A Joined-Up Bot-Fighting Strategy

Continued from page 1

Publishing their results in the latest issue of the journal Pattern Recognition, the researchers show that some of the best OCR programs can recognize the characters less than 1 percent of the time. "Before a computer can try to recognize a character, it first has to locate it," Oommen Thomas says, so having characters joined together should make this process (known as segmentation) more challenging.

However, Yan worries that such handwriting could also be much harder for humans to read. "My main concern is usability," he says. Currently, the system has a human success rate of 75 percent, meaning that one in four times, a human can't read the text. "That's way too low," says Yan.

Luis von Ahn, a computer scientist at Carnegie Mellon University, in Pittsburgh, and a member of the team that first coined the term CAPTCHA, agrees. Von Ahn's latest system, called reCAPTCHA, has a human success rate of 96 percent. "And still people complain," he admits.

Oommen Thomas concedes this but says that his team is looking at ways to improve the success rate. "There is a region where humans and machines both do badly, but there is also a sweet spot where humans do well and machines do badly," he says, and this is what he and his team are now trying to find. "There's a lot of money to be made circumventing CAPTCHAs to generate spam," he adds, meaning that spambots are likely to get better and better at breaking existing CAPTCHAs.

"It's a worthy thing to look at," says von Ahn, but he is not sure that there's a need for a completely new kind of CAPTCHA. Systems like reCAPTCHA (currently one of the most widely used systems: it's running on more than 100,000 websites) are regularly improved to stay ahead of the curve. One trick is to scan in characters from old books, with all their imperfections. "We only use the ones that computers cannot recognize," von Ahn says. Because of this, reCAPTCHA is extremely good at keeping the bots out, he says, with the best known attacks achieving a success rate of no better than one in 1,000.

"Humans are just not that good at recognizing handwriting," von Ahn adds, noting that, as we use handwriting less and less in modern life, our ability to recognize squiggly text may fade further still.