understanding captcha

image source: shareware plaza

If you would notice in most web forms such as blog comments and registration forms, there are mysteriously rambled and blur letters/numbers that you need to input in order for your comments to be approved or to complete your registration. They are called Captcha.

Captcha, according to Wikipedia is “a type of challenge-response test used in computing to determine whether the user is human. "CAPTCHA" is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart", trademarked by Carnegie Mellon University.

The term CAPTCHA was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University), and John Langford (then of IBM). A common type 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.”

Here are the main characteristics of CAPTCHA
1. Current computers are unable to accurately solve.
2. It can be solved mostly by humans.
3. It doesn’t rely on the attacker never having seen the given type of CAPTCHA before.
4. Its can automatically generate new challenges that require artificial intelligence techniques to solve.

Aside from the notion that Captcha only exists to stop spammers, here are great reasons for you to understand more its importance.

1.It is used to disable automated software from performing actions that can degrade the quality of service of a particular system.
2.It is installed as a response to infringement by commercial interests.
3.It is used to protect the systems that are prone to email spam.
4.It is serve a significant function in rate limiting
5.It implements automated usage policies as set by the administrator when certain usage metrics exceed a given threshold.

So the next time you see alphanumeric text that requires your input, don’t be amazed or confused. It is just testing you if you’re human. Don’t worry, you can guess and input endless times until you solve it.