An external device transfers a key to an implantable medical device over a proximity communication and then establishes a first far field communication session with the implantable medical device where the key is used for the first communication session. This first communication session may occur before implantation while the implantable medical device is positioned outside of the sterile field so that using a proximity communication is easily achieved. Once the implantable medical device is passed into the sterile field for implantation, the external device may then establish a second far field communication session with the implantable medical device where the last key that was used for the first communication session is again used for the second communication session which avoids the need for another proximity communication to occur within the sterile field.