## 5 Security and Privacy  
The human physiological parameters collected or transmitted by WBANs are highly sensitive and confidential because these determine the outcomes of clinical diagnoses and belong to the patients. With the development of Internet of Things technology, WBANs contain an increasing number of nodes, resulting in more and more critical data transmission in the networks. Therefore, security and privacy are the utmost considerations for BANs and should be guaranteed carefully to prevent information leakage and tampering. However, security schemes proposed for other networks are not suitable for WBANs due to strict resource constraints, such as energy consumption, communication rate and computing power. Hence, addressing security in WBAN environment is a vital research topic and brings additional challenges to the design of WBANs. The IEEE 802.15.6 standard defined three security levels for WBANs, as shown in Table 11. The specific level of security is determined by the types of data and their levels of the privacy. More precisely, for level 0, the lowest level of security, no algorithm or  
Figure 4: BAN interference. mechanism is used during communication. At level 1, secured authentication is provided during the data transmission. Level 2 requires both authentication and encryption. This standard also included four elliptic-curve-based protocols to guarantee security. However, researchers found that those four protocols are not secure enough in some practical applications and are vulnerable to multiple attacks [110]. Therefore, based on this standard, several security schemes for WBANs have been created to enhance security in recent studies. Table 12 shows a comparison of the recent security techniques in WBANs [111]. WBAN security schemes can be mainly divided into two groups: authentication and encryption. In terms of identity authentication, the WBAN system first needs to authenticate nodes trying to join the network and check whether they have the right to access the network to prevent illegal users from intruding. Nowadays, as the application scenarios of WBANs are becoming more and more diverse, authentication schemes need to be improved according to different application requirements. In response to the security requirements of WBANs in cloud-assisted environments, [PERSON] and [PERSON] proposed an identity-based anonymous authentication and key agreement protocol scheme, which was proven to be secure and achieved the required security properties [119]. A three-tier \begin{table}
\begin{tabular}{c c c} \hline \hline
**Security Levels** & **Protection Levels** & **Transmitted Frames** \\ \hline Level 0: lowest security level & Unsecured Communication & Data are transmitted in unsecured frames without encryption and authentication. \\ \hline Level 1: medium security level & Authentication but no Encryption & Data are transmitted in plaintext form but secured authentication are involved. \\ \hline Level 2: highest security level & Authentication and Encryption & Data are transmitted in secured authentication and encryption. \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Three security levels in the IEEE 802.15.6 standard for WBANs. \begin{table}
\begin{tabular}{c c c c} \hline \hline  
security approach was presented that uses lightweight cryptography to address security in a three-layered WBAN system [122]. In order to improve the adaptability to human motion and the integrity of data transmission, a multi-hop WBAN authentication method based on a lightweight physical unclonable function was proposed [123]. [PERSON] et al. [124] performed a certifiateless signature scheme to construct two efficient remote anonymous authentication (AA) schemes for WBANs. [PERSON] et al. [125] proposed a new AA scheme to avoid imitative attacks on this basis of [PERSON] et al. 's scheme. [PERSON] et al. [126] designed an improved AES algorithm for identity authentication in BAN by using one-dimensional cellular automata to replace the traditional look up table (LUT)-based S-Box method.