By way of this Coding Horror article discussing recent CAPTCHA breaks in Yahoo, Hotmail, and Google, I stumbled across the Asirra (Animal Species Image Recognition for Restricting Access) project at Microsoft Research. It’s a CAPTCHA system that uses pictures of cats and dogs, taken from Petfinder, to filter out bots from real people. I decided to try using it instead of my current CAPTCHA for comments, and, after some wrangling and debugging, got it to work. I have to say I find it much more amusing and stress-free than picking out bizarrely-mangled words amongst a field of visual noise.
Another blogger seems to take a dim view of Asirra, but I think his main criticism (being able to defeat the CAPTCHA by enlisting users of a legitimate site and storing the results keyed to an image hash) is something that could be addressed in several different ways. I thought about cropping and rotating the images, and then, after a brief search, found this paper whose results suggest that those may be viable options to defeat image hashing algorithms. This is definitely another “arms race” situation where spammers will keep developing new techniques to defeat “human authentication,” but I think Asirra is a pretty decent idea, and, like I said earlier, a lot more fun than a traditional CAPTCHA.
I hope you like it!