CAPTCHAs are a type of challenge-response test used in computing to determine whether the user is human and can often be found when subscribing to an online service like an email account, responding to a blog or similar. The right-hand side shows how facebook relies on CAPTCHAs when one tries to create a new account.
Since I sometimes get asked about CAPTCHA controls for .NET I want to introduce two .NET CAPTCHA controls together with some - hopefully interesting - background information:
A stand-alone comment spam filtering library. Includes Akismet (a class for interfacing with the Akismet service), Invisible Captcha (An invisible CAPTCHA validator control as first described here: http://haacked.com/archive/2006/09/26/Lightweight_Invisible_CAPTCHA_Validator_Control.aspx), and Visible Captcha (standard visible CAPTCHA control based on the work of Jeff Atwood http://codinghorror.com/). The Akismet library included is based on code in use by Subtext and DasBlog.
- MSR Asirra
Asirra (Animal Species Image Recognition for Restricting Access) is a really cool project developed by Microsoft Research. Asirra is a human interactive proof that asks users to identify photos of cats and dogs. It's powered by over three million photos from our unique partnership with Petfinder.com. The idea behind Asirra is that most common HIPs (Human Interactive Proof) ask users to identify text that has been distorted or obscured, like the facebook example shown above. Unfortunately, such challenges can be difficult and frustrating for people, yet are often easily solved by computers. Asirra is a HIP that works by asking users to identify photographs of cats and dogs. This task is difficult for computers, but our user studies have shown that people can accomplish it quickly and accurately. Many even think it's fun! Test Asirra on your own! Would you recommend the images (Click the Asirra image to see a live example)?