Encryption for channel modulation techniques on a physical basis is considered. A channel modulation scheme is proposed in which the selected channel depends on both transmitter and receiver configuration. The planned selections of the receiver become the encryption key. The communication system is prototyped in an anechoic chamber, where channels are distinguished based on phase differences. It is explored that, as the data is transmitted solely through the selection of the channel, the receiver could transmit the pilot signals and the transmitter could remain passive, i.e. the transmitter does not transmit electromagnetic waves.