I've added a small setting which makes it possible to choose if the response of the possible spammer just has to contain the configured response. My changes are in a git-repository at Github.com: http://github.com/the-kenny/chall...engereponse

I hope I'm allowed to make this changes.

A patch is ready if you want to merge this into your SVN-Repository. Just contact me.
(There is a small code-duplication, but this isn't a big issue...)