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You're using your computer to purchase tickets to see a concert at a local venue. Before you can buy the tickets, you first have to pass a test. This sort of test is a CAPTCHA. The term CAPTCHA is an acronym that stands for Completely Automated Public Turing Test to Tell Computers and Humans Apart. It's not a hard test -- in fact, that's the point. For human users, the test should be simple and straightforward. But for a computer, the test should be almost impossible to solve. The goal of CAPTCHAs is to identify malicious bots. Human users should be able to pass with no problems. You've probably seen these tests on lots of websites. The most common form of CAPTCHA is an image of several distorted letters. It's your job to type the correct series of letters into a form. If your letters match the ones in the distorted image, you pass the test!
Another common type is image CAPTCHAs. An image CAPTCHA test involves a series of photos of common scenes. This may include highways, city streets, or parks. The user will be asked to select only the photos that contain certain objects, such as street lights, fire hydrants and bicycles. After selecting the right photos, you'll pass the test. Image recognition captchas are typically harder for bots to decipher than text based captchas. Distorted or blurry images are even better as they can easily frustrate the bot's recognition techniques. Carefully crafting CAPTCHAs that only humans can solve is key. Why would anyone need to create a test that can tell humans and computers apart? It's because of people trying to game the system -- they want to exploit weaknesses in the computers running the site. While these individuals probably make up a minority of all the people on the internet, their actions can affect millions of users and websites. For example, a free e-mail service might find itself bombarded by account requests from an automated program.
That automated program could be part of a larger attempt to send out spam mail to millions of people. The CAPTCHA test helps identify which users are real human beings and which ones are computer programs. One interesting thing about CAPTCHA tests is that the people who design the tests aren't always upset when their tests fail. That's because for a CAPTCHA test to fail, someone has to find a way to teach a computer how to solve the test. In other words, every CAPTCHA failure is really an advance in artificial intelligence. One of the ironies of the CAPTCHA program is that a CAPTCHA application can generate a test that even it can't solve without already knowing the answer. Alan Turing, sometimes called the father of modern computing, proposed the test as a way to examine whether or not machines can think -- or appear to think -- like humans. The classic test is a game of imitation.
In this game, an interrogator asks two participants a series of questions. One of the participants is a machine and the other is a human. The interrogator can't see or hear the participants and has no way of knowing which is which. If the interrogator is unable to figure out which participant is a machine based on the responses, the machine passes the Turing Test. Of course, with a CAPTCHA, the goal is to create a test that humans can pass easily but machines can't. It's also important that the CAPTCHA application is able to present different CAPTCHAs to different users. If a visual CAPTCHA presented a static image that was the same for every user, it wouldn't take long before a spammer spotted the form, deciphered the letters and programmed an application to type in the correct answer automatically. Computers lack the sophistication that human beings have when it comes to processing visual data. We can look at an image and pick out patterns more easily than a computer.
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