Term:CAPTCHA [Completely Automated Public Turing test to tell Computers and Humans Apart]

« Back to Glossary Index

CAPTCHA is an acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart.” It’s trademarked by Carnegie Mellon University.

Technically, a CAPTCHA is any problem or test displayed by a computer that, in theory, is beyond the capabilities of current computational technology to figure out in a reasonable amount of time, and yet something that is relatively easy for an actual human being to understand.

The most common forms of CAPTCHA are visually distorted letters, where the user is instructed to type in the letters displayed. This type of analysis is currently practically impossible for computers to perform.

CAPTCHAs are most frequently used to prevent automated programs from creating new accounts at various services, or to prevent automated programs from sending spam.

CAPTCHA (Wikipedia)

This CAPTCHA (Version 1) of "smwm" obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient.

A CAPTCHA (/kæp.ə/, a contrived 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 (displayed as Version 1.0) was first invented in 1997 by two groups working in parallel. This form of CAPTCHA requires someone to correctly evaluate and enter a sequence of letters or numbers perceptible in a distorted image displayed on their 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.

This user identification procedure has received many criticisms, especially from people with disabilities, but also from other people who feel that their everyday work is slowed down by distorted words that are difficult to read. It takes the average person approximately 10 seconds to solve a typical CAPTCHA.

« Back to Glossary Index