CAPTCHA stands for Completely Automated Public Turing Test to tell Computers and Humans Apart. At one glance, it sounds like rocket science; however, in another five minutes time you will agree that you not only knew about it but have also used it a dozen times.

What is it?

Whenever you fill up a form on the web, the last section consists of a box with a hazy background and a string of malformed alphabets and numbers. The instructions above or below indicate that this is a test of your humanness; to test whether the person filling a form is a person and not a computer. This is what is called a CAPTCHA. You are allowed access to a website only if you type and submit those twisted alphabets and numbers and if the typed letters match those in the provided image.

This CAPTCHA technology is also called as a reverse Turing Test. The Turing Test is the brainchild of Alan Turing. The main premise of the Turing test was to determine whether or not computers have the ability to think or appear to think like humans. The reverse Turning Test tries to show that the computer is definitely not human. The term CAPTCHA has been trademarked by Carnegie Mellon University.

Why are CAPTCHAs used?

Entering the CAPTCHA is a criterion if you want to post a comment to a website or blog. It is a kind of security code and was introduced with the intention of reducing the amount of spam by preventing bots from participating in online polls and generating free emails. Spammers, although they exist in very small numbers, their activity can intercept the activity of millions of website users.

You might wonder why the characters in a CAPTCHA are distorted, sometimes stretched, blurred or reduced in size. Also, every image is different each time. It is done intentionally to make it more unreadable to computers. It obscures the image from interpretation. Also, if it is static it would be easy for the spammer to spot the form, decipher the letters and develop an application that would allow even computers to enter the pattern and pass the test.
You will be surprised to know that your computer that does five digit calculations in minutes and stores huge amounts of data has a hard time passing the simple test. We use words and numbers when interacting on the computer, but it cannot understand it. Computers make use of a special coded language. Raising the level of difficulty further the CAPTCHA technology makes use of images of a language and further obscures the image from interpretation. There are audio CAPTCHAs as well.

ReCAPTCHA is the most popular CAPTCHA. A computer cannot verify the contents of a scanned piece of paper. Thus, it uses the help of humans to identify it. Here's how it works. The application selects two words from the digitized image one of which it already recognizes. When you type in first word correct, it recognizes you as human and not a computer and then waits for your response to the next word. The second word is sent to many other users and the correct interpretation is based on general consensus. The word is then added to the verified pool.

The CAPTCHA technology has evolved over the years. Microsoft has recommended ASIRRA (Animal Species Image Recognition for Restricting Access). It requires users to identify photographs of cats and dogs which humans can do easily but computers can't.

Originally posted on Life Forums