# Security Establishment in ADS-B by Format-Preserving Encryption and Blockchain Schemes
## 2 Literature Review
In practice, the sender needs to have prior knowledge of the ground stations or other airplanes, and data can be sent to only one receiver at a time, which is not appropriate. The Message Authentication Code (MAC) encryption technique was proposed by [PERSON] et al. [18] to protect ADS-B messages. In MAC, all of the nodes perform authentication by appending the authentication code to the plain ADS-B data to produce an authentication identity for every node; irrespective of the verification result, all nodes can receive the data. Therefore, this approach maintains the openness feature of the ADS-B system. Taking into consideration that the ADS-B ciphertext with public key encryption has a 1024-bit length, and Universal Access Transceiver (UAT) data has only a 272-bit length, [PERSON] et al. [18] conclude that encryption of the data by applying the public key is a viable alternative. After investigating the possibility of encrypting the ADS-B data, [PERSON] et al. [19] also suggested the same encryption scheme, but both proposals only provided a high-level encryption structure; no detailed encryption schemes were offered. In 2018, LASSENA's researchers presented an approach to secured ADS-B by utilizing the combination of phase shift keying (PSK) modulation with the Pulse Position Modulation (PPM) of legacy ADS-B [20]. The aim was to increase the payload efficiency of the standard ADS-B. PSK modulation provides extra payloads to address the digital signature data requirement. Although the system keeps the openness feature of standard ADS-B, this scheme's PSK modulation is less robust than PPM modulation, and so the system may be more vulnerable under high-traffic. In the article [9], machine learning algorithms such as Support Vector Machine (SVM) are proposed in order to detect three types of message injection attack: path modification, ghost aircraft injection, and velocity drift. The OpenSky network dataset is used to train and validate the proposed model performance [21]. According to the results, the model accuracy for detecting attacks is over 95%. Another new study is that referred to in [22], which had the objective of protecting against message injection attacks. The technique is based on physical layer signal analysis, which involves estimating the range and direction of the signal source. A multi-channel receiver on ADS-B is part of the proposed system architecture, along with a directional slotted antenna system. The results are presented as the probability of detection versus the signal's direction of arrival (DoA). In [10], a supervised deep learning approach was used to identify injection attacks that changed the ADS-B messages' positioning and velocity elements. The OpenSky dataset is used to train and evaluate the Long Short-Term model for prediction of any error in flight path data. However, a simulation result shows about 99 percent of accuracy; this approach can be used to detect message injection threats. Recently, a holistic framework was proposed to overcome the security concerns of ADS-B [11]. The solution used a cryptographic module based on a keyed-Hash Message Authentication Code (HMAC) to protect data integrity and authenticity. In addition, the cryptographic keys exchange mechanism is defined to share the key to the ADS-B nodes. However, the proposed model tries to address security issues, and it imposes an extra load on the ADS-B traffic, which causes capacity degradation. To address the limited payload capacity (112 bits) of the legacy ADS-B, and because ciphering messages that use a conventional encryption algorithm lead to longer cipher texts, [PERSON] et al. [12] proposed a format-preserving encryption (FPE) method to encrypt ADS-B data and to reduce ciphertext length. Fixed-length messages and confidential fixed-length data such as credit card numbers and social insurance numbers do not fit into the standard block size (64 and 128 bits), and so the FPE method is primarily deployed to encrypt data without changing the data format [23].