In various examples, a method of establishing a communication session between an external device and an implantable medical device is described. The method includes generating at the external device a first private key and a first public key. A start session order is sent over a long-range communication channel. Evidence of physical proximity is sent from the external device to the implantable medical device over a short-range communication channel. A second private key and a second public key are generated at the implantable medical device. A first shared key is generated by the implantable medical device using the first public key and the second private key. A second shared key is generated by the external device using the second public key and the first private key. The first and second shared keys are used to encrypt and decrypt one or more messages between the external device and the implantable medical device.