A Dog or a Cat? New Tests to Fool Automated Spammers
On the Internet, nobody knows you’re a human — until you fill out a captcha.

Captchas are the puzzles on many Web sites that present a string of distorted letters and numbers. These are supposed to be easy for people to read and retype, but hard for computer software to figure out.

Most major Internet companies use captchas to keep the automated programs of spammers from infiltrating their sites.

There is only one problem. As online mischief makers design better ways to circumvent or defeat captchas, Web companies are responding by making the puzzles more challenging to solve — even for people.

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“You can make a captcha absolutely undefeatable by computers, but at some point, you are turning this from a human reading test into an intelligence test and an acuity test,” said Michael Barrett, the chief information security officer at PayPal, a division of eBay. “We are clearly at the point where captchas have hit diminishing returns.”

If that is true, at least captchas had a good run. Though several researchers devised similar tests early in the decade, credit for inventing the technology usually goes to Carnegie Mellon University, which was asked by Yahoo in 2000 to create a method to prevent rogue programs from invading its chat rooms and e-mail service.

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Yet some of that activity can be ethically murky. Aleksey Kolupaev, 25, works for an Internet company in Kiev, Ukraine, and in his spare time, with his friend Juriy Ogijenko, he develops and sells software that can thwart captchas by analyzing the images and separating the letters and numbers from the background noise. They charge $100 to $5,000 a project, depending on the complexity of the puzzle.

He lives in the former Soviet Union there’s a surprise.

On the bright side, with the mob penetration of those countries, hitmen are cheap and plentiful.

Microsoft researchers have developed an alternative captcha that asks Internet users to view nine images of household pets and then select just the cats or the dogs.

“For software, this is wildly hard,” said John Douceur, a Microsoft researcher. “Computers are tripped up by all the photos at different angles, with variable lighting conditions and backgrounds and the animals in different positions.”

The project, called Asirra (for Animal Species Image Recognition for Restricting Access), uses photographs of animals from Petfinder.com, a site that finds homes for homeless pets and has more than two million images in its database.

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It would be nice if this were to get some of those animals adopted.

Adopt a stray. Mutts and alley cats are just as good pets, and they don’t have the flaws from inbreeding.

He added: “No single defensive technology is forever. If they were, we would all be living in fortified castles with moats.”

Not everyone feels that the traditional captcha is finished. Luis von Ahn, a professor at Carnegie Mellon and a member of the team that invented captchas, recently unveiled an effort to give them new usefulness.

His reCaptcha project (recaptcha.net) seeks to block spam while handling the challenge of digitally scanning old books and making them available in Web search engines.

When character recognition software fails to decipher a word scanned in a book — when the page is yellowed or the letters are smudged, for example — Mr. von Ahn’s project makes it part of a captcha. After the mystery word has been verified by several people, it is fed back into the digital copy of the book.

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That is an insanely good “out of the box” application for this technology.