An implantable medical device (IMD) configured to communicate with an external device (ED). The ED supports two way RF communications and has a light source. The IMD includes a processor coupled to an optical detector, the processor is configured to verify that light is being received from the ED light source and that the ED is a trusted device, establishing a unidirectional optical channel from the ED to the IMD. An RF transceiver is coupled to the processor, the processor being configured permit two way RF communications with the ED only under a condition that the ED is verified as a trusted device. The processor may be configure to wake up periodically or aperiodically to check for the presence of light from the ED light source. The processor may be configured to detect a multi-bit message from the ED via the unidirectional optical channel. The multi-bit message may include a key.
