http://en.wikipedia.org/wiki/Challenge%E2%80%93response_spam_filtering http://en.wikipedia.org/wiki/Challenge%E2%80%93response_spam... Further reading if anyon

http://en.wikipedia.org/wiki/Challenge%E2%80%93response_spam_filtering http://en.wikipedia.org/wiki/Challenge%E2%80%93response_spam... Further reading if anyone is interested in challenge-response systems.