What Is CAPTCHA | What Are Different Types Of CAPTCHAs And Why Is CAPTCHA Used

What Is CAPTCHA | What Are Different Types Of CAPTCHAs And Why Is CAPTCHA Used

If you go by dictionary definition “A computer program or system intended to distinguish human from machine input, typically as a way of thwarting spam and automated extraction of data from websites”. In simple words “CAPTCHA is a program used to distinguish between human and a computer”. CAPTCHA is an abbreviation for “Completely Automated Public Turing test to tell Computers and Humans Apart”. It is test which only a human can solve. CAPTCHA was founded from get rid of the computer bots who started spamming the Internet in early 90’s. CAPTCHA is such a difficult test that if a machine solves the CAPTCHA then it will be a next milestone in the field of Artificial Intelligence.


As we mentioned earlier CAPTCHA is used to distinguish between a human and a computer bot. From the dawn of the Internet, it is prone to vulnerabilities and threats. Hackers and spammers used to steal the information or used to do some illegal activity on the Internet by using their computer bots. Computer bot is a program written for a particular use. For example, few year back Yahoo suffered from an attack where a computer bot used to create several email account on its own. So to solve such problems CAPTCHA was invented to distinguish between human and a computer.

Project reCAPTCHA

reCAPTCHA is a most popular implementation of the CAPTCHA. In reCAPTCHA the spam is stopped along with the digitization of books. In reCAPTCHA implementation, an image from a book is shown to the user and user has to enter the letters displayed in the image. This way a spam is stopped along with digitization of the books. Later, CAPTCHA was acquired by Google and then the implementation changed from digitization of book to recognizing the street signs captured from Google Street view cameras. This helped Google in gathering the data for the street view.

Different Types Of CAPTCHA

The Standard CAPTCHA with an audio option : 


This is the standard CAPTCHA where a user is shown with a scrambled word which he/she has to enter in the box below. User gets an option for next word if the current one is difficult. A visually disabled user can also identify the CAPTCHA image by hearing the audio.

Recognize Images :


In this format user is shown with images and is asked to select specific type of images. In the above example user is asked to select the images of earth.

Math Solving CAPTCHA :


In this form of CAPTCHA user is asked to solve a simple math problem. Like adding or subtracting two numbers.



This is hardest form of CAPTCHA where an image is shown is shown in 3-D form and user has to recognize it. This is also called as “Super CAPTCHA” .

Ad Injected CAPTCHA :


If a user is spending 10 seconds over a CAPTCHA the why not to display an ad beside it? Yes, the same goes this one. While the user solves CAPTCHA he/she is shown with an Ad. This helps website to earn that extra penny and the advertiser gets brand recognition.

Applications Of CAPTCHA

  1. Preventing Comment In Blog : It is spammers favorite job to leave a comment on any blog. Spammers does not care about the relevancy of the blog. They do this task using computer bots and to avoid such spam from those bots the CAPTCHA is used.
  2. Protecting Website Registration : As we mentioned earlier Yahoo was attacked by bots and several fake email accounts were created. Then CAPTCHA was used while registering of new user. This helped in saving those extra email id’s from getting created.
  3. Protecting Email Addresses From Scrapers :  Spammers are on continue watch for new email addresses. They keep crawling the web for it. And email addresses in plain text are real easy to find on any website. So to avoid this chaos user is asked to pass the CAPTCHA before showing him/her the email address.
  4. Online Polls :  If you are conducting polls then there are high chances that a computer bot might poll several times. This problem can be easily avoided by introducing a CAPTCHA while polling.
  5. Preventing Dictionary Attacks : In dictionary attack a hacker continuously tries to enter combination of password and he/she iterates this several times. To prevent this, locking of account after certain attempts was used. But again hacker might use this to lock an account anytime. To solve the problem, CAPTCHA is used every time for the login.


We have explained you all the basics of the CAPTCHA. If we have missed anything let our learners know by commenting below and as always you are free to ask questions.

Source : findexamples.com, captcha.net

Happy Learning!

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