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The Bouncy Castle Java S/MIME APIs for handling S/MIME protocols. This jar contains S/MIME 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. The Jakarta Mail API and the Jakarta activation framework will also be needed.
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package org.bouncycastle.mail.smime;

import java.io.IOException;
//import java.security.NoSuchAlgorithmException;
//import java.security.Provider;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;

import jakarta.activation.DataHandler;
//import javax.crypto.KeyGenerator;
import jakarta.mail.Header;
import jakarta.mail.MessagingException;
import jakarta.mail.Multipart;
import jakarta.mail.Session;
import jakarta.mail.internet.MimeBodyPart;
import jakarta.mail.internet.MimeMessage;

import org.bouncycastle.cms.CMSEnvelopedGenerator;
import org.bouncycastle.util.Strings;

/**
 * super class of the various generators.
 */
public class SMIMEGenerator
{
    private static Map BASE_CIPHER_NAMES = new HashMap();

    static
    {
        BASE_CIPHER_NAMES.put(CMSEnvelopedGenerator.DES_EDE3_CBC, "DESEDE");
        BASE_CIPHER_NAMES.put(CMSEnvelopedGenerator.AES128_CBC, "AES");
        BASE_CIPHER_NAMES.put(CMSEnvelopedGenerator.AES192_CBC, "AES");
        BASE_CIPHER_NAMES.put(CMSEnvelopedGenerator.AES256_CBC, "AES");
    }

    protected boolean useBase64 = true;
    protected String encoding = "base64";  // default sets base64

    /**
     * base constructor
     */
    protected SMIMEGenerator()
    {
    }

    /**
     * set the content-transfer-encoding for the CMS block (enveloped data, signature, etc...)  in the message.
     *
     * @param encoding the encoding to use, default "base64", use "binary" for a binary encoding.
     */
    public void setContentTransferEncoding(
        String encoding)
    {
        this.encoding = encoding;
        this.useBase64 = Strings.toLowerCase(encoding).equals("base64");
    }

    /**
     * Make sure we have a valid content body part - setting the headers
     * with defaults if neccessary.
     */
    protected MimeBodyPart makeContentBodyPart(
        MimeBodyPart content)
        throws SMIMEException
    {
        //
        // add the headers to the body part - if they are missing, in
        // the event they have already been set the content settings override
        // any defaults that might be set.
        //
        try
        {
            MimeMessage msg = new MimeMessage((Session)null)
            {
                // avoid the call of updateMessageID to prevent
                // DNS issues when trying to evaluate the local host's name
                protected void updateMessageID()
                    throws MessagingException
                {
                    // do nothing
                }
            };

            Enumeration e = content.getAllHeaders();

            msg.setDataHandler(content.getDataHandler());

            while (e.hasMoreElements())
            {
                Header hdr = (Header)e.nextElement();

                msg.setHeader(hdr.getName(), hdr.getValue());
            }

            msg.saveChanges();

            //
            // we do this to make sure at least the default headers are
            // set in the body part.
            //
            e = msg.getAllHeaders();

            while (e.hasMoreElements())
            {
                Header hdr = (Header)e.nextElement();

                if (Strings.toLowerCase(hdr.getName()).startsWith("content-"))
                {
                    content.setHeader(hdr.getName(), hdr.getValue());
                }
            }
        }
        catch (MessagingException e)
        {
            throw new SMIMEException("exception saving message state.", e);
        }

        return content;
    }

    /**
     * extract an appropriate body part from the passed in MimeMessage
     */
    protected MimeBodyPart makeContentBodyPart(
        MimeMessage message)
        throws SMIMEException
    {
        MimeBodyPart content = new MimeBodyPart();
    
        //
        // add the headers to the body part.
        //
        try
        {
            // JavaMail has a habit of reparsing some content types, if the bodypart is
            // a multipart it might be signed, we rebuild the body part using the raw input stream for the message.
            try
            {
                if (message.getContent() instanceof Multipart)
                {
                    content.setContent(message.getRawInputStream(), message.getContentType());

                    extractHeaders(content, message);

                    return content;
                }
            }
            catch (MessagingException e)
            {
                // fall back to usual method below
            }

            content.setContent(message.getContent(), message.getContentType());

            content.setDataHandler(new DataHandler(message.getDataHandler().getDataSource()));

            extractHeaders(content, message);
        }
        catch (MessagingException e)
        {
            throw new SMIMEException("exception saving message state.", e);
        }
        catch (IOException e)
        {
            throw new SMIMEException("exception getting message content.", e);
        }

        return content;
    }

    private void extractHeaders(MimeBodyPart content, MimeMessage message)
        throws MessagingException
    {
        Enumeration e = message.getAllHeaders();

        while (e.hasMoreElements())
        {
            Header hdr = (Header)e.nextElement();

            // normalise some headers
            if (hdr.getName().equals("Message-Id"))
            {
                content.addHeader("Message-ID", hdr.getValue());
            }
            else if (hdr.getName().equals("Mime-Version"))
            {
                content.addHeader("MIME-Version", hdr.getValue());
            }
            else
            {
                content.addHeader(hdr.getName(), hdr.getValue());
            }
        }
    }

//    protected KeyGenerator createSymmetricKeyGenerator(
//        String encryptionOID,
//        Provider provider)
//        throws NoSuchAlgorithmException
//    {
//        try
//        {
//            return createKeyGenerator(encryptionOID, provider);
//        }
//        catch (NoSuchAlgorithmException e)
//        {
//            try
//            {
//                String algName = (String)BASE_CIPHER_NAMES.get(encryptionOID);
//                if (algName != null)
//                {
//                    return createKeyGenerator(algName, provider);
//                }
//            }
//            catch (NoSuchAlgorithmException ex)
//            {
//                // ignore
//            }
//            if (provider != null)
//            {
//                return createSymmetricKeyGenerator(encryptionOID, null);
//            }
//            throw e;
//        }
//    }
//
//    private KeyGenerator createKeyGenerator(
//        String algName,
//        Provider provider)
//        throws NoSuchAlgorithmException
//    {
//        if (provider != null)
//        {
//            return KeyGenerator.getInstance(algName, provider);
//        }
//        else
//        {
//            return KeyGenerator.getInstance(algName);
//        }
//    }
}