This habilitation thesis (``Sammelhabilitation’’) collects and summarisesoriginal research by the author, primarily in the area of securityfor spontaneous interaction. Spontaneous interaction is one of thekey aspects of ubiquitous computing, and securing such spontaneousinteractions between devices that typically communicate over wirelessand therefore invisible channels requires human-verifiable authentication.Sub-topics discussed in this thesis include interaction methods,cryptographic protocols, and sensor data analysis.The thesis consists of two parts: Part~I defines the focus of thisspecific research area, methodically reviews the current state ofthe field, puts the collected publications into perspective, andsummarises the author’s contributions. Part~II contains reproductionsof the twelve publications collected in this thesis.