  Unmanned Aerial Systems (UAS) have a wide variety of applications, and their
development in terms of capabilities is continuously evolving. Many missions
performed by an Unmanned Aerial Vehicle (UAV) require flying in public
airspace. This requires very high safety standards, similar to those mandatory
in commercial civil aviation. A safe UAV Traffic Management (UTM) requires
several communication links between aircraft, their pilots and UTM systems. The
integrity of these communication links is critical for the safety of
operations. Several security requirements also have to be met on each of these
links. Unfortunately, current cryptographic standards used over the internet
are most often not suitable to UAS due to their limited resources and dynamic
nature. This survey discusses the security required for every communication
link in order to enable a safe traffic management. Research works focusing on
the security of communication links using cryptographic primitives are then
presented and discussed. Authentication protocols developed for UAVs or other
constrained systems are compared and evaluated as solutions for UAS security.
Symmetrical alternatives to the AES algorithm are also presented. Works to
secure current UTM protocols such as ADS-B and RemoteID are discussed. The
analysis reveals a need for the development of a complete secure architecture
able to provide authentication and integrity to external systems (other
aircraft, UTM systems...).
