Ultrasound has been proposed as out-of-band channel for authenticationof peer devices in wireless ad hoc networks. Ultrasound can implicitlycontribute to secure communication based on inherent limitationsin signal propagation, and can additionally be used explicitly bypeers to measure and verify their relative positions. In this paperwe analyse potential attacks on an ultrasonic communication channeland peer-to-peer ultrasonic sensing, and investigate how potentialattacks translate to application-level threats for peers seekingto establish a secure wireless link. Based on our analysis we proposea novel method for authentic communication of short messages overan ultrasonic channel.