## 3 Related Work  
[PERSON] et al. [6] propose an architecture focused on privacy in LPR systems. The central premise of the architecture proposed by the authors is that the user is the one who decides when he/she wants to be monitored, and this rule can be changed only with a court order from the competent authorities. As a solution, the authors propose a smart contract that stores the privacy preferences of the user. Asymmetric cryptography is used with the contract to guarantee the anonymity of the entities present in the architecture. This work is an evolution of the architecture presented by [PERSON] et al. [6], by assessing the security level of the smart contract developed, as it is shown in the last column (SSC) of Table 1.  
When performing a systematic literature review to identify other works proposing the use of blockchain for LPR systems, the results of this review pointed to the lack of such studies. In this sense, this section gives a brief review of other IoT applications. The summary of the identified works is presented in Table 1.  
From Table 1, it can be seen that no work other than [6] was identified to employ--in the same application--smart contracts, encryption, anonymization, and blockchains. Table 1 summarizes and compares the main features observed in the literature review. The SC column identifies whether the solution proposed uses smart contracts to provide privacy. As we can see, this approach is employed by around 50% of the authors. These works use smart contracts to store the privacy preferences of the users or to provide access control to the information. The E column identifies the works that adopted encryption techniques to improve user privacy. If we do not consider this article and our previous work [6], we observe that few works (only six) employed encryption to manage data privacy with the use of public keys to identify the entities present in each architecture. The next column (A) marks the works that applied anonymization techniques to provide privacy to the users, ensuring that his/her data could not be traced/mapped. As before, few studies (only five) use this technique to anonymize the users' identity, making it impossible to identify them. Following, BC indicates the works that chose to use private and consortium blockchains \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Work** & **SC** & **E** & **A** & **BC** & **SSC** \\ \hline [PERSON] et al. [22] & \(\bullet\) & & & & \(\bullet\) \\ [PERSON] et al. [23] & \(\bullet\) & \(\bullet\) & & & \\ [PERSON] et al. [24] & \(\bullet\) & & & & \\ [PERSON] et al. [25] & \(\bullet\) & & & & \\ [PERSON] et al. [26] & & & & & \(\bullet\) \\ [PERSON] et al. [27] & \(\bullet\) & & & \(\bullet\) & \\ [PERSON] and [PERSON] [28] & \(\bullet\) & & & & \\ [PERSON] et al. [29] & \(\bullet\) & & & & \\ [PERSON] et al. [30] & & & \(\bullet\) & & \(\bullet\) \\ [PERSON] et al. [31] & \(\bullet\) & & & & \(\bullet\) \\ [PERSON] and [PERSON] [32] & & & & & \(\bullet\) \\ [PERSON] et al. [33] & & & \(\bullet\) & & \\ [PERSON] et al. [34] & & & \(\bullet\) & & \(\bullet\) \\ [PERSON] et al. [35] & & & & & \(\bullet\) \\ [PERSON] et al. [36] & & & & & \(\bullet\) \\ [PERSON] et al. [37] & & & & & \\ [PERSON] et al. [38] & & & & & \(\bullet\) \\ [PERSON] et al. [39] & & & & \(\bullet\) & \\ [PERSON] et al. [40] & \(\bullet\) & & & & \\ [PERSON] et al. [41] & \(\bullet\) & & & & \\ [PERSON] et al. [6] & \(\bullet\) & \(\bullet\) & \(\bullet\) & \(\bullet\) & \\ This work & & & & & \(\bullet\) \\ \hline \hline \end{tabular} Note: SC: Uses Smart Contract, E: Applies encryption; A: Employs anonymization; BC: Uses Blockchain; SSC: Evaluates the security of the smart contract.  
\end{table} 
Table 1: Comparison and characterization of related work on privacy in IoT environments. in their architectures. Half of the works used this solution to eliminate computational overhead and ensure privacy so that only specific users could access the data stored in the blockchain.