On this WordPress blog, I'm running the WP Hashcash plugin to prevent spam. Recently, I've seen a lot more spam, I guess spambots now recognise WPHC encryption.
My solution is to include a few filters in WPHC. This allows for weblog owners to write a tiny plugin to have slightly different antispam than other weblogs. For instance, at this moment, my extension is as simple as:
Plugin Name: WPHC Extension
Plugin URI: https://www.geeklab.info/2010/04/wordpress-spam-filtering/
Author URI: https://www.geeklab.info
for($i = 0; $i < count($js); $i++)
You may use substraction, addition, xor, byte-swapping or any other method you like to improve security. The security is not so much what is done to the key, but that the routine is slightly different from other blogs.
Changes to WP Hashcash required for this to work:
Patch file (only changes)