Authenticating spontaneous interactions between devices and users is challenging for several reasons: the wireless (and therefore invisible) nature of device communication, the heterogeneous nature of devices and lack of appropriate user interfaces in mobile devices, and the requirement for unobtrusive user interaction. The most promising approach that has been proposed in literature involves the exploitation of so-called auxiliary channels for authentication to bridge the gap between usability and security. This concept has spawned the independent development of various authentication methods and research prototypes, that, unfortunately, remain hard to compare and interchange and are rarely available to potential application developers. We built a system which implements and unifies these approaches. In this paper, we present OpenUAT, an open source toolkit that implements our novel, unified cryptographic authentication protocol (UACAP), and a comprehensive range of specific auxiliary channels. We evaluated OpenUAT based on a user study in which we compared four authentication methods implemented by the toolkit. The user study showed that users tend to prefer the visual channel in spite of its comparatively poor performance.