“Captchas” sniff out the stealth bots
By Paul
Have you ever noticed that some people in Internet chat rooms seem more like hucksters for some online casino, porn site or get-rich-quick schemes? Usually there is a policy against using the chat room for spamming, imposed by the moderator.

But these spammers aren’t even human. They are “web robots”, automated programs that trawl the Internet gathering email addresses and free accounts from which to send email spam.

To try to counter the web bots, researchers at Carnegie Mellon University are designing software that serves as an online gatekeeper. Unless you can prove that you are human, you can’t get in to the web site.

The method involves a test used to distinguish real people from intelligent computer programs. Researchers term these tests 'captchas' — shorthand for 'completely automated public Turing tests to tell computers and humans apart.'

One test Carnegie Mellon called Gimpy selects a word from an 850-word dictionary and converts it to a mangled image of itself, warping letters and adding distracting ink spots, colors and backgrounds.

To pass the test, a user has to identify the word correctly. It's simple enough for a 5-year-old, but computer programs — even ones that can read — are stumped by the distortions and distractions.

Internet portal Yahoo started using Gimpy last year to weed out bots trying to obtain free e-mail accounts. This month, Microsoft's Hotmail started using another captcha that uses random letters and numbers.

Search engine AltaVista uses one to block bots from automatically submitting sites for listings. And Internet auctioneer eBay uses one for people who wish to sign up for the PayPal payment service.

However, the tests are not foolproof. Gimpy’s dictionary only holds 850 words, and if hackers stole them, a sophisticated web bot could simply guess and guess again until it gets the answer right – almost instantaneously.

Although the tests aren’t perfect, they are another layer of defence, said Peter Norvig, director of search quality at Google and author of a book on Web robots.

'All they have to do is make it so expensive their opponents don't want to do it,' Mr. Norvig said, adding that time is money. 'If you are doing this battle you want to have a lot of tools and frustrate your opponent step-by-step.'

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