He says another key problem to using the software for Captcha lies in the test procedure. It's not clear how it would determine whether a user has accurately identified the image. Asking users to describe what they see would be far too complicated. One person might write "dog" to describe the subject, while another might write "doggy," "puppy," or "Dalmatian." There are far too many correct answers. "We cannot do multiple choice, either," Cohen-Or says. "Then the computer could guess."

Other researchers have found ways around this problem. James Wang, an associate professor of information sciences and technology at the Pennsylvania State University worked on a still image Captcha system that asks users to select one image from a collage and then annotate an unrelated image by selecting the correct response from a list. The approach reduces the likelihood of a spambot bluffing the system. "The success rate for a random attack can be controlled to as low as one in 210,312, if these steps are applied twice," says Wang.

Wang admits that the emergence system does have a "cool factor." Users might enjoy finding the hidden animal in a scene. But he says it will take more development and experimentation to create a practical Captcha system.

"Whereas this work is interesting, and the encoding scheme appears to be novel, only time will tell if image and vision scientists can find a way to break it," Wang says. "Besides, in order to make a practical Captcha system with a proven low brute-force attack rate, more development and experimentation will have to be done."

Wang notes that the system would have to incorporate a lot of different animals to work, and he wonders how many could be easily identified by humans. "For example, will I be able to tell a tiger from a leopard when only body silhouettes are shown?"

Luis von Ahn, a professor of computer science at Carnegie Mellon University and one of the people who first started building Captcha systems, agrees that the animal selection might be too limited to make Cohen-Or's approach practical. And while he finds the research interesting, he's not sure emergent images are really more secure than the standard distorted text systems currently used. "Nobody has actually tried to break this really hard," von Ahn says.