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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 to JDK 1.8. 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.cms;

import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStream;

import org.bouncycastle.asn1.ASN1Encodable;
import org.bouncycastle.asn1.ASN1EncodableVector;
import org.bouncycastle.asn1.ASN1OctetStringParser;
import org.bouncycastle.asn1.ASN1SequenceParser;
import org.bouncycastle.asn1.ASN1Set;
import org.bouncycastle.asn1.ASN1SetParser;
import org.bouncycastle.asn1.BERTags;
import org.bouncycastle.asn1.DERSet;
import org.bouncycastle.asn1.cms.AttributeTable;
import org.bouncycastle.asn1.cms.EncryptedContentInfoParser;
import org.bouncycastle.asn1.cms.EnvelopedDataParser;
import org.bouncycastle.asn1.cms.OriginatorInfo;
import org.bouncycastle.asn1.x509.AlgorithmIdentifier;

/**
 * Parsing class for an CMS Enveloped Data object from an input stream.
 * 
 * Note: that because we are in a streaming mode only one recipient can be tried and it is important 
 * that the methods on the parser are called in the appropriate order.
 * 
 * 
 * Example of use - assuming the first recipient matches the private key we have.
 * 
 *      CMSEnvelopedDataParser     ep = new CMSEnvelopedDataParser(inputStream);
 *
 *      RecipientInformationStore  recipients = ep.getRecipientInfos();
 *
 *      Collection  c = recipients.getRecipients();
 *      Iterator    it = c.iterator();
 *      
 *      if (it.hasNext())
 *      {
 *          RecipientInformation   recipient = (RecipientInformation)it.next();
 *
 *          CMSTypedStream recData = recipient.getContentStream(new JceKeyTransEnvelopedRecipient(privateKey).setProvider("BC"));
 *          
 *          processDataStream(recData.getContentStream());
 *      }
 *  
 *  Note: this class does not introduce buffering - if you are processing large files you should create
 *  the parser with:
 *   *          CMSEnvelopedDataParser     ep = new CMSEnvelopedDataParser(new BufferedInputStream(inputStream, bufSize));
 *  
 *  where bufSize is a suitably large buffer size.
 */
public class CMSEnvelopedDataParser
    extends CMSContentInfoParser
{
    RecipientInformationStore recipientInfoStore;
    EnvelopedDataParser envelopedData;
    
    private AlgorithmIdentifier encAlg;
    private AttributeTable unprotectedAttributes;
    private boolean attrNotRead;
    private OriginatorInformation  originatorInfo;

    public CMSEnvelopedDataParser(
        byte[]    envelopedData) 
        throws CMSException, IOException
    {
        this(new ByteArrayInputStream(envelopedData));
    }

    public CMSEnvelopedDataParser(
        InputStream    envelopedData) 
        throws CMSException, IOException
    {
        super(envelopedData);

        this.attrNotRead = true;
        this.envelopedData = new EnvelopedDataParser((ASN1SequenceParser)_contentInfo.getContent(BERTags.SEQUENCE));

        // TODO Validate version?
        //ASN1Integer version = this._envelopedData.getVersion();

        OriginatorInfo info = this.envelopedData.getOriginatorInfo();

        if (info != null)
        {
            this.originatorInfo = new OriginatorInformation(info);
        }

        //
        // read the recipients
        //
        ASN1Set recipientInfos = ASN1Set.getInstance(this.envelopedData.getRecipientInfos().toASN1Primitive());

        //
        // read the encrypted content info
        //
        EncryptedContentInfoParser encInfo = this.envelopedData.getEncryptedContentInfo();
        this.encAlg = encInfo.getContentEncryptionAlgorithm();
        CMSReadable readable = new CMSProcessableInputStream(
            ((ASN1OctetStringParser)encInfo.getEncryptedContent(BERTags.OCTET_STRING)).getOctetStream());
        CMSSecureReadable secureReadable = new CMSEnvelopedHelper.CMSEnvelopedSecureReadable(
            this.encAlg, encInfo.getContentType(), readable);

        //
        // build the RecipientInformationStore
        //
        this.recipientInfoStore = CMSEnvelopedHelper.buildRecipientInformationStore(
            recipientInfos, this.encAlg, secureReadable);
    }

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

    /**
     * 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 the content encryption algorithm details for the data in this object.
     *
     * @return AlgorithmIdentifier representing the content encryption algorithm.
     */
    public AlgorithmIdentifier getContentEncryptionAlgorithm()
    {
        return encAlg;
    }

    /**
     * Return the originator information associated with this message if present.
     *
     * @return OriginatorInformation, null if not present.
     */
    public OriginatorInformation getOriginatorInfo()
    {
        return originatorInfo;
    }

    /**
     * return a store of the intended recipients for this message
     */
    public RecipientInformationStore getRecipientInfos()
    {
        return recipientInfoStore;
    }

    /**
     * return a table of the unprotected attributes indexed by
     * the OID of the attribute.
     * @exception IOException 
     */
    public AttributeTable getUnprotectedAttributes() 
        throws IOException
    {
        if (unprotectedAttributes == null && attrNotRead)
        {
            ASN1SetParser             set = envelopedData.getUnprotectedAttrs();
            
            attrNotRead = false;
            
            if (set != null)
            {
                ASN1EncodableVector v = new ASN1EncodableVector();
                ASN1Encodable        o;
                
                while ((o = set.readObject()) != null)
                {
                    ASN1SequenceParser    seq = (ASN1SequenceParser)o;
                    
                    v.add(seq.toASN1Primitive());
                }
                
                unprotectedAttributes = new AttributeTable(new DERSet(v));
            }
        }

        return unprotectedAttributes;
    }

    private byte[] encodeObj(
        ASN1Encodable obj)
        throws IOException
    {
        if (obj != null)
        {
            return obj.toASN1Primitive().getEncoded();
        }

        return null;
    }
}