This habilitation thesis (``Sammelhabilitation’’) collects and summarises original research by the author, primarily in the area of security for spontaneous interaction. Spontaneous interaction is one of the key aspects of ubiquitous computing, and securing such spontaneous interactions between devices that typically communicate over wireless and 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 this specific research area, methodically reviews the current state of the field, puts the collected publications into perspective, and summarises the author’s contributions. Part~II contains reproductions of the twelve publications collected in this thesis.