Asirra (Animal Species Image Recognition for Restricting Access) is a captcha technology developed by Microsoft. This article will explain how it's different from other captcha systems, the benefits of using Asirra, and how to set it up on your PHP-based website.
Unlike other types of captcha that utilize difficult text obfuscation techniques (such as Google reCaptcha), this system utilizes images of dogs and cats, such as those shown in the screen shot below:
The following are the main benefits of using the Asirra captcha system:
1. It is very user friendly to website users. Unlike other systems that display almost unrecognizable text (which can be difficult and time consuming for visitors to figure out), Asirra uses real photos of cats and dogs. The user will simply select all images of cats from the array presented to them on the web form, and then submit the form for authentication.
2. This is a very difficult system for spammers to break using automated methods, because the image database provided by www.petfinder.com (in partnership with Microsoft) includes at least three million photos -- and is still growing, at a rate of 10,000 images per day. So even if a spammer hires humans to reconstruct the database, keeping up would be an impossible task.
3. It does not use session ID. Asirra captcha is completely stateless. There is no need to create sessions just for captcha purposes, or to be concerned about the possibility of a hijacked session.
4. Asirra is very easy to integrate with PHP, and there is no need to register your website to use Asirra services. If you want to start using Asirra captcha to protect your web application/form, it is as simple as copying the code and then pasting it to your existing application with very minor tweaks.
5. Lastly, this captcha solution is free and provided by Microsoft.
This article talks about integrating this technology with PHP, particularly emphasizing its important applications: bot prevention for forms, and human person validation.