Find out more
Here are more online resources:
Anatomy and Performance of SSL Processing http://www.cs.ucr.edu/~bhuyan/papers/ssl.pdfb
CAPTCHA: Telling Humans and Computers Apart Automatically http://www.captcha.net/
Alternatives to CAPTCHA http://www.w3.org/TR/turingtest/
Usability 101: Introduction to Usability http://www.useit.com/alertbox/20030825.html