Simple Text-based CAPTCHA Implementation

wiras@slepi.net (Wiras Adi) — Sun, 03 Feb 2008 18:43:26 -0600

Spambots are automated scripts that crawl on the net searching for URLs containing some kind of application forms - such as forums, guestbooks, or comment form on popular blogs -, and then automatically posting whatever its initial launcher (spammer) wants everybody to know. It usually carries commercial messages, offers, or simply just site promotions. This annoying practice has been one of the biggest problems of the Internet since the early days.

There are several known ways to fight this kind of spambot, like applying moderation mechanism to allow moderators of the site doing some sort of manual checking and validation against every post submitted. Despite being an effective (yet not too efficient) way to prevent spams, there is in fact a more preferred method called CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart). As being indicated by the word automatic in the name, this AI-based testing attempts to eliminate manual validations existed in a moderated system, adding a higher degree of efficiency.

Different algorithms have been developed to implement CAPTCHA. The most popular one is by challenge users to rewrite a certain text or word presented as a distorted image, assuming such text will be difficult for computer to read but still recognizable to human. Another algorithm is to present users a sound and challenge them to write what they've heard. But my favorite CAPTCHA implementation is the old and simple text-based challenge. It works by asking users to answer a randomly generated question, like "What is the color of the sky at night?" or simple math question like "What is twenty divided by five?". Personally I'd prefer this kind of question-answer interaction to a system that asking me to write down something it shows. It feels more "human", and it works at roughly the same security level as the other methods.

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Simple Text-based CAPTCHA Implementation