# Controller-Pilot Data Link Communication Security
## 1 Introduction  
The aeronautical telecommunications network is utilized to provide air traffic communication (ATC) within the domestic airspace. The standard method of communication between an air traffic controller and a pilot is voice radio, using either very high frequency (VHF) bands for line-of-sight communication or High Frequency (HF) bands for long-distance communication. Controller-pilot data link communications is a two-way data-link system by which controllers can transmit strategic messages to an aircraft as an alternative to voice communications. CPDLC enables controllers to issue ATC clearances (level assignments, lateral deviations/vectoring, speed assignments, etc. ), radio frequency assignments, and various free text requests for information. It also plays an instrumental role in reducing mid-air collision risk, while also decreasing voice traffic on radio frequencies. Voice communications provide a situational awareness that the data link does not, since unlike the voice communications, CPDLC clearances are intended to be seen by only the controller and the pilot. Some studies [1] have discovered there is an urgent need to protect air traffic management (ATM)-related technologies from a wide spectrum of cyberattacks, and the solutions need to be implemented in a timely manner. With the widespread availability of cheap and powerful tools such as software-defined radios (SDRs), the aviation community has lost the considerable technical advantage that protected its communication over the past decades. Anyone possessing relatively cheap radio equipment (e.g., SDRs) can access and monitor data link communications and easily decode these messages. As outlined in [2], hobbyists create websites to share what they are hearing and distribute the data they collect via websites and social networks, creating a complete operating picture including aircraft ownership, aircraft type, flight plan, flight destination, and private information. Aircraft communications addressing and reporting systems (ACARS) in general [3], and CPDLC in particular, have become an easy target for cybersecurity attacks. According to the Civil Air Navigation Service Organization (CANSO) 2020 security strategy [4]: \"The introduction of increased automation and reliance on system-wide exchange of data means that the associated cybersecurity issues are now a key area that requires joint industry solutions\". In this paper we review the technological characteristics and parameters of the CPDLC technology, based on which we construct a threat model and propose a set of possible countermeasures. We emphasize the importance of creating cybersecurity threat model which identifies all possible cyberthreats to ATM information systems, enabling the ATC authorities to be proactive and comprehensive in their approach to cybersecurity. We propose several solutions for the improved CPDLC security such as elliptical curve cryptography (ECC), protected aircraft communications addressing and reporting systems (PACARs) and the Host Identify Protocol (HIP), and discuss their performance and deployability in the view of the current state of the data messaging system. In addition, we consider identity-defined networking ( IDN) as an example genuine security solution, and conclude that implementing such a standard within the whole communication system at the scope of the air traffic control unit or broader implies global system changes and may require several years of standardization and adaptation, and significant resource consumption. The rest of the article is organized as follows. In Section 2 we analyze the related work. We review the technical characteristics of CPDLC and the main principles of this communication technology in Section 3. In Section 4 we present the threat model and analyze possible attacks on data link messaging system. We propose several solutions for CPDLC protection and consider an example genuine security proposal applicable to the whole aeronautical system in Section 5.