Web services are often protected with a challenge that's supposed to be easy for people to solve, but difficult for computers. Such a challenge is often called a CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) or HIP (Human Interactive Proof).
It is used for many purposes, such as to reduce email and blog spam and prevent brute-force attacks on web site passwords.
Today, the most common HIPs ask users to identify text that has been distorted or obscured, such challenges can be difficult and frustrating for people, yet are often easily solved by computers.
Asirra (Animal Species Image Recognition for Restricting Access) is a HIP that works by asking users to identify photographs of cats and dogs. This task is difficult for computers, but people can accomplish it quickly and accurately.
It's easy to add an Asirra HIP to your web site. Microsoft Research is providing it as a FREE web service.
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