## 4 Secure, and Private Data Filtering, and Aggregation
### Data Filtering and Aggregation
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \(n\) & **10** & **20** & **30** & **40** & **50** & **60** & **70** & **80** & **90** & **100** \\ \hline Layered object (bytes) & 840 & 1680 & 2520 & 3360 & 4200 & 5040 & 5880 & 6720 & 7560 & 8400 \\ Total (bytes) & 3340 & 4180 & 5020 & 5860 & 6700 & 7540 & 8380 & 9220 & 10,060 & 10,900 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Size of the layered object at the selected OM path lengths \(n\). The row total gives the OM total size, including the payload of 2.5 kB. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Operation**} & **ECC** & **ECC** & **ChaCha20** & **ChaCha20** & **Data** \\  & **Decryption** & **Decryption** & **Encryption** & **Decryption** & **Processing** \\ \hline Data & 1 B & 1 kB & 2.5 kB & 2.5 kB & 15 kB \\ Execution time & 18.4 ms & 18.9 ms & 1.2 ms & 1.1 ms & 9.8 ms \\ \hline \hline \end{tabular}
\end{table} 
Table 2: OM execution broken in individual operations; the operation execution time was measured on the ESP32-DevKitC V4. Cryptographic operations were carried out using the Libsodium library [57]. The public-key cipher is ECC based using Curve25519 [58] and the symmetric key cipher is ChaCha20. ### Blockchain

As described in Section 5, the privacy-preserving framework relies on a PoA Ethereum blockchain maintained by a selected group of validator nodes operating in server farm-like settings. Therefore, the smart contracts responsible for RBAC and OM creation are executed on high-performance machines. Several studies [44, 59] provide evidence that RBAC could function in similar settings, and reported results show that RBAC operations require low resource consumption. On the other hand, OM creation requires several public-key cryptography operations. We measured the time to create an OM of 100 encryption layers using Curve25519 [58] on a standard laptop (CPU: Intel i5, RAM: 16 GB). The OM construction took 19 ms of CPU time. However, the time required to execute the mentioned contracts is negligible in the assessment of the framework response time since the blockchain state is propagated only at new block creation. Therefore, the nodes of the floor system running the light client can detect a new OM only after a new block is added to the blockchain. The PoA Ethereum block period is usually in the range from 2 to 15 s [60]. ### Discussion

We provided the validation of the WSN framework for privacy-aware indoor location by assessing its response time. The reported results show that applying the PoA Ethereum on the floor system does not introduce significant latency in response times. Nonetheless, it binds the detection of new OMs and the result transaction to a discrete basis imposed by the block period. Moreover, the results show the applicability of the General Purpose Data and Query Privacy Preserving Protocol [13] to the indoor location floor system. The data in Figure 8 and Table 3 show that in the extreme scenario of OM path length \(n=100\), the OM Round-Trip-Time is generally less than 30 s. However, in practical implementations, the system will rather rely on multiple smaller OMs executed in parallel than one large OM. Therefore, the framework response time is reduced to approximately 10 s + two block periods if parallel OM execution is applied at \(n=50\). ## 7 Conclusions and Future Work

In this paper, we present a system for privacy-preserving, non-intrusive, and secure indoor location monitoring. We specifically design the system to not allow identification through data filtering. We present an innovative way of passively approximating location by measuring the force applied to the floor. We are able to distinguish objects from persons by observing the activity at the local level. The sensitized floor forms a WSN that is secure

Figure 8: Required time for an OM to travel the selected path length. Measurements do not include the OM processing delay. Statistics are computed for 30 OMs at each OM path length. from both external and internal adversaries.