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Another image-based Captcha method

Captcha the Dog offers a simple yet affective image-based method to fight against computer-generated inputs.

Screenshot by Dong Ngo/CNET

Captcha, or Completely Automated Public Turing test to tell Computers and Humans Apart, is a method used by many Web sites to fight against computer-generated input. As computers get smarter, Captchas hves become more of a nuisance because most of them are now tough for us humans to pass.

I recently blogged about a new and more humane way to create a Captcha by using 3D images and the implementation of that method at I've just run into a different approach, similar to the Asirra tool revealed by Microsoft in 2007, that seems even easier for humans to pass while remaining impossible for machines to figure out.

It's called Captcha the Dog from a Web site of the same name. Like the 3D-based Captcha, this method uses images instead of text for the challenge. However, the challenge is always the same: clock on the one different object on the screen, i. e., click on the photo of a dog among eight photos of cats.

With Captcha the Dog you are required to do this multiple times in a row. Each time, the position of the dog is changed and if you click on the wrong picture once, the process starts over from the beginning.

Once you have clicked on the right one enough times, all the photos will be those of cats. This is when you know you have passed the Captcha.

As it is currently impossible for a computer to distinguish between these photos, it's virtually impossible for a machine to randomly select the right image multiple times in a row.

This method of Captcha costs $25 per year with customized images. Beyond that, you can get its codes for free. The new method is said to be compatible with any browser (including that of the iPhone) and can be implemented within 15 minutes without the use of cookies.

This seems a simple yet effective alternative to the text-based Captcha that's so popular and so frustrating to use. However, like other image-based Captcha methods, Captcha the Dog doesn't currently offer a way to support people with disabilities. However, the site states that its new version will offer an audio component for vision-impaired people.

Try out the new Captcha method at and leave your thoughts in the comments.