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...).