A system for authenticating messages transmitted on a bus based on physical location of transmitting units, comprising a reflector adapted to inject a plurality of reflection signals at a first point of a line topology bus, each in response to each of a plurality of messages transmitted by a plurality of bus connected units and a probe adapted to intercept the messages and the reflection signals at a second point of the bus. The probe calculates propagation timing between a reception time of the message and a reception time of an associated reflection signal transmitted in response to the message and determines validity of the message according to a match between the calculated propagation timing and a predefined propagation timings associated with the bus connected units. Wherein the bus connected units are statically connected to the bus between the first point and the second point.