Spontaneous interaction is a desirable characteristic associated withmobile and ubiquitous computing. The aim is to enable users to connecttheir personal devices with devices encountered in their environmentin order to take advantage of interaction opportunities in accordancewith their situation. However, it is difficult to secure spontaneousinteraction as this requires authentication of the encountered device,in the absence of any prior knowledge of the device. In this paperwe present a method for establishing and securing spontaneous interactionson the basis of emphspatial references that capture the spatialrelationship of the involved devices. Spatial references are obtainedby accurate sensing of relative device positions, presented to theuser for initiation of interactions, and used in a peer authenticationprotocol that exploits a novel mechanism for message transfer overultrasound to ensures spatial authenticity of the sender.