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How CAPTCHA Works


CAPTCHA and Artificial Intelligence
Hackers have found ways to teach computers how to recognize the text in EZ-Gimpy CAPTCHAs.
Hackers have found ways to teach computers how to recognize the text in EZ-Gimpy CAPTCHAs.
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Luis von Ahn of Carnegie Mellon University is one of the inventors of CAPTCHA. In a 2006 lecture, von Ahn talked about the relationship between things like CAPTCHA and the field of artificial intelligence (AI). Because CAPTCHA is a barrier between spammers or hackers and their goal, these people have dedicated time and energy toward breaking CAPTCHAs. Their successes mean that machines are getting more sophisticated. Every time someone figures out how to teach a machine to defeat a CAPTCHA, we move one step closer to artificial intelligence.

As people find new ways to get around CAPTCHA, computer scientists like von Ahn develop CAPTCHAs that address other challenges in the field of AI. A step backward for CAPTCHA is still a step forward for AI -- every defeat is also a victory [source: Human Computation].

But what about Web administrators? They might not find von Ahn's philosophy to be nearly as attractive. From their perspective, they still have to deal with a massive problem -- spammers and hackers. People who maintain Web sites or create online polls need to be aware that several CAPTCHA systems are no longer effective. It's important to do a little research on which CAPTCHA applications are still reliable. And it's equally important to keep up to date on the subject. If one CAPTCHA system fails, the administrator might need to remove the code from his or her site and replace it with another version.

As for CAPTCHA designers, they have to walk a fine line. As computers become more sophisticated, the testing method must also evolve. But if the test evolves to the point where humans can no longer solve a CAPTCHA with a decent success rate, the system as a whole fails. The answer may not involve warping or distorting text -- it might require users to solve a mathematical equation or answer questions about a short story. And as these tests get more complicated, there's a risk of losing user interest. How many people will still want to post a reply to a message board if they must first solve a quadratic equation?

Eventually, we might reach a point where computers and humans perceive puzzles the same way. If that happens, tests like CAPTCHA will become useless lines of code. Until then, we'll just have to squint (or listen) carefully while trying to decipher CAPTCHA codes.

To learn more about Web site security and related topics, test out the links on the next page.