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Two Cal engineers stump "Gimpy" bot blocker

An ingenious computer security system designed to stop automated Internet robots from trying to impersonate humans was cracked by two Berkeley computer scientists, in response to an open challenge from the researchers who created it.

gimpy screen image
A picture is worth a thousand words. You need just one word to get past Gimpy, but it must be the right one. While humans can decipher it with ease, computer-generated bot programs have difficulty reading the distorted text.

Known as "Gimpy," the program was originally created by researchers at Carnegie Mellon University to stop computer-automated robots or "bots" from taking online polls, creating new e-mail accounts, signing up for free Web-based mail, and all the other things human beings use their computers for. Bot programs can produce e-mail accounts that are difficult to trace, making them ideal vehicles for proliferating unwanted spam messages to legitimate e-mail users.

Gimpy adds a step in an online registration process asking the user to read a word on the screen that has been distorted by a fuzzy background. Most people have no trouble with this, while computer programs based on optical character recognition can’t pass the test.

Yahoo, one of the largest providers of free Web-based e-mail, implemented Gimpy last year in its new account registration process. Users who pass the test can proceed to the next step to get an account, but those who cannot read the word are blocked.

"We were able to crack Gimpy because of our previous research on a technique called ‘shape contexts’ for object recognition," says Jitendra Malik, the Arthur J. Chick professor of EECS. "The idea is to match shapes based on relative configuration of contours in a way that can tolerate small distortions."

It took Malik and computer science doctoral student Greg Mori just five days to create a program that empowered their computer to read the Gimpy text. They then called Manuel Blum, professor of computer science, and his graduate student Luis von Ahn at Carnegie Mellon to announce their result.

"I was delighted when I heard from them," Blum said. "They were the first ones to successfully take up the challenge." Blum taught computer science at Berkeley for 30 years before joining Carnegie Mellon. His project there, of which Gimpy is a part, is called CAPTCHA, or "Completely Automated Public Turing Test to Tell Computers and Humans Apart."

Malik and Mori stumped EZ-Gimpy, the simpler of two versions of the program. They also devised a program to beat a more difficult version, which requires users to identify three words instead of just one, but it works only about a third of the time.

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