Many times you have tried to register on many sites and you come across a thing which asks you to type in certain words and letters, which may be alphanumeric too. They are known as CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart).CAPTCHA often makes you wonder why is it kept there, and even if it is kept there, why is it so easy, to be precise why it is there on the first place when it is going to be so general. It is to ensure that the response is not generated from a computer.

The goal is to create a test that humans can pass but not a computer. The CAPTCHA application provides different CAPTCHAs for different users on the same site. If the image was same for the website, a spam computer won’t take time to decipher the CAPTCHA. The CAPTCHAs are not only different for every user; there are different types of CAPTCHAs also like, visual CAPTCHA, mouse movement CAPTCHA and many more.

Following is how a CAPTCHA works:

• The website having the registration form or download specifications create some random string as random values are often hard to guess and predict. An image is generated where in the random value is inserted in an awkward manner as it is tough for any computer to read such images. Different techniques such as zigzag , twist and turn and many other are used to make the task of reading more difficult, but it is kept within the limits of reading for a human.

• If a person cannot read it, they opt for a different CAPTCHA and even some sites offer the service of an audio CAPTCHA. Then this random value is stored and even displayed to the user to check it later on when the person enters it. Then when the user enters the CAPTCHA, the data from the stored CAPTCHA and the data entered by user are submitted as a form. They are called CAPTCHA string and input string from user. Now both the strings are passed on to the next page where they are checked.

• If they are similar its ok, else the user is provided with an error stating “THE INPUT DIN’T MATCH, PLEASE ENTER AGAIN”. Again the same process starts and the user is again provided a new CAPTCHA. Always the CAPTCHA is generated and sent back to requesting user, but only in the form of an image. And if the user is a human they’d never have enough of a trouble inputting the required text though it is not in a clear manner.

But if it’s a bot it might face difficulties guessing what’s in the image. This CAPTCHA technology has made our lives secure by reducing the number of spam computers worldwide. They are deployed to protect systems vulnerable to e-mail spam, such as the webmail services of Gmail, Hotmail, and Yahoo! Mail.