We introduce Integrity (I) regions, a novel security primitive that enables message authentication in wireless networks without the use of pre-established or pre-certified keys. Integrity regions are based on the verification of entity proximity through time-of-arrival ranging techniques. We demonstrate how I-regions can be efficiently implemented with ultrasonic ranging, in spite of the fact that ultrasound ranging techniques are vulnerable to distance enlargement and reduction attacks. We further discuss how I-regions can be used in key establishment applications in peer-to-peer wireless networks. Categories and Subject Descriptors C.0 [Computer-Communication Networks]: [Security and protection] General Terms Security, Wireless Networks Keywords Authentication, Key Establishment, Distance Bounding