CAPTCHA Programming

This chapter presents the most advanced algorithms which are used to successfully design a CAPTCHA test. In the first part, the most important techniques which are employed for designing a text CAPTCHA are described. In fact, although it is the most attacked type of CAPTCHA, it is still used in many web sites. Then, the attention will be moved toward the design of the image-based CAPTCHA, for which different techniques of image transformation are usually employed. The third part will be dedicated to algorithms and methods for designing other types of CAPTCHA, such as text-based reCAPTCHA, NoCAPTCHA reCAPTCHA, and game-based CAPTCHA. Finally, the last part of the chapter provides a practical mini-guide on how to design a simple text and image-based CAPTCHA in JavaScript and PHP.