CAPTCHA (Completely Automated Public Turing Test to Tell Computers and Humans Apart)

Contributor(s): Linda Rosencrance

A CAPTCHA is a type of challenge-response system designed to differentiate humans from robotic software programsCAPTCHAs are used as security checks to deter spammers and hackers from using forms on web pages to insert malicious or frivolous code. 

The need for CAPTCHAs began as far back as 1997. At that time, the internet search engine AltaVista was looking for a way to block automated URL submissions to the platform which were skewing the search engine's ranking algorithms. To solve the problem, Andrei Broder, AltaVista's chief scientist, developed an algorithm that randomly generated an image of printed text. Although computers couldn't recognize the image, humans could read the message the image contained and respond appropriately. Broder and his team were issued a patent for the technology in April 2001. 

In 2003, Nicholas Hopper, Manuel Blum, Luis von Ahn of Carnegie Mellon University and John Langford of IBM perfected the algorithm and coined the term CAPTCHA. The name stands for Completely Automated Public Turing Test to Tell Computers and Humans Apart.

Captcha example

How CAPTCHAs work

CAPTCHAs are a kind of Turing test. Quite simply, end users are asked to perform some task that a software bot cannot do. Tests often involve JPEG or GIF images, because while bots can identify the existence of an image by reading source code, they cannot tell what the image depicts.  Because some CAPTCHA images are difficult to interpret, users are usually given the option to request a new test. 


The most common type of CAPTCH is the text CAPTCHA, which requires the user to view a distorted string of alphanumeric characters in an image and enter the characters in an attached form. Text CAPTCHAS are also rendered as MP3 audio recordings to meet the needs of the visually impaired. Just as with images, bots can detect the presence of an audio file, but only a human can listen and know the information the file contains. 

Picture recognition CAPTCHAs, which are also commonly used, ask users to identify a subset of images within a larger set of images. For instance, the user may be given a set of images and asked to click on all the ones that have cars in them.

Other types of CAPTCHAs include:

  • Math CAPTCHAs - require the user to solve a basic math problem, such as adding or subtracting two numbers.
  • 3D Super CAPTCHAs - require the user to identify an image rendered in 3D.
  • I am not a robot CAPTCHA -  requires the user to check a box.
  • Marketing CAPTCHAs - require the user to type a particular word or phrase related to the sponsor's brand. 

Bypassing CAPTCHA

Users who prefer not to solve CAPTCHAs can use any of several browser add-ons that allow users to bypass CAPTCHAs. Popular browser add-ons include AntiCapture, CAPTCHA Be Gone and Rumola. 

The AntiCaptcha automatic CAPTCHA solver plug-in for Chrome and Firefox automatically finds a CAPTCHA on a webpage and solves it for the user. This extension is promoted as being helpful for users with vision impairments, as well as for users who prefer to bypass CAPTCHA codes. As of this writing, the cost of the service starts at $0.70 for 1,000 CAPTCHA images.

The CAPTCHA Be Gone extension detects CAPTCHAs on webpages, solves them and copies the result to a user's clipboard. At this time, the utility is available for Firefox, Chrome and Internet Explorer for a $3.50 per month subscription fee.

The Rumola add-on for Firefox, Chrome and Safari automatically searches for CAPTCHAs on the webpages a user visits. Currently, the cost is either $0.95 for 50 CAPTCHA solutions or $1.95 for 150 solutions. There is also a JavaScript bookmarklet that can be used for devices that connect to the internet.

Because CAPTCHA bypass add-ons are created by third parties, end users should be aware that the browser extension could expose their browsing activity to untrusted sources. Another reason not to use CAPTCHA bypasses is that the performance of the extensions is inconsistent. This is primarily because as bots get smarter, CAPTCHAs are also evolving and it can be difficult for the add-on programs to keep up. 

This was last updated in August 2017

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Are CAPTCHAs getting too complicated for people to use easily?
Get that many robots (bots) applying for jobs or posting comments on articles or videos do you? I think this filtering out program to be stupid. It's like the goverment funding the building of highways that go no where. I can get a job but they have people actually surveying whether or not a person is a human, STOP the INSANTIY.
yes, you are correct.
it is like government funding the building of highways and regulating from the blocking the road. 


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