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). HIPs are 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, like the example seen to the right.
Unfortunately, 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 our
user studies have
shown that people can accomplish it quickly and accurately. Many even
think it's fun!
Past projects have used photographs to tell computers and humans apart.
Examples include Carnegie Mellon's
PIX CAPTCHA,
Oli Warner's
KittenAuth, and work done by
Chew
and Tygar.
These projects have a common weakness: they use relatively small image
databases. There's a fundamental reason for this. It's difficult for a
computer to automatically classify pictures with high accuracy —
that's why the task is useful as a HIP. An image database small enough to
be constructed manually by a researcher is also small enough to be
manually reconstructed by an attacker.
Asirra is different because of our unique partnership with
Petfinder.com, the world's largest
site devoted to finding homes for homeless pets. They've provided us
with over
three million images of cats and dogs, manually
classified by people at thousands of animal shelters across the
United States. In exchange, we provide a small
"Adopt Me!" link
beneath each photo, supporting Petfinder's primary mission of finding
homes for homeless animals.
Our work was inspired in part by Frozen Bear's
HotCaptcha, which similarly uses
a large database of manually classified images as the basis for a HIP.
We were motivated to improve on the idea because HotCaptcha isn't
really appropriate for widespread deployment. Asking users to rate
people's attractiveness is potentially offensive, and the challenge
does not have any ground truth.
It's easy to add an Asirra HIP to your web site. Microsoft
Research is providing it as a free web service. Be warned that
Asirra is still in beta-testing; the service and its API may both be
unstable.
Asirra consists of two components:
- A JavaScript client component that you add to your web page inside a form.
Our code will add an Asirra challenge to your web page. If the challenge is
solved correctly, the client code gets an Asirra Ticket from our server, and
adds it to your form as a hidden input field.
- A web service at Microsoft Research that your form processor should call
each time a user form is submitted, to check that the ticket provided is valid.
The JavaScript works in all major browsers; it has been tested in IE6,
IE7, Firefox 2, Safari 2.0.4, Opera 8.54 and Opera 9.23.
For detailed instructions, please see our
Installation Page.
If you use Asirra on your web site, we'd love to hear from you! Contact information is
at the bottom of the page.
For more information, please contact us at
