Dominique Stender A blog about my thoughts and experiences in Information Technology

The CAPTCHA arms race

CAPTCHAs... we have all seen them. CAPTCHA means Completely Automated Public Turing test to tell Computers and Humans Apart and is a family of techniques to make sure a user (typically on a website) is indeed a human being and not a program trying to act like one.

When you leave your comment on this blog you will be asked to type in two words which are displayed as distorted graphic. Most bulletin boards and free mail providers ask you to do the same before they allow you to create an account.

CAPTCHA 101

The reason behind is the same most of the time: Preventing SPAM. Spammers use forums, blog comments and contact forms to post their ads. They use bots (quite similar to the bots that update the search index on Google, Yahoo and all other search websites) to automate that process.

So the idea of CAPTCHAs is to present a task to a website visitor that is difficult to solve for a machine, but easy to solve for a human. The graphical CAPTCHA is the most commonly used one.

There are other CAPTCHA variants such as audio-based ones or image recognition based CAPTCHAs. I've even seen a simple math question as CAPTCHA.