A system for authenticating messages transmitted on a bus based on physical location of transmitting units, the system 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 said plurality of messages and said plurality of reflection signals at a second point of said bus, said probe having at least one processor adapted to: calculate a propagation timing between a reception time of said each message and a reception time of an associated one of said plurality of reflection signals transmitted in response to said each message, and determine validity of said each message according to a match between said calculated propagation timing and each of a plurality of predefined propagation timings associated with said plurality of bus connected units; wherein said plurality of bus connected units are statically connected to said bus between said first point and said second point, wherein said calculated propagation timing is indicative of a physical location on said bus of an originating unit of said plurality of bus connected units which transmitted said each message, and wherein said plurality of reflection signals is functionally transparent to said plurality of bus connected units. 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 said plurality of messages and said plurality of reflection signals at a second point of said bus, said probe having at least one processor adapted to: calculate a propagation timing between a reception time of said each message and a reception time of an associated one of said plurality of reflection signals transmitted in response to said each message, and determine validity of said each message according to a match between said calculated propagation timing and each of a plurality of predefined propagation timings associated with said plurality of bus connected units; wherein said plurality of bus connected units are statically connected to said bus between said first point and said second point, wherein said calculated propagation timing is indicative of a physical location on said bus of an originating unit of said plurality of bus connected units which transmitted said each message, and wherein said plurality of reflection signals is functionally transparent to said plurality of bus connected units. calculate a propagation timing between a reception time of said each message and a reception time of an associated one of said plurality of reflection signals transmitted in response to said each message, and determine validity of said each message according to a match between said calculated propagation timing and each of a plurality of predefined propagation timings associated with said plurality of bus connected units; wherein said plurality of bus connected units are statically connected to said bus between said first point and said second point, wherein said calculated propagation timing is indicative of a physical location on said bus of an originating unit of said plurality of bus connected units which transmitted said each message, and wherein said plurality of reflection signals is functionally transparent to said plurality of bus connected units.