CAPTCHA stands for “Completely Automated Public Turing test to tell Computers and Humans Apart”. CAPTCHA is defined as a program that can tell whether its user is a human or a computer. You’ve probably seen them — colorful images with distorted text at the bottom of Web registration forms. CAPTCHA are used by many websites to prevent abuse from “bots,” or automated programs or Scripts usually written to generate spam. No computer program can read distorted text as well as humans can, so bots cannot navigate sites protected by CAPTCHA. About 200 million CAPTCHA are solved by humans around the world every day.
How CAPTCHA works
The working of CAPTCHA is very simple, first the program code generates the random string (combination of numeric and characters). then user has to fill the generated CAPTCHA in text box. and when he submits the form the CAPTCHA validate the user on client side. if the entered CAPTCHA gets matched with the random generated string then the user can proceed to nest step otherwise try again. This is well explained in the given flowchart below:
- CAPTCHA prevents the website or blogs from comment spams. Many bloggers face they receive more than 100 spam comments per day. And these spam comments are controlled with the help of CAPTCHA plugins.
- Most of the Scripts are generated in order to fill up the signup or registration forms available on various websites. So when CAPTCHA is encountered by those automatically generated scripts at the end of the web forms, it not get submitted. In general, free services should be protected with a CAPTCHA in order to prevent abuse by automated programs.
- CAPTCHA also prevents dictionary attacks. These are the type of attacks under which a automatically generated scripts or computer program tries random passwords on login forms in case on unsuccessful login.
- It helps search engines to avoid crawl the spam contents that may be harmful for keeping database safe or sensitive data of being exploited.