New CAPTCHA for smartphones and tablet PC

The most common CAPTCHA technique, Gimpy which requires a user to type in twisted characters has following problems. Characters are over twisted because some spammer uses OCR to bypass verification. Type in operation on mobile and tablet devices is difficult where keyboard are not utilized. Our method uses multiple noise images where invisible objects or message is hidden. The hidden objects appear when two images are overlapped at certain position. Finding the point is a key to pass verification which is difficult for none human intelligence. This enhances usability on tablet devices since a user only has to use one finger to get verified instead of using a keyboard.