Secure device pairing is especially diﬃcult for spontaneous interactionin ubiquitous computing environments because of wireless communication,lack of powerful user interfaces, and scalability issues. We demonstratea method to address this problem for small, mobile devices that doesnot require explicit user interfaces like displays or key pads. Byshaking devices together in one hand for a few seconds, they aresecurely paired. Device authentication happens implicitly as partof 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 mobilephones.