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• 1What is Captcha?
• 2Available Captchas
  • 2.1Pligg Default Captcha
  • 2.2Recaptcha
  • 2.3WhiteHat Method
  • 2.4Math Question

What is Captcha?

A captcha is a program that can tell whether its user is a human or a computer. You've probably seen them — colorful images with distorted text at the bottom of Web registration forms. Captchas are used by many websites to prevent abuse from "bots," or automated programs usually written to generate spam. No computer program can read distorted text as well as humans can, so bots cannot navigate sites protected by captchas [source].

Available Captchas

Currently Pligg supports 4 different captcha methods. These four methods are all modularized so that they can be updated without altering the Pligg core code.

Pligg Default Captcha

The standard Pligg captcha is one of the more advanced methods for generating a captcha image. It generates a common captcha image with alphanumeric characters that users have to decifer in order to proceed.

Recaptcha

About 60 million CAPTCHAs are solved by humans around the world every day. In each case, roughly ten seconds of human time are being spent. Individually, that's not a lot of time, but in aggregate these little puzzles consume more than 150,000 hours of work each day. What if we could make positive use of this human effort? reCAPTCHA does exactly that by channeling the effort spent solving CAPTCHAs online into "reading" books.
To archive human knowledge and to make information more accessible to the world, multiple projects are currently digitizing physical books that were written before the computer age. The book pages are being photographically scanned, and then transformed into text using "Optical Character Recognition" (OCR). The transformation into text is useful because scanning a book produces images, which are difficult to store on small devices, expensive to download, and cannot be searched. The problem is that OCR is not perfect.
reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher. More specifically, each word that cannot be read correctly by OCR is placed on an image and used as a CAPTCHA. This is possible because most OCR programs alert you when a word cannot be read correctly.
But if a computer can't read such a CAPTCHA, how does the system know the correct answer to the puzzle? Here's how: Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one. The system then gives the new image to a number of other people to determine, with higher confidence, whether the original answer was correct.
Currently, we are helping to digitize books from the Internet Archive and old editions of the New York Times.

WhiteHat Method

The WhiteHat captcha is similar to the Pligg default captcha. It is based on teh work of http://www.white-hat-web-design.co.uk. You can see the source for the original captcha code here.

Math Question

The math question captcha asks a user to solve a simple math formula. You specify a 2 number ranges for the captcha to pull a random number from using the Pligg configuration page. The end result would look something like this:

What is the answer to this question: What is 4 + 4 =

This captcha method assumes that a computer is not able to detect that the captcha field is for bot checking and that the computer won't try to solve the math question. It is much less secure than randomly generated images because it can be read by computers much easier. At the same time it is partially more user friendly because a user doesn't have to decipher a partially scrambled alphanumeric image.