Security and Usability Issues in Captcha Design

Captcha stands for Completely Automated Public Turing test to tell Computers and Humans Apart. Captcha is a challenge response test which determines whether the user on Internet is human or a spam robot. They are also called as Human interactive proof. Captcha is used to prevent the automated attacks by the computer robots. The Captcha test generates a simple task which can be easily solved by humans and hard for bots to complete the task. Thus Captcha prevents the unauthorized entry of bots into the websites and web services.