An authentication system comprising: a bus; a transmitter connected to the bus; and a receiver connected to the bus; and wherein: a physical layer signal is applied by the transmitter to a message on the bus for authenticating the transmitter; the physical layer signal incorporates an identifier of the transmitter; the receiver receives the message and decodes the physical layer signal on the message; a decoding of the physical layer signal on the message reveals the ID of the transmitter sending the message; the receiver looks up the ID on a list of IDs corresponding to transmitters approved to send the message, to determine whether the ID of the transmitter sending the message matches an ID on the list; and if the ID of the transmitter matches an ID on the list, then the transmitter is authenticated and authorized to send the message; and wherein: the bus is a controller area network; the message is a CAN message; the CAN message has a dominant portion and a recessive portion; and the physical layer signal is applied to a dominant portion, a recessive portion, or both a dominant and a recessive portion of the CAN message. a bus; a transmitter connected to the bus; and a receiver connected to the bus; and wherein: a physical layer signal is applied by the transmitter to a message on the bus for authenticating the transmitter; the physical layer signal incorporates an identifier of the transmitter; the receiver receives the message and decodes the physical layer signal on the message; a decoding of the physical layer signal on the message reveals the ID of the transmitter sending the message; the receiver looks up the ID on a list of IDs corresponding to transmitters approved to send the message, to determine whether the ID of the transmitter sending the message matches an ID on the list; and if the ID of the transmitter matches an ID on the list, then the transmitter is authenticated and authorized to send the message; and wherein: the bus is a controller area network; the message is a CAN message; the CAN message has a dominant portion and a recessive portion; and the physical layer signal is applied to a dominant portion, a recessive portion, or both a dominant and a recessive portion of the CAN message. a physical layer signal is applied by the transmitter to a message on the bus for authenticating the transmitter; the physical layer signal incorporates an identifier of the transmitter; the receiver receives the message and decodes the physical layer signal on the message; a decoding of the physical layer signal on the message reveals the ID of the transmitter sending the message; the receiver looks up the ID on a list of IDs corresponding to transmitters approved to send the message, to determine whether the ID of the transmitter sending the message matches an ID on the list; and if the ID of the transmitter matches an ID on the list, then the transmitter is authenticated and authorized to send the message; and wherein: the bus is a controller area network; the message is a CAN message; the CAN message has a dominant portion and a recessive portion; and the physical layer signal is applied to a dominant portion, a recessive portion, or both a dominant and a recessive portion of the CAN message. the bus is a controller area network; the message is a CAN message; the CAN message has a dominant portion and a recessive portion; and the physical layer signal is applied to a dominant portion, a recessive portion, or both a dominant and a recessive portion of the CAN message.