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(also human verification)

CAPTCHA definition

CAPTCHA is a security measure used to verify that a user is not a bot (computer program) trying to access the site. The term is an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart.“ CAPTCHAs present a user with a task that a human can easily solve but that would be difficult for a computer program. For example, a person may see a distorted image of letters and numbers and then have to type the correct answer.

See also: spambot

How CAPTCHA works

  1. A user visits a website that requires a CAPTCHA to verify that they are human.
  2. The website presents the user with a challenge (e.g., a distorted image of text or a puzzle to solve).
  3. The user completes the challenge, verifying that they are human and not a bot.
  4. The website verifies the user’s response and allows them to enter the site if the answer is correct. If not, the website asks the user to try again.
  5. If the user fails to complete the CAPTCHA after several attempts, they may be blocked from accessing the site.


  • Image-based CAPTCHA. This type of CAPTCHA shows the user an image with distorted text and asks them to correctly identify it.
  • Audio-based CAPTCHA. Audio-based CAPTCHAs are used when users have difficulty seeing or interpreting image-based CAPTCHAs.
  • Checkbox CAPTCHA. Checkbox CAPTCHAs ask the user to tick a box to demonstrate they are not a robot. They may also be asked some additional questions or puzzles to verify they’re human.
  • reCAPTCHA. Developed by Google, reCAPTCHA uses a combination of image and text recognition. The user may be asked to identify objects in an image or transcribe distorted text.
  • Game-based CAPTCHA. These CAPTCHAs require users to solve a puzzle to access the site.
  • Mathematical CAPTCHA. This type of CAPTCHA gives the user a simple mathematical problem to solve.

Further reading

Ultimate digital security

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