Computer – CAPTCHA

A CAPTCHA is a program that can generate and grade tests that humans can pass but current computer programs cannot. For example, humans can read distorted text (as the one given  below),  but current computer programs can’t

What is full form of CAPTCHA :

The full form of CAPTCHA is “Completely Automated Public Turing test to tell Computers and Humans Apart”.  This is  a trademark of Carnegie Mellon University,  and can be termed as a type of challenge-response test used in computing to determine whether or not the user is human. The term was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper of Carnegie Mellon University, and John Langford of IBM.

The most common type of CAPTCHA was first invented by Mark D. Lillibridge, Martin Abadi, Krishna Bharat, and Andrei Z. Broder. This form of CAPTCHA requires that the user type the letters of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, a CAPTCHA is sometimes described as a reverse Turing test. This term is ambiguous because it could also mean a Turing test in which the participants are both attempting to prove they are the computer.

What are the uses of CAPTCHA ? 

CAPTCHAs have several applications for practical security.  Some of the important uses of CAPTCHA are as follows:-

•             It Prevents Comment Spam in Blogs – We have heard that in some cases bogus comments are submitted either as a spam or to to raise search engine ranks of some website.   Usually, such spam comments are generated through computer programmes.   Thus, by using a CAPTCHA websites can eliminate such spam comments as in that case only humans can enter comments on a blog.  

•             To Prevent Fake Registration –  There are many companies which  offer free email services.  Till few years back,  most of such services experienced bots attack  that would sign up for thousands of email accounts every minute.   The problem was resolved by introduction of CAPTCHA before final submission of the details as it ensured that only humans obtain free accounts.   In general, free services should be protected with a CAPTCHA in order to prevent abuse by automated programs.

•             Online Polls  -   In order to prevent  fake polling votes , CAPTCHA plays an important role.

•             Preventing Dictionary Attacks.   CAPTCHAs can also be used to prevent dictionary attacks in password systems.   The idea is simple: prevent a computer from being able to iterate through the entire space of passwords by requiring it to solve a CAPTCHA after a certain number of unsuccessful logins.

•             Worms and Spam. CAPTCHAs also offer a plausible solution against email worms and spam: “I will only accept an email if I know there is a human behind the other computer.” A few companies are already marketing this idea.