What is CAPTCHA?

CAPTCHA is an acronym for Completely Automated Public Turing Test to Tell Computers and Humans Apart. As the name suggests, it's a kind of test that can reliably determine is the solver human, allowing webmasters to tell bots and humans apart.

Captcha software is a program automatically generating and grading Captcha tests that:

  • Most people (i.e. real human users) can easily pass
  • Current bots (i.e. various computer programs) can't pass

Bots are the Problem and CAPTCHA is the Solution

With the exponential growth of online services and businesses, website security has become a serious concern for webmasters and web application developers.

The Problem: Bots

Many software programs are written and used to impersonate human beings, mimic their surfing patterns, and imitate their online activities for various malicious purposes. These "pretending to be human" programs are referred to as bots (short for "robots"), spambots or spam bots (since they are most commonly used for spamming).

The Solution: BotDetect CAPTCHA

BotDetect Captcha is an efficient out-of-the-box security solution designed to deal with this threat, available as an ASP.NET Captcha Control for .NET web applications, an ASP Captcha Component for Classic ASP forms, a Java Captcha Library for Java forms, or a PHP Captcha Library for PHP forms.

BotDetect CAPTCHA Component v3 Security

Using Captcha images specifically designed to be out of the reach of currently available computer vision and OCR technologies, bots can be prevented from accessing protected website functionality.

While computer programs cannot read the textual code from the image, it is easy to read for most humans. So the answer to the webmaster's question "is the current client submitting this page a real person, and not a bot?" will be the same as the Captcha validation result.

To keep websites accessible to the blind and people with impaired vision – for whom reading the Captcha images could be a problem – BotDetect also provides a sound Captcha alternative, pronouncing the textual code to users who prefer to hear it.

CAPTCHA Applications

  • Prevent automated spam submission: Captcha images ensure that an email, blog comment, forum post, guestbook entry or social networking message is sent by a human, not a bot.
  • Prevent automated registrations: Captcha images block bots that sign up for thousands of email, forum or social networking accounts every minute.
  • Prevent automated friend requests: Captcha images block bots that send thousands of fake friend requests on social networking and instant messaging sites every minute.
  • Secure login pages: Captcha images prevent dictionary attacks and brute force attacks trying to guess login information
  • Block malicious crawlers: Captcha images guarantee that bots trying to leech content or find vulnerabilities to exploit won't access protected website sections.
  • Prevent poll manipulation: Captcha images protect online polls by ensuring only humans can vote and automated voting scripts can't manipulate the results.
  • Prevent resource harvesting: Captcha images protect online games and virtual worlds with economic systems from automated virtual resource ("gold") harvesting.
  • Prevent cheating in contests: Captcha images protect skill-based online games from fraudulent users who employ bots to achieve unfair advantage over other players.

CAPTCHA Accesibility Guidelines


A CAPTCHA (an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart") is a type of challenge-response test used in computing to determine whether or not the user is human.

The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford. The most common type of CAPTCHA was first invented in 1997 by Mark D. Lillibridge, Martin Abadi, Krishna Bharat, and Andrei Z. Broder. This form of CAPTCHA requires that the user type the letters of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, a CAPTCHA is sometimes described as a reverse Turing test.


The old Recaptcha was a not particularly good captcha implementation originally developed by CMU and later acquired by Google.

However, since its (2014/12) NoCaptcha incarnation, Recaptcha seems to became a spyware whose primary purpose might be improved ad targeting.

It might be also that it's primary purpose is doxxing of US residents and spying on everybody else. Only the time will show.

Captcha and Recaptcha should not be confused under any circumstances.