The FAA and other Air Navigation Service Providers (ANSPs) plan to share the existing cockpit data radio for NextGen data communication applications. This radio is currently used for supporting airline operations. Sharing this radio, which operates in a relatively open network environment, with mission critical air traffic control communications creates a need to address air-ground security. Most of the data to be shared over air-ground communication is tactical and transient in nature. In addition, secure communication between the controller and the pilot provides situational awareness to all receivers listening on the voice radio channel. In this paper we provide a rationale for securing air-ground communication and explore some of the issues in implementing a secure air-ground communication channel between the controller and the pilot over the shared radio. We recommend a lightweight approach based on Identity Based Encryption (IBE) that meets the security paradigms of confidentiality, integrity, and availability of the system without compromising situational awareness.