Hacking For Beginners – Manthan Desai

2010

40. What is CAPTCHA and how it works?

CAPTCHA or Captcha (pronounced as cap-ch-uh) which stands for "Completely Automated Public Turing test to tellComputers and Humans Apart” is a type of challenge-response test to ensure that the response is only generated byhumans and not by a computer. In simple words, CAPTCHA is the word verification test that you will come across the endof a sign-up form while signing up for Gmail or Yahoo account. The following image shows the typical samples ofCAPTCHA.

Almost every Internet user will have an experience of CAPTCHA in their daily Internet usage, but only a few are aware ofwhat it is and why they are used. So in this post you will find detailed on how CAPTCHA works and why theyare used.

What Purpose does CAPTCHA Exactly Serve?

CAPTCPA is mainly used to prevent automated software (bots) actions on behalf of actual humans. Forexample while signing up for a new email account, you will come CAPTCHA at the end of the sign-up form so as toensure that the form is filled out only by a legitimate human and automated software or a computerbot. The main goal of CAPTCHA is to put forth a test which is simple and straight forward for any human to answer but fora computer, it is almost impossible to solve.

What is the Need to Create a Test that Can Tell Computers and Humans Apart?

For many the CAPTCHA may seem to be silly and annoying, but in fact it has the ability to protect systems from maliciousattacks where people try to game the system. Attackers can make use of automated software’s to generate a h ugequantity of requests thereby causing a high load on the target server which would degrade the quality of service of agiven system, whether due to abuse or resource expenditure. This can affect millions of legitimate users and theirrequests. CAPTCHAs can be deployed to protect systems that are vulnerable to email spam, such as the services fromGmail, Yahoo and Hotmail.

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