A precautionary term, CAPTCHA is nothing but a fully computerized communal touring test to be implemented during on line form submission process. The purpose behind such a development is to avoid unwanted emails that might be sent intentionally in view of spamming. Simply as a forbidden step initially, CAPTCHA signs up for consequent web site accounts and to be unrecognizable, usually, captcha is made up of a combination of somehow fuzzy and random letters, surrounded by a mixture of background colours.

Most recently, under the concept of modern captcha, to be most effective, not only unrecognizable but also unfeasible captcha images are generated. Due to formulation of entirely crowded cluster of strings, these images become enough efficient to remain impracticable to identify always.

Under the captcha mechanism, an indiscriminate string has been generated illogically by assembling content and information together and the whole mixture has been entrenched into images. Ultimately the mixture as a captcha, stored in the session or cookies of the web pages has been verified by users during online form filling process.

Flow chart for captcha mechanism as follows:

• Creating indiscriminate text
• Encrypting image throughout by written text
• Storing image in a session/cookie
• Demonstrating image during form filling.
• Accepting and substantiating user input after matching.
• Submitting form

Realistic uses of captcha as follows:

• Prohibition of “comment spam” in user blogs.
• Defensive website registration through anticipation of bots, as for spaming purpose only many accounts can be registered using these “bots” only.
• Protection of email addresses from “Scrapers”, scraper is a software bot that searches the Internet for email addresses in plain text.
• Helps in preventing Dictionary Attacks.