1 encoded representation of this object.
     */
    public byte[] getEncoded()
        throws IOException
    {
        return contentInfo.getEncoded();
    }
}