Trust architecture and reputation evaluation for internet of things

Internet of Things (IoT) represents a fundamental infrastructure and set of techniques that support innovative services in various application domains. Trust management plays an important role in enabling the reliable data collection and mining, context-awareness, and enhanced user security in the IoT. The main tasks of trust management include trust architecture design and reputation evaluation. However, existing trust architectures and reputation evaluation solutions cannot be directly applied to the IoT, due to the large number of physical entities, the limited computation ability of physical entities, and the highly dynamic nature of the network. In comparison, it generally requires a general and flexible architecture to manage trust in such a dynamic environment as IoT. In this paper, we present IoTrust, a trust architecture that integrates Soft Defined Network (SDN) in IoT, and a cross-layer authorization protocol based on IoTrust. IoTrust and the protocol together provide a new insight for research on trust management in the IoT. For trust establishment, we further propose a Behavior-based Reputation Evaluation Scheme for the Node (BES) and an Organization Reputation Evaluation Scheme (ORES). Both our theoretical analysis and simulation results validate the efficiency of BES and ORES.


Introduction
Internet of Things(IoT) creates a world where physical entities are seamlessly integrated into information networks to provide advanced and intelligent services for human beings (Alrawais et al. 2017;Guo et al. 2017;Dabbagh and Rayes 2017).The IoT entities generally include sensor nodes, RFID tags and wireless communicating devices (e.g.readers, mobile phones) connected to the Internet in a smart environment (Memos et al. 2017;Sedjelmaci et al. 2017).The proliferation of IoT greatly empowers people to control their lives.Generally, a tag is attached to an object and can only communicate with a reader nearby.Until now, a huge number of readers deployed by business or government organizations to provide service for commercial or public use (Yan et al. 2014).Since physical entities including readers, tags and various application servers in IoT need to collaborate with each other, it is important for them to identify the trustworthy partners.
Despite the wide use of trust protocols for P2P (Chen et al. 2014;Cho et al. 2012) and ad hoc sensor networks (Ganeriwal et al. 2008;Jiang et al. 2015), there is little work on trust management for IoT (Sicari et al. 2015).Reputation is a concept closely related to trust relations and has been widely used in many knowledge domains ranging from social sciences to digital sciences.In fact, reputation is often seen as one measure by which trust or distrust can be built based on good or bad past experiences and observations based on collected referral information.In recent years, the concept of reputation has proven useful in many areas of research in computer science, particularly in the context of distributed and collaborative systems where trust and security issues are critical.We summarize the main challenges of trust management for IoT as follows.First, traditional trust management solutions cannot be simply and directly applied to the IoT due to the different standards, communication stacks, and weak computation ability of entities (Shen et al. 2018;Li et al. 2018;Gao et al. 2018;Liu et al. 2017).Second, most works about trust in IoT systems are designed for specific applications and 1 3 therefore cannot be applied to other applications (Chen et al. 2016;Jayasinghe et al. 2016;Nitti et al. 2014).Third, IoT has a huge number of heterogeneous entities with limited storage space and computation resources while existing work does not scale well to accommodate this requirement (Nitti et al. 2014).Fourth, IoT represents a dynamic environment which evolves with new nodes joining and existing nodes leaving.Thus, it requires a flexible infrastructure to allow newly joining nodes to build up trust quickly with a reasonable degree of accuracy.Lastly, they are dependent on specific assumptions such as the availability of feedback and known ownership (Hellaoui et al. 2016;Bernabe et al. 2016).
We aim to design a scalable and general trust management framework for IoT to address the above challenges.To this end, we first present a general trust architecture integrating Soft Defined Network (SDN), an emerging technology that can meet the need of current IoT requirements of heterogeneity and flexibility (Kuang et al. 2016).Then, we present a cross-layer authorization protocol based on the architecture and two reputation evaluation schemes for the node and organization, respectively.In a nutshell, we make the following contributions in this paper: -We present IoTrust, a trust architecture integrating SDN for IoT, which consists of the object layer, the node layer, the SDN control layer, the organization layer, and the reputation management layer.Since the SDN control layer decouples the control functionality from the data routing and processing, IoTrust facilitates optimization and configuration of a network in an efficient and automated manner and provides interoperability among heterogeneous IoT network.The general and flexible infrastructure can be applied to various types of applications to solve the scalability issues in a dynamic IOT environment.-We present a cross-layer authorization protocol based on IoTrust.Specifically, only the reader authorized by the tag related organization can access to the tag.Moreover, the reader's operations on the tag will be recorded for the node's reputation evaluation.-We present a Behavior-based Reputation Evaluation Scheme for the Node (BES) and an Organization Reputation Evaluation Scheme (ORES).Based on the node's behavior, BES decides the node's state by which node's reputation is evaluated.Then, ORES evaluates the organization's reputation based on all its nodes' current states.
The theoretical analysis supports our simulation results, indicating the efficiency of BES and ORES.
The rest paper is organized as follows.Section 2 introduces typical trust management work in IoT.We then present the soft-defined trust architecture with a cross-layer authorization protocol in Sect.3. Section 4 proposes reputation evaluation schemes for node and organization in details followed by the performance evaluation.The simulation is given in Sect. 5. Finally, we conclude the paper in Sect.6.

Related work
There are two approaches for trust in computer networks: the first based on policies.For example, SPINS (Perrig et al. 2002), develops a trusted network.The second approach is based on reputation (Chen et al. 2014;Cho et al. 2012), which is defined as a probability that an agent is trustworthy.
In recent years, the concept of reputation has shown itself to be useful in many areas of research in computer science particularly in the wireless body area networks, cloud computing (Wu et al. 2016(Wu et al. , 2014;;Huang et al. 2017), and social networks (Yuan et al. 2017;Chen et al. 2017;Shen et al. 2015), where interesting issues of trust and security manifest themselves.
Despite active research efforts in the related topics (Ganeriwal et al. 2008;Jiang et al. 2015), few researchers focus on the trust management in the context of the IoT.In fact, reputation is often seen as one measure by which trust or distrust can be built based on good or bad past experiences and observations (Chen et al. 2016;Raya et al. 2008) or based on collected referral information (Bernabe et al. 2016;Hellaoui et al. 2016;Jayasinghe et al. 2016).Chen et al. 2016 proposed a trust management framework for service oriented architecture (SOA) based IoT systems.Trust is based on entities' previous interactions and experiences.It uses distributed collaborative filtering to select trust recommendations.It dynamically adjusts the protocol's parameters for different environments.Also, it considers four types of malicious attacks.Jayasinghe et al. (2016) propose a novel trust computational model based on three trust metrics (TMs); Knowledge, Recommendations, and Reputations for Social Internet of Things.However, both (Chen et al. 2016) and (Jayasinghe et al. 2016) are designed for specific applications, e.g.service provision.
Jorge (Bernabe et al. 2016) focused on the trust control technologies.Hellaoui (Hellaoui et al. 2016) presented an efficient adaptive security model for the IoT.It allows evaluating the trust related to the presence of security threats and performing, consequently, adaptive security decisions.Both (Hellaoui et al. 2016) and (Bernabe et al. 2016) are dependent on specific assumptions, such as feedback must be available, ownership is known, and so on.Raya et al. (2008) evaluates data reports with corresponding trust levels using Bayesian inference.However, Bayesian inference depends on prior knowledge about events which may be unavailable.
Other related work either does not scale well or falls unsuitable for dynamic Soft-defined IoT network (Nitti et al. 2014).Thus, trust management for Soft-defined IoT remains an open issue.

Trust architecture with a cross-layer authorization protocol
We first present IoTrust, a trust architecture integrating SDN for IoT according to different functions of entities.Then, a cross-layer authorization protocol is proposed based on IoTrust.IoTrust with the cross-layer authorization protocol provides new insights for research on trust-based interaction in IoT.This is because our soft-defined trust architecture enables trust relationship establishment among highly dynamic entities managed by different organizations.

IoTrust
There are five types of entities: tag-attached objects or tags, nodes, controllers, organizations and the RMC (Reputation Management Center).According to the functions of different entities, IoTrust divides the IoT into five layers including the object layer, the node layer, the SDN control layer, the organization layer and the reputation management layer.Figure 1 shows the bottom object layer consists of a large number of moving tag-attached objects.Before joining the IoT, each tag or object must choose and then register with an organization.This layer is the data source.Above the object layer is the node layer which consists of different kind of nodes such as readers, sensors and so on.This layer manages data collection from the object layer.Specifically, nodes retrieve data from nearby tags and then return the required results to the organization layer.The SDN control layer lies between the node layer and the organization layer.This layer can program the bottom node network to react differently depending on the nature of the node, its potential for maliciousness, and the resources it requires.Moreover, this layer is in charge of predicting network traffic and implement mobility management in the IoTrust architecture.Specifically, controllers in this layer should be carefully deployed and designed according to their different functions such as flow scheduling, mobility management and so on.The organization layer is composed of different commercial or government organizations.Each organization deploys a certain number of nodes to perform operations on tags such as data retrieval.Since IoT is spread across a large area which can not be covered by nodes from one organization, it is necessary for different organizations and nodes to cooperate with each other.However, a malicious node or an organization among good ones can launch different attacks after the node gains access to the tag, thereby severely damage the network.In order to identify the good nodes and organizations from malicious ones, reputation is used to measure how good the node or organization is.IoTrust thus evaluates the reputation of each node and organization by the reputation evaluation schemes introduced in Sect. 4 by RMC on the top reputation management layer.The tag related organization will grant the authorization to the good node which receives an operation request from good organization.The authorization protocol will be introduced in the following subsection.

Cross-layer authorization protocol
The cross-layer authorization protocol is used to authorize the good node to access to the tag.The node can interact with the tag directly if it stays within the tag's communication range.Note that only the node authorized by the tag related organization can be trusted by the tag.Therefore, before accessing to the tag, the node must obtain the authorization from the tag related organization.The tag related organization decides whether or not to authorize the node's access according to the node's reputation and the user registered organization's reputation.The reputation evaluation for nodes and organizations will be introduced in Sect. 4. The main idea of the cross-layer authorization protocol is shown in Fig. 2. Specifically, the cross-layer authorization protocol performs the following nine steps.-Once authorized, R can perform either a good or a malicious operation.For safety consideration, the attacked nodes that often perform malicious operations should be prevented from accessing to tags.However, it is difficult for a tag to defend against the attacked node.On the one hand, a tag usually has limited energy.On the other hand, a malicious operation may be performed by either an attacked node or (occasionally) an unattacked node in case of a temporary breakdown as it is impractical for a tag with limited computation capability to identify an attacked node from a node in temporary breakdown.Therefore, we choose the RMC with powerful energy and computational ability to identify the attacked node according to its reputation.Each node's reputation is determined by its previous behavior such as its operations on different tags.

Reputation evaluation schemes
For safety consideration, attacked nodes must be identified and then prevented from accessing to the tag.Different from unattacked nodes, attacked nodes usually perform a malicious behavior.So, we identify an attacked node according to its behavior which is measured by its reputation.We then propose a Behavior-based reputation Evaluation Scheme (BES) for nodes in Sect.4.1.Furthermore, an Organization Reputation Evaluation Scheme (ORES) will be introduced in the following Sect.4.2.

Behavior-based reputation evaluation scheme for nodes
Based on the node's behavior, BES decides the node's state by which node's reputation is evaluated (See Fig. 3).Firstly, tag T in the object layer records evidence ED which includes the operations performed by reader R.Then, when interacting with the next node, T will include ED as part of the message which will be sent to O T by the node.After that, O T determines and then submits R's behaviors to RMC.Finally, RMC updates R's reputation by R's state which can be determined by R's behaviors at regular intervals.In all, the node reputation evaluation process includes node behavior verification, node state verification, and node reputation evaluation.Specifically, BES consists the following three steps as described in Algorithm 1.
-Node's behavior determination based on evidence Before performing operations on a tag T, the node R must be authorized by the tag's organization O T .Thus, R sends an authorization request message AUTH_REQ to O T .Once being authorized, the node can have the right to do the operation on the tag.When the operation is completed, the tag will generate an evidence ED which is used to record the operation of the node.Specifically, ED=< ID R , OP, seq, rand > where ID R is the ID of the node.'OP' is the performed operation such as reading, writing or deleting data.'rand' is a random number generated by the tag.'seq' is a sequence number which is initialized to 1 and will be increased by one after each operation.When the tag is requested by the next node R', ED will then be sent to the tag's organization O T by R' as part of AUTH_REQ=< ID T , ID R ′ , OP ′ , ED,  > .Spe-  We then have that the state of R is Attacked according to Table 1.-Node's reputation evaluation After obtaining the state of R, namely P R , RMC will compute R's reputation p R according to Algorithm 1. Specifically, if R is in a good state, P R will be updated to the maximum reputation value p 0 , where p 0 is the initialization reputation value of a node; otherwise, if R is being temporarily breakdown or attacked, P R will be reduced to * p 0 or the lowest value 0. The parameter is an impact factor affecting the reputation of breakdown node, where 0 <  ≤ 1.

Organization reputation evaluation scheme
Before joining in the network, an organization requests for a certificate from RMC and then it can be trusted by other organizations as well as the RMC.Since the number  of organizations is far less than that of nodes or tags, the computation or storage cost for reputation evaluation of an organization won't cause too much burden to RMC.Specifically, the organization's reputation Φ will be cal- culated at regular intervals based on all its nodes' current state.Given that there are N total nodes deployed by the organization.The number of good, attacked and breakdown nodes are N good , N attacked and N breakdown .The reputa- tion of the organization will be computed according to Eq.
(1).Specifically, Φ 0 is the initialization reputation value of an organization.and are reputation decay factors for an attacked and breakdown node respectively, where 0 < ,  ≤ 1 and  < .

Performance analysis
We use the detection accuracy of abnormal (malicious and fault) behavior to measure the performance of BES and ORES.In particular, the evidence ED's protection level including integrity, authenticity, freshness, and nonrepudiation is chosen to measure the detection accuracy of abnormal behavior.This is because each node's behavior is recorded in each evidence, based on which BES and ORES evaluate the reputation of nodes and organizations.Therefore, only when the security of the evidence ED is guaranteed, BES and ORES are effective.The evidence will be verified by the tag's organization as described in the following Algorithm 2.
Specifically, both BES and ORES are capable of thwarting three types of attacks to ensure the integrity, authenticity, freshness, and non-repudiation of ED according to the following analysis. (1) -BES and ORES can defend against the modification attack and protect ED's integrity and authenticity.This is because the organization O T can discover ED's modifica- tion by checking = E k (H(ED)) according to Sect. 4.1.-BES and ORES can defend against the replay attack and thus ED's freshness is preserved.This is because the replayed ED can be verified if the 'rand' which is a unique identifier is a previously used one.-BES and ORES can defend against the message dropping attack and ensure the non-repudiation of ED.The organization O T can verify that ED is discarded if seq ≠ T.seq +1, where 'seq' and 'T.seq' are sequence numbers included in two consecutive evidences received from the same tag.

Simulation results
In this section, we apply our trust architecture and reputation schemes to the IoT system covering over 1000 × 800 m 2 .The system includes one RMC, 3 organizations, 30 tags and a large number of nodes.In order to be tracked by authorized users, each tag has to register at one organization.All tags are moving at a speed of 3 m/s in the network.Each organization deploys its own nodes in areas where it requests data frequently.The available communication distance between a node and a tag is no more than 30 m.The maximum communication distance between two nodes is 150 m.Both the reputation of an organization and a node are initialized to 1. Figures 4 and 5 show the effect of different node densities on the organization reputation evaluated by ORES and Bayes-based method (Raya et al. 2008).Given that the node density is the number of nodes which are able to listen to the communication between a tag and a node on average.
We can observe from Fig. 4 how the organization reputation changes over time when the node density is low.Each organization deploys and manages 30 nodes and thus the node density is = 0.32.Fifty percent of nodes has been attacked.Both ORES and Bayes-based method evaluate the organization reputation by the number of attacked nodes being detected.Then we have that the more accurate the detection result is, the more accurate the organization reputation is.It is observed that the organization reputation for ORES decreases over time as the number of detected attacked nodes grows.This is because ORES is able to detect the attacked nodes successfully by their malicious behaviors recorded in evidence.That evidence will be successfully transmitted to the tag related organization.We can also observe that after a period of time which is 120 s in Fig. 4, the organization reputation for ORES becomes stable.After 120 s, all attacked nodes (45 nodes) have been detected.Different from ORES, the organization reputation for the Bayes-based method keeps unchanged since attacked nodes cannot be detected.In the Bayesbased method, the communication between a node and a tag can hardly be monitored by another node if nodes are sparsely deployed.So the node's behavior (whether good or malicious) during the communication process cannot be observed by any other node.It is obvious that ORES outperforms the Bayes-based method with a low node density deployment.
Figure 5 shows how the organization reputation changes over time when the node density is high.Each organization deploys 100 nodes, 80% of which is attacked.Other parameters' setting is the same as Fig. 4. It can be seen that the organization reputation decreases over time for ORES and Bayes-based method overall.Take note that the organization reputation for Bayes-based fluctuates greatly.This is because ORES can detect each malicious behavior while Bayes-based method cannot.Specifically, for the Bayesbased method, the communication process between a node and a tag will be missed, if no other node within the communication range of the node and the tag.Thus, ORES can detect attacked nodes with a higher probability than the Bayes-based method even in a high node density network.In all, ORES can detect attacked nodes with a higher probability whether in a low or high node density network.
We can observe from Fig. 6 how the organization reputation changes with different , where is the percentage of attacked nodes.Each organization deploys 100 nodes, each of which is attacked or breakdown.Figure 6 illustrates that the organization reputation decreases over time since more attacked nodes and temporary breakdown nodes have been detected.We can further observe that as decreases, the organization reputation decreases obviously.The results show clearly that the number of attacked nodes has a more significant impact on the organization reputation than that of temporary breakdown nodes.This is because temporary breakdown nodes may recover and can then return to a normal state while attacked nodes cannot.Thus, the more attacked nodes the organization owns, the lower the organization reputation is.
Figure 7 shows how the moving speed of tags affects the number of detected attacked nodes for BES.In this simulation, we set that 30% of the nodes has been attacked.Each organization deploys 100 readers and 5 tags.It can be seen from Fig. 7 that the number of attacked nodes being detected increases with the growth of the speed of tags.This is because tags can encounter readers frequently and then capture the readers' behavior with a high possibility if tags are moving quickly.

Conclusion
In this paper, we have presented a trust architecture integrating SDN, called IoTrust, with a cross-layer authorization protocol.IoTrust can be applied to various types of applications to solve the scalability issue in an IoT dynamic environment.In addition, we propose two reputation evaluation schemes for node and organization, respectively.Theoretical analysis shows that the proposed reputation evaluation schemes can defend against modification attack, replay attack, and message dropping attack and achieve higher detection accuracy of attacked nodes.Simulation results support our theoretical analysis and validate the efficiency of the proposed reputation evaluation schemes.
Our future work includes extension and further validation of the proposed techniques to address the remaining challenges in the trust management for IoT.For example, we will enhance the proposed model to adapt our reputation architecture and its reputation schemes to other IoT protocols.Another important research direction is the detection of malicious user and organization behaviors.Typically, such a malicious behavior could be the collusion across those entities with the aim of generating fake reputation values for a targeted node.Other promising directions include designing a mechanism for managing reputation for RMC and exploring how variations in the presence ratio of ill-behaved and well-behaved entities would lead to a notion of reputation reflecting the wider system.

1.
The user sends a request to the user related organization O U for the specified operation on a tag-attached object known as the target tag.2. The organization O U sends the request message INFO_REQ = < ID U , ID T , O per > which will then be broadcasted to the node layer, where ID U , ID T and O per stand for the ID of O U , the ID of the target tag T and the requested operation.3. The node R which discovers that the target tag T stays within its communication range is the target node.R will send INFO_REQ to T for accessing request.

Fig. 3
Fig. 3 Three steps for node reputation evaluation

Fig. 4 Fig. 5 Fig. 6
Fig. 4 Organization reputation changes over time under low node density

Table 1
Node state based on its