Have you ever completed a CAPTCHA? You know, the process of carefully typing in a random string of numbers or letters or a combination or both on a website? From banking to blogging, websites are plagued by spammers. As protection against the onslaught of malware online, many websites feature this handy defense mechanism that was specially designed to defeat spammers.

CAPTCHA is a relatively short acronym for the rather drawn out complete description: Completely Automated Public Turing Test To Tell Computers and Humans Apart. The phrase was created by a few brainiacs from Carnegie Mellon University: Luis von Ahn, Manuel Blum and Nicholas J. Hopper. John Langford of IBM also played a significant role in trying to actually patent the acronym.

A Turing test is a technical check of a computer’s capacity to demonstrate intellect. Test results are measured by whether or not a human being can consistently tell the difference between the computer from another human. So CAPTCHA utilizes the same technology to produce tests that humans can crack, but computers can’t. In reality, a CAPTCHA could technically be called a reverse Turing test because it’s a computer that administers the test to a human.

The tests are generally a distorted set of arbitrary numbers or letters that humans may have no trouble reading, but which computers have lots of trouble seeing them properly. This is because computers rely on character recognition to read information. Wavy or otherwise distorted information is nearly impossible for a computer to decode – at this time. As you read this article, technology is being perfected that will eventually allow computers to read unclear figures. Until that technology is faultless, computers are still stumped by the CAPTCHA, which alienates the characters enough to be unreadable.
How the tests are evaluated is also determined by a real human versus computer infiltrator scale. CAPTCHA uses technology that determines whether or not a real human is “taking the test” (entering the letters and numbers) or whether it’s an automated program of some sort. This actually works much like a spam filter – a sort of virtual sifter for websites that keeps out the riffraff, or in this case, spammers.

Do CAPTCHAs really work? Well... for now. A newer version of this technology, aptly named RECAPTCHA, is being fine tuned to defeat spammers once again. Spammers, as expected, have figured out how to maneuver past current CAPTCHA technology, so fairly soon, RECAPTCHA will be the standard to stump spammers once again.