I’ve done it! I have a working S/MIME plugin for Wordpress! It really only makes sense if you only allow registered users to use your site, but I can see other
uses for it as well.

Prerequisites: S/MIME uses openssl function calls, you must have openssl compiled in or enabled as an extension. WP 1.5 is required - I seriously doubt that < 1.5 will work, but you’re welcome to try.

In order to use it, download the smime.zip file, uncompress it into your wp-content/plugins folder, and enable it. Administrator options are available under Options->Crypto Options, and user keys are available under Users->Encryption Keys. In order to enable the encryption, you must edit wp-includes/functions.php and look for the wp_mail function. Within the wp_mail function, change mail( to smimemailer(. It appears that this will not be necessary with 1.5.1, but I’m not sure yet, so this is the current safest way.

Currently, you still have to cut and paste the PEM file, but I’d like to implement file upload. The weblog does not sign e-mails yet. And I’ve commented out the options form which stores the blog’s private key. There are probably a lot of bugs, please let me know. I have the plugin registered on wp-plugins.org as smime, so maybe you can post bug reports there. I’m not sure how it works yet, so be patient as I figure it out.

A better README will be coming soon.