Our days computer systems and Web have developed to the point when software tools (such as standalone programs or web scripts) can perform automated job on the web, such as browsing from website to website (spiders) filling and sending web types.
This task was hard several years back, and individuals did not thought much about it at that time. Now, with the increasing amount of SPAM and the advanced script languages, many hackers and spammers are writing and using scripts to automatically fill and submit forms.
Lots of sites (all the significant ones) are using a way to obstruct automatic submissions that is continuously growing in appeal: CAPTCHA.
A CAPTCHA (an initialism for “Completely Automated Public Turing test to tell Computers and Humans Apart”, trademarked by Carnegie Mellon University) is a kind of challenge-response test used in calculating to identify whether or not the user is human.
A common kind of CAPTCHA needs that the user type the letters of a distorted image, sometimes with the addition of an obscured series of letters or digits that appears on the screen. This usually guarantees that only people can enter the appropriate response, because is tough (but possible!) for makers to draw out the letters and digits from the image.
CAPTCHAs are typical used particularly on web kinds, where the user will enter some sort of details, such as an URL, a remark, a post, or to signup for some service, this will avoid automated software from performing actions such as: publishing to blogs or forums, doing a big number of scrap automated submissions, signups.
CAPTCHAs are by meaning totally automated, needing little human maintenance or intervention in administering the test. This has apparent benefits in cost and dependability.
How Captcha Works
The website has to generate a brand-new, distinct captcha image every time the page containing the web type is visited; this image will NOT contain the letters and digits in any part of the html code of the page. A common mistake of captcha images is to consist of the code displayed also in the filename of the image (ex. [http://www.myserver.com/captcha-SAQSK0.gif], where SAQSKO is the code that needs to be entered by the visitor ).
The Captcha images are produced by a script (generally made in PHP) which choses a Random string of letters and digits, a random typeface (from numerous), and some random sound to be shown on the image, and after that produces a new image with these.