# H2O: Secure Interactions in IoT via Behavioral Fingerprinting
## 4 Behavioral Fingerprint for Reliability
### Human-to-Human Interaction  
In this section we describe the second reliability mechanism our H2O network is equipped with. Indeed, in Section 4.2, we detailed how the interaction between two objects can be made more robust against attacks aiming at corrupting or damaging a node. Indeed, our approach provides a way for a node to assess the reliability of a second node it is interacting with. At the same time, if a node requests a service from another one for the first time, it can rely on our approach to know what its neighbors think about that node and assess whether it can be considered reliable or not. By applying a similar reasoning, our approach can be extended also to human-to-human interactions. A sequence diagram showing our solution in this particular case is reported in Figure 5.  
In particular, if a human wants to interact with another human in the H2O network our protocol will perform a sequence of steps useful to assess the reliability of the second human. In particular, as showed in Figure 5, let us hypothesize that [PERSON] wants to interact with [PERSON] to send a personal file. For this task, [PERSON] relies on one of his object, let us say \(n_{c}\), whereas [PERSON]'s object, that has to receive the file, is identified with \(n_{h}\). In this scenario, two cases may happen. The first possibility is that \(n_{h}\) is an object with sufficient computational power (i.e., it belongs to [PERSON]'s \(N_{PPAN}\)) and it is equipped with an algorithm for the computation of biological or behavioral biometrics mechanisms (showed in Section 4.3). In this case, before \(n_{c}\) sends the file to \(n_{h}\), it will request the result of the biometrics authentication mechanism \(n_{h}\) is equipped with. At this point, our scheme provides \(n_{c}\) with a way to assess \(n_{h}\) reliability, too. In order to do so, it will perform the sequence of steps described in Section 4.2 for object-to-object reliability assessment. If \(n_{h}\) is reliable and its response about [PERSON] reliability is positive, the file exchange can safely take place. The second case happens if \(n_{h}\) is a lower power object (i.e., it belongs to [PERSON]'s \(N_{LPAN}\)), hence, it is not equipped with a way to authenticate its owner, [PERSON]. Our approach allows \(n_{h}\) to rely on more powerful objects belonging to [PERSON]'s \(PAN\) for this computation. To do so, an approach based on consensus, similar to the one described in Section 4.2, can be adopted. Specifically, let \(\Delta^{H}(n_{h})\) be the set of objects belonging to the \(PAN\) of \(n_{h}\), which have the capability of computing biological or behavioral biometrics mechanisms to authenticate their owner. Let \(\widehat{\Delta^{H}(n_{h})}\subseteq\Delta^{H}(n_{h})\) be the set of nodes of \(\Delta^{H}(n_{h})\) that can be considered reliable by \(n_{h}\) according to the strategy described in Section 4.2. To authenticate its owner, \(n_{h}\) can relay on any node belonging to \(\widehat{\Delta^{H}(n_{h})}\); moreover, \(n_{c}\) must adopt again the strategy described in Section 4.2 to confirm the reliability of \(n_{h}\)'s answer. Finally, if all the involved nodes are reliable and the response about Hope's reliability is positive, the file exchange can safely take place. Figure 5: The sequence diagram for a human-to-human communication. Algorithm 2 summarizes the steps of our approach for human-to-human reliability assessment. Concerning the computational complexity of this functionality, we can consider two contributions. The former is the cost of the continuous authentication mechanism, say \(auth(h_{i})\), that strictly depends on the specific biometrics approach adopted. For these reason, we generically refer to this cost as \(C_{auth}\) and we do not include it in our analysis. The latter, which is the cost introduced by our approach, can be expressed, once again, in terms of the number of messages exchanged by the objects involved in this task. In particular, the human starting the communication will use one of his object for this task.