The Captcha plugins are used to prevent spam submissions on your forms such as registration, contact and login. You basic installation of Joomla includes one Captcha plugin which leverages the ReCaptcha service but you may install other plugins connecting to different Captcha systems.
Note: ReCaptcha is a the trademark of Google Inc. and is an independent product not associated with or endorsed by the Joomla Project. You will need to register and agree to the Terms of Service at Recaptcha.net to use this plugin. Complete instructions are available if you edit the ReCaptcha plugin in the Plugin Manager.
A CAPTCHA is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a person. The process usually involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are assumed to be unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. Thus, it is sometimes described as a reverse Turing test, because it is administered by a machine and targeted at a human, in contrast to the standard Turing test that is typically administered by a human and targeted at a machine. A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen.
The term "CAPTCHA" was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie Mellon University). It is an acronym based on the word "capture" and standing for "Completely Automated Public Turing test to tell Computers and Humans Apart". Carnegie Mellon University attempted to trademark the term, but the trademark application was abandoned on 21 April 2008.
Characteristics: A CAPTCHA is a means of automatically generating challenges which intends to:
- Provide a problem easy enough for all humans to solve.
- Prevent standard automated software from filling out a form
A check box in a form that reads "check this box please" is the simplest (and perhaps least effective) form of a CAPTCHA. CAPTCHAs do not have to rely on difficult problems in artificial intelligence, although they can.
CAPTCHAs are used in attempts to prevent automated software from performing actions which degrade the quality of service of a given system, whether due to abuse or resource expenditure. CAPTCHAs can be deployed to protect systems vulnerable to e-mail spam, such as the webmail services of Gmail, Hotmail, and Yahoo! Mail.
Most interactive sites today are run by databases and become quickly clogged and sluggish when a database table exceeds capabilities the operating server can handle. A website's Google Page Rank can also be reduced by excessive commercial links created by automated posting.
CAPTCHA's are also used to minimize automated posting to blogs, forums and wikis, whether as a result of commercial promotion, or harassment and vandalism. CAPTCHAs also serve an important function in rate limiting. Automated usage of a service might be desirable until such usage is done to excess and to the detriment of human users. In such cases, administrators can use CAPTCHA to enforce automated usage policies based on given thresholds. The article rating systems used by many news web sites are another example of an online facility vulnerable to manipulation by automated software.