April 20, 2018 03:46 am

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What is CAPTCHA and How it Works?

captcha

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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, CAPTCHA is the word verification test that you will come across 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 a detailed information on how CAPTCHA works and why it is used.

What Purpose does CAPTCHA Exactly Serve?

CAPTCPA 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 which is simple and straight forward 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.

Who Uses CAPTCHA?

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 designed on the fact that, the computers lack the ability that human beings have when it comes to processing visual data. It is more easily possible for humans to look at an image and pick out the patterns than a computer. This is because, computers lack the real intelligence that humans have by default. CAPTCHAs are implemented by presenting users with an image which contains distorted or randomly stretched characters which only humans should be able to identify. Sometimes, characters are stroked out or presented with a noisy background to make it even more harder for computers to figure out the patterns.

Most, but not all, CAPTCHAs rely on a visual test. Some Websites implement a totally different CAPTCHA system to tell humans and computers apart. For example, a user is presented with 4 images in which 3 contains picture of animals and one contain a flower. The user is asked to select only those images which contain animals in them. This Turing test can easily be solved by any human, but almost impossible for a computer.

Breaking the CAPTCHA:

The challenge in breaking the CAPTCHA lies in 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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Comments

  1. Nice post i really wanna know that. Thanks, but i have one request that can you give a post on “How to hack someone’s computer using TELNET”
    please.

  4. Very nice post indeed !!! can you tell me what is a artificial intelligence(AI). Please give a post on it.

    Thank You
    Mars M

  7. Dear Sir,
    As far as I know, Alen Turing is the father of “Turing Test”, am I right? Thanks for your nice post.

    Mars M

  8. hey srikantht..first let me congratulate u fr this wonderful website
    I have a problem..
    I have an unsecured wireless network..and recently i noticed that some one else is using my wireless internet ‘coz i found one more pc in network and sharing centre.. when i tried to ping it was replying..
    but whenever i try to access it,it shows that it has a password.Is there a way by which i can access it without the password.. or a method by whicj i could get the usernames and password lists of the computer…….because i want to know which guy is using my internet

  11. @Shubhendu

    If someone is using you unsecured wireless network then you should implement security. First disable broadcasting you SSID then implement WPA2 with AES and finally secure your network with MAC based authentication.

    Just put in ur MAC in wireless router and only you will be able to use it then no one else.

  12. i hate captha’s cause my internet is very slow and sometimes they wont load even when i refresh the page

  13. oh. . it was great. i don’t know how to say a thank u. anyway you are the genius. 🙂 thank u. VERY MUCH

  19. Hi.
    We have a software that will help you a lot in you captcha entry work! The good thing is that this software will generate the captchas 24 hours every day, whenever you like to work and the captchas are appearing very fast!

  21. wow thanks … i alwys wondered about that.. thought about it..
    thnks posting this article.. can u plz post a article on procedure taken by a hacker while hacking a website or mail.

  22. man you have it all.but please i am a beginner and i really need your help to go about most of your advice.please i am a real novice.thnx

  24. Very informative. I understood the importance after reading this. Thanks for the information.

  25. Theres actually services where you can pay say $5 per 1,000 captchas entered so as far as im aware they just pay people in countries like india a dollar a day to sit and enter captchas lol.

  27. Good stuff.we usually come across these codes. i was having such idea that these codes serve this purpose..but i was not sure..thanks for the information
    —–ravs———

  28. its nice nd useful……….
    9ce job……thank u this info nd I want 2 know why is my mail not opening in time.

  29. hey srikanth….
    i am not genius in computers and related stuffs….
    but why cant we use OCR(OPTICAL CHARACTER RECOGNITION)

  32. very very important post…really i came across many websites but this is the only website i found all usefull posts with easily understandable…..U ROck!!! 😀

  34. sir realy thanks 4 you i got knowlege about captchas and spamming
    otherwise cant

  35. Captcha is the most ignorant thing I have ever seen?! It serves little no purpose other than inconvenience! When you type over and over different captchas that appear on the screen – but aren’t accepted, you are simply blocked from sending your e-mail and/or comment. Ironically some of these are used by government sites for which the captchas aren’t discernible or legible. That simply stops citizens from purchasing licenses or obtaining important government information. What is the point of trying to make it difficult for citizens to obey the law or purchase goods and/or services?!

  36. I appeared to be extremely pleased to search out this web-site.I desired to appreciate it for your precious time for this great learn!! I definitely enjoying each little it and I’ve got you saved as a favorite to see fresh things you text.

  37. Nice Srikanth, very nice. Loved the way you’ve established your site. Had some confusion on captcha thing, but thanks buddy lots of quires are solved 🙂

    Keep up the good work…

  38. Good post. I learn something new and challenging on sites
    I stumbleupon on a daily basis. It’s always helpful to read through articles from other writers and practice something from
    their web sites.

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