How to Sign Javamail with DKIM

Question

Is there a library or a way to do this without an external library? I am using apache james as my mail server and currently send email like this:

public void sendMessage(String to, String subject, String content) {
    MimeMessage message = new MimeMessage(session);
    try {
        message.addRecipients(Message.RecipientType.TO, to);
        message.setFrom(new InternetAddress(from));
        message.setSubject(subject);
        message.setContent(content, "text/html; charset=utf-8");
        Transport.send(message);
    } catch (MessagingException e) {
        e.printStackTrace();
    }       
}

But i'd like to sign the email with DKIM before hand. I understand I need to implement DKIM signing into the james server and plan on use jDKIM to do this, I also understand I need to create the keys using something like www.port25.com, but how do I actually sign the email in java before I send it out?

Answer 1

I ended up using DKIM for Javamail which can be downloaded at: DKIM For Javamail

Here is an example (Its pretty well documented in the examples in the download):

public void sendMessage(String to, String subject, String content) {
    //Create DKIM Signer
    DKIMSigner dkimSigner = null;
    try {
        dkimSigner = new DKIMSigner(properties.getProperty("mail.smtp.dkim.signingdomain"), properties.getProperty("mail.smtp.dkim.selector"), properties.getProperty("mail.smtp.dkim.privatekey"));
        dkimSigner.setIdentity(properties.getProperty("mail.user") + "@" + properties.getProperty("mail.smtp.dkim.signingdomain"));
        dkimSigner.setHeaderCanonicalization(Canonicalization.SIMPLE);
        dkimSigner.setBodyCanonicalization(Canonicalization.RELAXED);
        dkimSigner.setLengthParam(true);
        dkimSigner.setSigningAlgorithm(SigningAlgorithm.SHA1withRSA);
        dkimSigner.setZParam(true);
    } catch (Exception e) {
    e.printStackTrace();
        }
    if(dkimSigner != null) {
        //Create message
        Message message = new SMTPDKIMMessage(session, dkimSigner);
        try {
            message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(to, false));
            message.setFrom(new InternetAddress(from));
            message.setSubject(subject);
            message.setContent(content, "text/html; charset=utf-8");
            Transport.send(message);
        } catch (MessagingException e) {
            e.printStackTrace();
        }   
    }           
}

Answer 2

Simple Java Mail recently added support for DKIM signing. Here's your code, but now with Simple Java Mail:

public void sendMessage(String to, String subject, String content) {
    final Email email = new Email.Builder()
            .from(null, from)
            .to(null, to)
            .subject(subject)
            .textHTML(content)
            .build();

    email.signWithDomainKey(new File(properties.getProperty("mail.smtp.dkim.privatekey")),
                            properties.getProperty("mail.smtp.dkim.signingdomain"),
                            properties.getProperty("mail.smtp.dkim.selector"));

    new Mailer(...).sendMail(email);
}

The private key argument can be a File, InputStream or a byte[].

Interestingly, Behind the scenes Simple Java Mail uses java-utils-mail-dkim (GitHub), which is an active fork of the dormant DKIM-for-JavaMail (GitHub), which was the continuation of the library you are using now, DKIM For Javamail (SourceForge). So, the one you are using is very old.