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Animated captcha’s

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Animated ink-blot images keep unwanted bots at bay
12:50 03 November 2009 by Colin Barras
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Captchas, the scrambled images used to separate humans from software bots online, could become harder for bots to solve – and easier for humans to handle – by animating them.
That is the claim of computer scientist Niloy Mitra at the Indian Institute of Technology Delhi, who along with colleagues has devised a system that should separate the bots from the humans.
With some captcha systems close to being cracked, website owners are having to make them ever more fiendish to thwart bots. That comes at a cost, however: it makes them difficult for humans to read too, says Mitra.
Working with Daniel Cohen-Or and others at Tel Aviv University, Israel, and colleagues from Taiwan, Mitra thinks he has found a way round that problem.
Ink-blot tests
The team’s new system uses so-called “emerging images” – seemingly random assortments of blotches from which a coherent image emerges after a few seconds (see image, above).
To produce the emerging image, they have developed an algorithm that identifies key features within an original image and converts them into an array of ink blots or “splats”. It then removes a number of the splats to make it harder for bots to reconstruct the original shape – while leaving enough information for a human brain to do so.
The number of splats and the noise in the background can be tweaked to make the emerging image easier or harder to spot. Tests with 310 volunteers showed that 98 per cent could recognise over 80 per cent of the emerging images at the easy setting, taking 6.4 seconds on average to do so.
Tough for bots
With the same images, three state-of-the-art software systems managed to identify whether an image contained a horse or a human between 51 and 60 per cent of the time – only slightly better than random guessing.
As the images generated moved towards the “hard” end of the scale, even humans began to struggle. The success rate fell to 74 per cent and users took on average 12.5 seconds to find the hidden image – making them as frustrating as some existing captcha systems.
Pass rates
That could be a problem, says Luis von Ahn at Carnegie Mellon University in Pittsburgh, Pennsylvania, co-creator of the written captchas found on the web today. His ReCaptcha update to the technology was recently bought by Google.
“For ReCaptcha, it takes approximately 10 seconds for users to do the test, and over 96 per cent of all attempts made by humans are correctly answered,” von Ahn says. “It’s hard to beat that.”
Adding a further element could make the emerging image system better yet, say Mitra and Cohen-Or. When they used their algorithm to convert 3D animations into emerging videos (see video above), they found that all volunteers could spot the animated figure, even when the emergence setting was set to very hard. If shown a single frame from the video, under 10 per cent of volunteers could identify the image.
“When we add motion we win on two fronts,” says Mitra. “Recognition becomes much easier for humans and much harder for bots.” He says the team plans to analyse exactly how well the animations perform as a captcha system.
Picture perfect
Greg Mori at Simon Fraser University in Burnaby, British Columbia, Canada, sees Mitra and Cohen-Or’s move towards animation as hugely promising. “Moving towards video-based captchas has a lot going for it,” he says. “In fact, I am involved with a small start-up that is trying to do just that.”
Cohen-Or and Mitra will present their work at the SIGGRAPH Asia conference in Yokohama, Japan, in December.

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