A CAPTCHA is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a person. The process usually involves a computer asking a user to complete a simple test which the computer is able to grade. These tests are designed to be easy for a computer to generate, but difficult for a computer to solve, so that if a correct solution is received, it can be presumed to have been entered by a human. Captchas are used by many websites to prevent abuse from "bots," or automated programs usually written to generate spam.
Currently Pligg supports 3 different captcha methods through a module add-on that comes standard and pre-configured with Pligg.
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.
The WhiteHat captcha is similar to the Pligg default captcha. It is based on the work of http://www.white-hat-web-design.co.uk. You can see the source for the original captcha code here.
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:
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.
What is the answer to this question:
What is 4 + 4 =