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What is CAPTCHA ?

CAPTCHA standing for Completely Automated Public Turing test to tell Computers and Humans Apart. In simple words which differenciate between the input generated by human and computers. I ‘m sure everyone uses atleast CAPTCHA once in a day. Sometimes they really annoyed us and sometimes we thanked for this  technology. In this article you will know Why we need these CAPTCHAs and How they are used and different types of CAPTCHAs nowadays.

Why we need CAPTCHAs

We visit lot of websites daily and frequently we saw them on websites and blogs. But we never think why website owner or developers use them on website. Because you will know the importance of this only if either you run websites or you are a spammer. Just think about your mailbox, when you open your email I ‘m sure you”ll see spam folder too. If you ever open these folders on your emails then you will come to know why email providers provide you this facility. Everyone receives lots of spam emails daily and it automatic goes into spam in your mailbox. In a similar manner when ever someone creates an account or login on some website or participate in online voting then some information is stored on server everytime when user login or votes. So if all these information that is stored on server is completely generated by spammers then its difficult to recognise which information is real and which is fake.  Its the same way if you receive 100 or more emails in your mail daily and you have to identify which one is realand which are spammed emails.   Then another question came to our mind that Why spammer do that? What they got after doing all these things? The answer is simple incentives, profit, name, satisfaction etc…

  • To access your personal information.
  • Disintegrate or defacing the website data.
  • To increase the profit from advertisements.
  • Increase Search engine rankings.

How CAPTCHAs are used and protection from spam

There are different types of CAPTCHAs that are present today, that we discuss in the next part. Basically its only the code or one can say script that website owner installed at the backend in a same manner as you install a software in your computer. User must have to give input according to the CAPTCHA data if your entered input matches with answer then BINGO! Have you any idea CAPTCHAs role in protecting spams for a single day ? According to data available on their respective sites, ReCAPTCHA and Akismet both individually protects 30+Million spams daily. Now you can think easily how much important is to have a protection against spams.

Different approaches of different type of CAPTCHAs

As the technology improves, in similar manner methods of cracking these technology also ameliorates. OCR technique is used to decode the characters from image. That why now much complicated and overlapped characters shown in images of CAPTCHAs. Lets talk about various types of CAPTCHAs used these days. Some of them are :

  • Text recognition based (eg: reCAPTCHA)
  • Image recognition based (eg: identiPIC)
  • Social Authentication/ Friend Recognition (rg: facebook CAPTCHA)
  • Logic questions based (eg: textCAPTCHA)
  • User interaction (eg: sliders type)
  • Honeypot

Text recognition

When image shows different character in CAPTHCA then it comes under the type Text recognition. One of the such service is provided by GOOGLE freely known as ReCAPTCHA. reCAPTCHA project started at Carnegie Mellon University in 2000. reCAPTCHA service that helps to digitize books and newspapers.

Google reCAPTCHA project

Google reCAPTCHA project

Now this project is run by Google, According to facts mentions on site it says about 200 Million CAPTCHAs are entered in single day by humans all over the world. reCAPTCHA uses OCR technique for this. In reCAPTCHA those words are inserted CAPTCHA which is not correctly read by OCR technique during the process of digitizing of books.



In reCAPTCHA also give you option of audio CAPTCHA, those who have problem visually they can take the benifit of this technology.

Text recognition is used by Google in Gmail,Orkut or on other services. Here shows an example of two different CAPTCHAs.

google or gmail captcha

google or gmail captcha

Yahoo also uses text recognition based CAPTCHAs.It also provide audio feature in CAPTCHAs same as reCAPTCHA. Letters are much conjusted so its difficult for bots and OCR techniqus to decode correcly hence protecs fom spams.



Image recognition

identiPIC is best example of such type of CAPTCHAs. Microsoft also working on Image recognition project called asirra


Microsoft asirra project : identiPIC

Microsoft asirra project : identiPIC

Here is identiPIC demo how it looks like:



In image recognisation CAPTCHAs there is no option of audio, So its the negative effect for those who arevisually impaired.

Social Authentication or Friend Recognition

Facebook CAPTCHA method comes under this category. May be sometime it happens with you that when you try to login facebook shows some images from your  photo gallary and ask you who is this person like that. Its actually method to protect your account and protect from spamming. Here is demo :


Facebook CAPTCHA

Social Authentication menthod is better because hacker may know your personal information but they don’t know who your friends are. This misht create problems for those who has long list of social friends and they even don’t know the name of all or not able to remember them, afterall we are humans 😛


Logic questions

Sometime you saw on websites they asked simple ques like “How much is 9+2= ?”. textCAPTCHA fall in this category. There are 180,243,205 simple question in textCAPTCHA database. At some places textCAPTCHAs are more effective than image recognisation CAPTHCAs.


  • Image recognisation challenge is inaccessible to visually impaired users. This problem generally overcome by adding audio CAPTCHAs. But audio also creates problem like pronunciation,background noise or difficult to hear.
  • These image  and audioCAPTCHAs are difficult to implement, reCAPTCHA is easy to implement but it requires javascripts and add code using iframes.
  • Image CAPTCHAs are not always effective. Some OCR software can often read the image.


As every coin has two sides, So Logic Questions also have some disadvantages:

  • Text based CAPTCHAs are usually less secure than image and audio recognisation based CAPTCHAs. Mean they are easier to break.
  • Logic CAPTCHAs need mental attention more. So it may create problems who have mentally imapired.
  • More over text based CAPTCHAs  question are language specific.




User interaction


This is something more interesting one. In such types of CAPTCHAs user has to dragg the small slider to perform task. TheyMakeApps uses such CAPTCHA where slider must be dragged to create rather than to click submit as shown in image.



CAPTCHA in apps

In similar manner Adafruit blog uses Slider CAPTCHA in which user need to drag the sliders to each color as shown in the resistor image. You can see image below.






This is smart method to detect bots and auto generated scripts. When someone fill forms then there is some hidden fields, which is not visible to users. So bots interact with code of page and they are not able to distinguish that field is hidden or not. So whenever data is inserted into this hidden field called “Honeypot” then its sure its not genuine user.

As everything is vulnerable so this method is also not fool proof. But we can make it more efficent through coding and using javascripts to fill dynamically hidden fields.


So this is all about CAPTCHAs. Hope this article is helpful for you and you found some valuable information about “what is CAPTCHAs”. If you familiar with any other types of CAPTCHAs then feel free to share.



Friend Recognition

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