# Security Establishment in ADS-B by Format-Preserving Encryption and Blockchain Schemes
## 5 Discussion  
The goal of this research was to address ADS-B threats based on FPE methodology and to design and implement SADS-B by deploying SDR. Blockchain features were utilized to manage key exchanges for encryption purposes within ADS-B nodes. System performance and major key requirements were compared with standard ADS-B. FFX was used here for encryption purposes, as it is a NIST standard algorithm, particularly FF1 and FF3-1. It has already been used for the exchange of sensitive information, including credit card numbers, ID numbers, and social insurance numbers. In addition, criteria for the measurement of the security complexity of FFX algorithms are defined for ADS-B for security level evaluation. For this purpose, the entropy of ADS-B information is computed for both standard ADS-B and SADS-B for various scenarios. The results indicate that the encryption of longer ADS-B data leads to higher entropy and higher security. In addition, after considering other factors such as serial correlation and plain and cipher data correlation, the optimal approach is to encrypt either 13- or 14-byte data sizes. Apart from security concerns, a sensitivity experiment was conducted to assess system performance and to compare the results with standard ADS-B. No changes in system sensitivity were observed; therefore, the FFX-integrated system is capable of detecting poor signals such as that of standard ADS-B. Perhaps the most cited advantages of the FAA's switch to ADS-B surveillance are the quick transmission rate and the related controller scope refresh rate. As mentioned in Figures 3 and 4, FFX adds an extra block of encryption and decryption subfunction to ADS-B In and ADS-B Out, respectively. The encryption increases the time required for message transmission and reception at both ends. Since the message generation time in SADS-B Out is far less than the transmission rate, adding an encryption block does not affect the SADS-B operation. However, the performance of SADS-B In might be challenged when there are many SADS-B signals. The computational time takes longer in SADS-B because ADS-B is based on cascading blocks. Figure 11: (**a**) Standard ADS-B waveform and spectrum, (**b**) SADS-B waveform and spectrum. In other words, each block operation depends on the previous block. This issue could be addressed by applying a threading feature to the FFX algorithm. Blockchain is deployed to provide a secured channel for key exchanges among the ADS-B nodes. Since Hyperledger fabric satisfies the requirements of ADS-B and has been deployed in known cases, it is selected here for key exchange and for sensitive information storage. The key could be scheduled to update periodically. Table 3 offers a comparison of the different types of solutions in terms of their implementation possibility (Achievability), security level, sensitivity degradation, and compatibility. Based on this comparison, FPE of size 14 and 13 bytes qualifies as the most achievable, as it is easy to implement, offers a high level of security without any drawback in sensitivity, and is compatible with standard ADS-B and only needs to be added to the software. Table 4 represents a comparison of the different types of solutions in terms of their security requirements, including data integrity, location integrity, confidentiality, and authentication. Based on the comparison, the paper solution qualifies to achieve all requirements without adding any extra payload to ADS-B data. On the one hand, implementation of the FFX algorithm secured the confidentiality and the integrity of ADS-B messages. Therefore, only the authorized nodes are capable of decoding the encrypted ADS-B data. The transmitted data is encrypted by a standard methodology, and it has high entropy to be decoded, thus attackers are not able to decode the data and manipulate it. The solution guarantees the integrity of the ADS-B data.