What is Captcha?

CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Human Apart.

A CAPTCHA is a program that can tell whether the user is a human or a computer.

A CAPTCHA is a program that protects websites against bots by generating and grading tests that humans can pass but current computer programs cannot.



An example for CAPTCHA is shown below:


You can see from the image that computer can't recognise these characters.


A CAPTCHA is sometimes called as a reverse Turing test as it is administered by a computer and targeted to a human.

The term CAPTCHA was invented by Luis von Ahn, Manuel Blum, Nicholas Hooper and John Langford of Carnegie Mellon University.

Application of CAPTCHA:

• Prevent Automatic E-mail Regestration
  
  Captchas are used to prevent automated mass registration of accounts to online services such as E-mail site,Online registration,forums and auction sites.
  
  
• Preventing Comment Spam in Blogs.
  
  
• Protecting E-mail Address from Scrapers.
  
  Mechanism to hide your email address, require users to solve a CAPTCHA before showing your email address.
  
  
• Online Polls.
  
  You cannot trust the results of an online roll because anybody could just write a program to vote for their favorite option thousands of times.
  
  
• Protecting Website Registration.
  
  E-mail services, Yahoo, Microsoft, Google
  
  
• Preventing Dictionary Attacks (in password systems).
  
  Prevent a computer to iterate through the entire space of passwords by requiring it to solve a CAPTCHA after a certain number of unsuccessful logins.
  
  
• Search Engine Bots.
  
  It is sometimes desirable to keep webpages unindexed to prevent others from finding them easily.
  
  
• CAPTCHAs are also used to minimize automated posting to blogs, forums and wikis, whether as a result of commercial promotion, or harassment and vandalism

Roughly 60 million CAPTCHAs are solved each day.

There are different types of CAPTCHAs too.

• Images
  
  
• Logical
  
  
• Mathematical
  

Accessibility of CAPTCHA:

CAPTCHAs prevent visually incapable users from accessing the protected resource.

CAPTCHAs tend to be inaccessible for all users that can not see the test image.

Imagine if you could not see the image. What would you write in the text field?

So, sites implementing CAPTCHAs may provide an audio version of the CAPTCHA in addition to the visual.

The official CAPTCHA site recommends providing an audio CAPTCHA for accessibility reasons, but it is not usable for deafblind people or for users of text web browsers(browser which doesn't support sound).

Even audio and visual CAPTCHAs will require manual intervention for some users, such as those who have disabilities.

Audio and visual CAPTCHA can provide ease for blind people to submit online application forms.

So, CAPTCHA is a wall between fake programmers and e-mail sits, bloggers etc.