A CAPTCHA is an abbreviation of Completely Automated Public Turing test to tell Computers and Humans Apart. It is a challenge-response system test designed to differentiate the humans from an automated programs. It differentiates between a human and bot by setting some task that is easy for most humans to perform but it is more difficult and time-consuming for current bots to complete.
A CAPTCHA is often used to stop bots and other automated programs from using the blogs to affect the rankings of the search engine, signing up for e-mail accounts to send out a spam or to take part in an on-line poll.
Frequently, a CAPTCHA features an image file of slightly distorted alphanumeric character. A human can usually read the characters in the image without any difficulty. A bot program has able to recognize that the content contains an image, but it has no idea about the image. To accommodate the visually-impaired, some CAPTCHAs uses audio files. In such a system, the human listens to a series of the letters or short words and types what he hears to prove he is not a bot.
The “E-Z Gimpy” is one of the most commonly used CAPTCHAs, and it is one of the easiest to crack by using a dictionary attack. The full edition of GIMPY is considered to be more secure. It randomly selects seven words from a dictionary and then presents them, but somewhat distorted, in an image. The user is asked to type three of the words that appear in the image on the screen.
Another test, “Pix,” presents the user with six images of a single subject, such as horses or babies, and asks them to define the subject of the pictures. A Math CAPTCHAs are very popular for the blog software. A Math CAPTCHA has uses the numbers instead of letters and asks the user (human) to solve a simple mathematical calculation (5 +5 =) and record the answer.
Once a user has completed the particular CAPTCHA challenge satisfactorily, he is granted to access the page. His address or login information may be added to whitelist so that he is not asked to take the test on his next visit.