What is CAPTCHA and How it Works?


CAPTCHA or Captcha (pronounced as cap-ch-uh) which stands for “Completely Automated Public Turing test to tell Computers and Humans Apart” is a type of challenge-response test to ensure that the response is only generated by humans and not by a computer.

In simple words, It is the word verification test that you will come to the end of a sign-up form while signing up for Gmail or Yahoo account. The following image shows the typical samples of CAPTCHA.

Captcha Sample

Almost every Internet user will have an experience of CAPTCHA in their daily Internet usage, but only a few are aware of what it is and why they are used. So, in this post, you will find detailed information on how CAPTCHA works and why it is used. What Purpose does CAPTCHA Exactly Serve?  

CAPTCHA is mainly used to prevent automated software (bots) from performing actions on behalf of actual humans. For example, while signing up for a new email account, you will come across a CAPTCHA at the end of the sign-up form so as to ensure that the form is filled out only by a legitimate human and not by any of the automated software or a computer bot.

The main goal of CAPTCHA is to put forth a test that is simple and straightforward for any human to answer but for a computer, it is almost impossible to solve.

What is the Need to Create a Turing Test?

For many, the CAPTCHA may seem to be silly and annoying! In fact, CAPTCHA has the ability to protect systems from malicious attacks where people try to game the system. Attackers can make use of the automated software to generate a huge quantity of requests thereby causing a high load on the target server.

This could lead to a degrade the quality of service of a given system, either due to abuse or resource expenditure. This can affect millions of legitimate users and their requests. CAPTCHAs can be deployed to protect systems that are vulnerable to email spam, such as the services from Gmail, Yahoo, and Hotmail.


CAPTCHAs are mainly used by websites that offer services like online polls and registration forms. For example, Web-based email services like Gmail, Yahoo, and Hotmail offer free email accounts for their users. However, upon each sign-up process, CAPTCHAs are used to prevent spammers from using a bot to generate hundreds of spam mail accounts.

Designing a CAPTCHA System:

 CAPTCHAs are outlined in the way that, the PCs do not have the capacity that people have with regards to handling visual information. It is all the more effectively feasible for people to take a gander at a picture and select the examples than a PC.

This is on account of PCs do not have the genuine insight that people have as a matter of course. CAPTCHAs are actualized by giving clients a picture that contains misshaped or haphazardly extended characters which just people ought to have the capacity to distinguish. At times, characters are stroked out or given a boisterous foundation to influence it, to considerably more, harder for PCs to make sense of the examples.

Most, however not all, CAPTCHAs depend on a visual test. A few Websites execute an entirely unexpected CAPTCHA framework to distinguish people and PCs from the other. For instance, a client is given 4 pictures in which 3 contains pictures of creatures and one contains a blossom. The client is made a request to choose just those pictures which contain creatures in them. This Turing test can undoubtedly be tackled by any human, however practically inconceivable for a PC.

Breaking the CAPTCHA

The challenge in breaking the CAPTCHA lies in a real hard task of teaching a computer how to process information in a way similar to how humans think. Algorithms with artificial intelligence (AI) will have to be designed in order to make the computer think like humans when it comes to recognizing the patterns in CAPTCHA images.

However, there is no universal algorithm that could pass through and break any CAPTCHA system. Thus, each CAPTCHA algorithm must have to be tackled individually. It might not work 100 percent of the time, but it can work often enough to be worthwhile to the spammers.

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