Beta automatic CAPTCHA
Friday, 29 October 2010 01:32:01 UTC
It's based on HashCash. The client performs a CPU-intensive task (it concatenates the URL, time accurate to the minute, and tries to find a number to tack onto the end that has a SHA1 hash with enough zeroes on the beginning). It's easy to tell (one hash and query to the clock), but hard to get (a ton of numbers won't generate enough zeroes).