Multi-touch tablets allow users to interact with computers through intuitive, natural gestures and direct manipulation of digital objects. One advantage of these devices is that they can offer a large, collaborative space where several users can work on a task at the same time. However the lack of privacy in these situations makes standard password-based authentication easily compromised. This work presents a new gesture-based authentication system based on users' unique signature of touch motion. This technique has two key features. First, at each step in authentication the system prompts the user to make a specific gesture selected to maximize the expected long-term information gain. Second, each gesture is integrated using a hierarchical probabilistic model, allowing the system to accept or reject a user after a variable number of gestures. This touch-based approach would allow the user to accurately authenticate without the need to cover their hand or look over their shoulder. This method has been tested using a set of samples collected under real-world conditions in a business office, with a touch tablet that was used on a near daily basis by users familiar with the device. Despite the lack of sophisticated, high-precision equipment, the system is able to achieve high user recognition accuracy with relatively few gestures, demonstrating that human touch patterns have a distinctive signature" that can be used as a powerful biometric measure for user recognition and personalization.