Secure device pairing is especially difficult for spontaneous interaction in ubiquitous computing environments because of wireless communication, lack of powerful user interfaces, and scalability issues. We demonstrate a method to address this problem for small, mobile devices that does not require explicit user interfaces like displays or key pads. By shaking devices together in one hand for a few seconds, they are securely paired. Device authentication happens implicitly as part of the pairing process without the need for explicit user interaction “just for security”. Our method has been implemented in two variants: first, for high-quality data collection using wired accelerometers; second, using built-in accelerometers in standard Nokia 5500 mobile phones.