Computers are poor at correctly orientating images like this one, but were the problem to be built into website security, spammers would soon start working hard to solve it (Image: Tatters / http://www.flickr.com/photos/tgerus/)

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CAPTCHAs – scrambled letters that separate real online users from software bots – protect online services from being overwhelmed by spam by posing problems only humans can solve. But they may also offer a way to coerce spammers into solving some important problems in artificial intelligence.

Spammers have already written software able to match humans at some CAPTCHAs. But when CAPTCHAs finally fail, their co-creator Luis von Ahn at Carnegie Mellon University says there will be reason for celebration as well as concern.


Software that can solve any text-based CAPTCHA will be as much a milestone for artificial intelligence as it will be a problem for online security.

Arms race

In the last couple of years, the arms race between CAPTCHA maintainers and their adversaries has escalated. To escape the bots able to solve some CAPTCHAs they have been made more difficult – at the expense of frustrating legitimate users. Various commentators have claimed CAPTCHAs will soon be obsolete.

“Actually, that’s not true,” von Ahn told New Scientist, but he admits that the spammers are highly motivated and making headway. The biggest target is von Ahn’s reCAPTCHA system, used by over 40,000 websites including Facebook and Craigslist.

He has seen bounties as high as $500,000 offered for software to break it – enough to attract people with the skills to the task and five times more than the Loebner Grand Prize offers to the programmer who designs a computer that can truly pass the Turing test.

Silver lining

The demise of reCAPTCHA could, however, be beneficial.

It has users decode distorted text taken from historic books and newspapers that is beyond the ability of optical character recognition (OCR) software to digitise. Humans who fill in a reCAPTCHA are helping translate those books, and spam software could do the same.

“If [the spammers] are really able to write a programme to read distorted text, great – they have solved an AI problem,” says von Ahn. The criminal underworld has created a kind of X prize for OCR.

That bonus for artificial intelligence will come at no more than a short-term cost for security groups. They can simply switch for an alternative CAPTCHA system – based on images, for example – presenting the eager spamming community with a new AI problem to crack.

Image puzzle

One example will be presented (pdf) by researchers from Google at the International World Wide Web Conference (WWW2009) in Madrid, Spain, next week. It’s a CAPTCHA system that asks users to correctly orientate images randomly spun around (see image, right).

Without a telltale horizon, image orientation is difficult for computers. But if this new CAPTCHA becomes common, it won’t be long before spammers turn their attention to cracking the problem, with potential fringe benefits to cameras and image editing software.

Speech recognition CAPTCHAs are already being used, and image labelling ones could follow, says von Ahn. AI researchers are already working in both these areas, but they could soon be joined by spammers also helping advance the technology.

Perhaps it is time to start designing CAPTCHAs in a different way – pick problems that need solving and make them into targets to be solved by resourceful criminals.

Journal references: Science, DOI: 10.1126/science.1160379

Full Google research WWW2009 research paper (pdf)

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