reCAPTCHA: Blocking Spam and More

A few months ago, I developed a ColdFusion/Mach-II implementation of the reCAPTCHA API.  This type of CAPTCHA requires the person to enter two words rather than a single sequence of random letters.  In addition to protecting your website from spam, reCAPTCHA helps digitize text books.  They do this by using a mystery word as one of the two words, until there is a consensus about what the correct answer for that word is.

First off, a CAPTCHA is a program that can tell the difference between a human user and a computerized bot.  reCAPTCHA is a free CAPTCHA service that helps to digitize books, newspapers and old time radio shows.

The reCAPTCHA service uses two layers of security by taking images that already can't be read by computers and then distorts them even further.

1. They start with a scanned book containing thousands of words.
2. Next, the extract a word that cannot be read by OCR (Optical Character Recognition).  The words are already somewhat distorted because of the font family, age of the book, or the quality of the scan.
3. For extra security, the word is then distorted even more, by using random lines and warps.
4. The CAPTCHA is generated with two distorted word images from books.

To learn more about reCAPTCHA, read their own explanation in their "What is reCAPTCHA?" section.  I also found a youtube video about reCAPTCHA that I enjoyed.