A Survey on Trust Computation in the Internet of Things
Nguyen B. Truong
Media Research Lab.
Gyu Myoung Lee
Internet of Things defines a large number of
diverse entities and services which
interconnect with each other and individually
or cooperatively operate depending on context,
conditions and environments, produce a huge
personal and sensitive data. In this scenario,
the satisfaction of privacy, security and trust
plays a critical role in the success of the
Internet of Things. Trust here can be
considered as a key property to establish
trustworthy and seamless connectivity among
entities and to guarantee secure services and
applications. The aim of this study is to
provide a survey on various trust computation
strategies and identify future trends in the
field. We discuss trust computation methods
under several aspects and provide comparison of
the approaches based on trust features,
performance, advantages, weaknesses and
limitations of each strategy. Finally the
research discuss on the gap of the trust
literature and raise some research directions
in trust computation in the Internet of Things.
With recent advanced technologies toward a
hyper-connected society from the increasing
digital interconnection of humans and objects,
big data processing and analyzing, the Internet
of Things (IoT)-related applications and
services are playing more and more significant
role in the convenience of human daily life.
However various problems occurred due to the
lack of trust which will hinder the development
of IoT. To cope with a large number of complex
IoT applications and services, it is needed to
create a trusted and secured environment in
order for sharing information, creating
knowledge and conducting transactions.
Trust concept is an abstract notion with
different meanings depending on both
participators and scenarios; and influenced by
both measurable and non-measurable factors.
There are various kinds of trust definitions
leading to difficulties in establishing a
common, general notation that holds, regardless
of personal dispositions or differing
situations. Generally, trust is considered as a
computational value depicted by a relationship
between trustor and trustee, described in a
specific context and measured by trust metrics
and evaluated by a mechanism. Previous research
has shown that trust is the interplay among
human, social sciences and computer science,
affected by several subjective factors such as
social status and physical properties; and
objective factors such as competence and
reputation . The competence is measurement
of abilities of the trustee to perform a given
task which is derived from trustee’s diplomas,
certifications and experience. Reputation is
formed by the opinion of other entities,
deriving from third parties' opinions of
previous interactions with the trustee. Trust
revolves around ‘assurance’ and confidence
that people, data, entities, information or
processes will function or behave in expected
ways. At the deeper level, trust is regarded as
a consequence of progress towards security or
Till now, most research on trust have focused
on trust computation models and trust
management systems for solving related-security
issues such as Access Control in decentralized
systems ,, Identity Management ,
and Public Key Certification ,. In these
research works, some network environments are
considered such as sensor networks, peer-to-
peer networks, ad-hoc network, social networks
and IoT. However, there are limited works on
trust computation in the IoT environments; and
most of them are related to security enhancement
for dealing with malicious entities or access
control. Nonetheless, the research of trust in
the IoT is very decent due to the need for a
trusted environment for the reach of IoT full
In this survey, some existing trust
computation methods are analyzed and discussed
based on our classification of a trust
computation in the IoT: network architecture,
system layered architecture, various kind of
trust models; and trust aggregation. We
summarize both pros and cons of each method and
make comparison among them in order to highlight
the effectiveness when applying trust to offer
more secure services. Finally, we discuss the
gap of state-of-the-art research directions in
developing trust computation in IoT, as a result,
suggest some future research areas.
II. Background and Trust Computation
A. Trust Attributes
Generally trust presents the confidence and
the assurance that entities, users, systems,
data and process behave as it is expected to be.
Therefore trust can be considered as a way of
achieving extra security and privacy objectives.
As trust can be interpreted in different ways,
here we present various meanings from
literature for more clear views on trust in
terms of Information and Communication
Technologies (ICT) .
Trust is dynamic:
as it solely depending on
the time and changing nature of entities. As an
example from human world, one who was
trustworthy for some time ago can be changed
over time and completely unreliable.
Trust is context-dependent:
contexts trust can be totally unlike and will
have different trust measures for each and every
dissimilar scenarios. For example one can get
advice from friend about his lessons but about
medical treatments as the knowledge, experience
is different in two scenarios.
Trust is not transitive in nature but maybe
transitive within a given context:
That is, if
entity A trusts entity B, and entity B trusts
entity C then entity A may not trust entity C.
However A may trust any entity that entity B
trusts in a given context although this derived
trust may be explicit and hard to be quantified.
Trust is an asymmetric relationship:
trust is a non-mutual reciprocal in nature. That
means if entity A trust entity B, then the
statement “entity B trusts entity A” is not
The nature of trust is fuzzy, dynamic and
complex. Besides asymmetry and transitivity,
there are additional key characteristics of
trust: implicitness, antonymy, asynchrony, and
Trust can have different form
depending on the context and entity and hence
it is difficult to clearly measure the
confidence, belief, capability, context, and
time dependency of trust.
The degree of seriousness in trust
relationships may differ between the entities.
For example, entity A may think that its trust
with entity B is important, however, entity B
may think it differently.
B. Trust in IoT
There are plentiful trust solutions have been
proposed for many network systems such as peer-
to-peer (P2P), multi-agent systems, and e-
commerce. In this section, we consider trust in
IoT: the networks of devices like household
appliances, office appliances, sensors and
vehicles which are interconnected seamlessly
and with ability of self-configuring capability.
These electronic devices, which are billions in
number and varied in size and computing
capabilities, are ranging from Radio Frequency
Identification tags (RFIDs) to vehicles with On
board Units (OBUs). IoT is expected to enable
advanced services and applications like smart
home, smart grid or smart city by integrating
a variety of technologies in many research areas
from embedded systems, wireless sensor networks,
service platforms, and automation to privacy,
security and trust.
Recently, trust in IoT is intensively
investigated and mostly divided into two types
direct trust and third party trust . The
direct trust is a situation where a trusting
relationship is nurtured by two entities and
formed after these entities have performed
transactions with each other. The third-party
trust is a trust relationship of an entity that
is formed from the third party recommendations
which could be no previous transaction ever
occurred between the two interacting entities.
For example, entity A trusts entity B because
B is trusted by entity C. In this example,
entity A derives trust of B from C, and A also
trusts entity C does not lie to him. As with
any types of trust relationship, there is a link
with the risk which affects the trusting
relationship between the entities. Author in 
stresses that an entity will only proceed with
the transaction if the risk is perceived as
Lately, the convergence of two emerging
network paradigms Social Networks and IoT as
Social Internet of Things (SIoT) has attracted
many researchers as a prospective approach for
dealing with challenges in IoT. The benefit of
SIoT is the separation in terms of the two
levels of humans and devices; allowing devices
to have their own social networks; offering
humans to impose rules on their devices to
protect their privacy, security and maximize
trust during the interaction among objects
assessing trust is imitated by modulating
Reputation, Recommendation, and Knowledge as
three basic Trust Metrics (TMs).
C. Trust Computation Objectives
To provide trust among entities in the IoT
environment, research on trust computation
should achieve some goals in accordance with
the deployment of a trust platform in the IoT
System Architecture and Network
Architecture of the environment in which
trust platform will be deployed. Based on
this, trust computation models are
developed and built.
Trust Model: a Trust Model in accordance
with TMs and TAs. This part should include
Trust Composition as Credentials, TMs,
Technical Attributes (TAs) and IoT
properties contributed to Trust Computation
such as network characteristics and social
Trust Aggregation techniques: methods to
examine a trust score or trust level once
all TAs and TMs are already collected and
III. System and Network Architecture of a
A. Network Architecture
With heterogeneous applications and services
in IoT, one must give special attention to the
architecture of the trust model with respect to
trust propagation. According to the literature,
studies on trust architectures can be mainly
categorized into centralized approach and
distributed approach. Some properties of each
approach are described in Table 1.
As the name implies centralized approach store
all the information about TMs, TAs, protocols
and algorithms and mathematical models, related
to trust computation in a central database and
provide the service on demand as shown in Figure
1(A). On the other hand in distributed approach
Figure 1(C), trust agents do all the computation
Figure 1. Centralized vs Decentralized vs
Table 1. Comparison of Trust Propagation Methods
Points of failure
Single point of
Finite number of
Ease of development/
More details needed
But for IoT applications, sticking in to only
one approach will not be sufficient as sometimes
calculations have to be done locally and some
are remotely depending on the resources
availability. Therefore fully distributed model
or fully centralized versions will not give
satisfactory results and combined methods also
to be considered in respect to trust computation.
In this regard, the decentralized model shown
in Figure 1(B) can be considered as an optimum
model for the trust computation with the
complexity of IoT services.
In the approach, each trust request and
service will go through a central node or TA
which can be accessed by all other nodes in
his domain as shown in Figure 1(A). TA will
be responsible for managing trust information
including trust negotiation, calculation and
decision making and/or assist users by
providing the initial information required
for trust computation.
In general, centrality based rating systems
are global rating systems. One of the most
prominent area where centralized trust
computation has been deployed is in the social
networks like Facebook™ and e-markets like
Amazon™ and eBay™ ,. In here,
reputation is a function of the cumulative
ratings on users by others. Furthermore, 
explains how the reputation system works in
social networks using a mathematical model.
Basically it introduces adjacency matrix
which represent rating from node “i" to node
“j” and method to solve this matrix
recursively to obtain the reputation of each
More evolved version of a reputation model
called SPORAS compared to eBay™ is developed
by  where only the most recent
recommendations have been taken into the
consideration. Here the mechanism is built in
such a way that the reputation update will
effect significantly for low reputed users and
rarely for the users with high reputation.
The underlying core principal is based on the
standard deviation of reputation values. Also
they suggest a method to incorporate
reputation mechanisms in online communities
to make it more reliable and more effective
the way users contribute in the community.
In , trust computation based on a
centralized cluster head is proposed.
Initially cluster head is responsible for
delivering trust values for every node in its
domain. After that local node will combine
locally calculated trust with initially
learned trust value from cluster head.
In ,, an agent based trust
computation method is suggested for mobile ad
hoc network (MANET). It uses the weighted
means to measure the nodes final trust and
then makes the corresponding decision.
A trust modelling scheme for a group of
nodes (group trust) based on cluster head
approach is proposed in ,. The entire
network is divided into number of small groups
and every group has a cluster head and all
the cluster heads are connected to the base
station. This trust value will be sent to
cluster head. The cluster head will determine
the trust value of other cluster heads based
on interactions and then forward all the
information to the base station. Base station
will then decide the trust factors (fully
trust, untrust or uncertain). Comparison of
different centralized trust computing schemes
with respect to research area, pros and cons,
complexity and performance limitations is
provided in Table 2.
Table 2. Comparison of different centralized trust computing mechanisms
This refers to IoT users independently
exchange trust matrices with neighboring
users without intervention of centralized
entity. In here the trust computation methods
can be categorized in to three parts as direct
trust, indirect Trust and hybrid methods as
shown in Figure 2.
A trustor node (X) is directly in contact
with trustee node (Y) and learns the trust
knowledge via direct negotiation between them
as shown in Figure 2(a). In an event, trustor
node compares this learned knowledge with
locally calculated trust values and best on
that final trust value will be generated. That
is, final trust value is a combination of both
locally generated trust values and direct
observations. Hence determination of trust
factor between these two entities is vital
and  proposes a mathematical model based
on probability theory to determine optimum
percentages from both entities.
Trust is measured in
the interval [0, 1]
The computed trust
is global and not
The computed trust
may not be precise
with respect to
node. Cluster head
can be single point
Nodes query the
agents for the
value based on
Trust is defined in
the interval [0, 1].
community sizes have
This scheme can
attack well as the
than one trust
agents and the
from the agents
to the nodes.
This scheme will
perform well as
long as number of
the final trust.
Trust is presented as
fuzzy logic in the
intervals [0 − 0.4,
0.4 − 0.6, 0.6 – 1].
have been analyzed.
cluster heads to
Cluster head can be
single point of
Based on a
Trust is confined in
the range [0, 1]. The
impact on trust
increasing the peer
numbers has been
algorithm can be
made adaptive by
of the Trust
cost of hosting
Trust Block could
be single point of
Figure 2. Distributed Trust Computation methods .
A direct trust computation method for
wireless sensor nodes is proposed in 
based on confidence interval concept. Final
trust value will be decided after observing
the behavior of adjacent node over
considerable time. Here trust is represented
as mean trust value and a conﬁdence interval
about the mean. Then based on the confidence
interval trustor will proceed with the
decision making process, i.e. if the
confidence interval is sufficiently narrow
enough. If not trustor will observe more
knowledge from trustee before calculation of
final trust value. In , table based trust
storage mechanism is used for each neighboring
node. Comparison of some other distributed
trust computing mechanisms with respect to
research area, pros and cons, complexity and
performance limitations is provided in Table
Table 3. Comparison of different direct trust computing mechanisms
Trust is a
fractional value in
[0, 1]. Convergence
time, memory cache
Accumulates the past
behaviors and weigh
them based on time.
Hence the trust
precise. No single
Requires memory to
store the past
determine the t-
local and biased.
Trust is a
fractional value in
[0, 1]. Performance
of AODV and DSR
protocol have been
analyzed with the
Works based on
existing request and
schemes in AODV and
No single point
to monitor the
event of neighbors.
neighbors all the
time to construct
and update trust
Computed trust is
Trust is measured
trust for various
No single point
requires memory and
may not be
In a situation where direct observation is
not possible with trustee node
trustworthiness can be calculated based on
recommendations from the peer users which have
records about trustee. However relying on
others recommendations involved high risk
compared to direct trust method as
recommenders can falsely provide dishonest
information which can lead to reduce trust
value of honest users and improve the trust
of malicious nodes. Therefore other than
calculating trust, validating them is also a
key research area in this method.
In regard to determine dishonest users, ,
 propose trust credibility evaluation
methods based on threshold values and
assigning lesser weights for the dishonest
users in future transactions. After filtering
out the false recommendation, the next step
is to calculate the effectiveness of each
honest recommendation. Authors in 
proposed several methods determine
creditability of trust by using fuzzy logic.
A trust calculation method based on threat
reports for MANETs is proposed in . In
this method, an alarming system is included
in each and every node. Then every node listen
to its adjacent nodes and generate a trust
report based on their behavior. This will be
broadcast to each and every node so that if
any node generate false report it can be
detected by the alarming system.
B. System Layered Architecture
With the definition of IoT, it is clear that
establishing trust in one particular layer is
not enough and in fact trust should be defined
as multidimensional property over all layers of
IoT layered architecture as shown in Figure 3.
Figure 3. Trust establishment procedures
That is, the final value of trust of specific
entity is determined not only by one single
parameter but trust matrices distributed among
users, applications, connections and devices.
Moreover, these aggregated data is essential
for the decision making process as shown in
As an example, smart city is considered to
elaborate layered trust architecture mentioned
above. With corresponding to three layer
structure, device layer represents physical
devices like various kind of sensors and
physical network. In our user case, these
sensors helps to gather information like
weather, location and traffic condition. In
similar manner, trust matrices like Quality of
Service (QoS), delay and routing is considered
in the network layer for trust computation and
at application layer, trust for services like
storage, processing, and etc. is calculated.
Then the locally calculated trust in each layer
will be send for the final decision making
process as shown in Figure 4.
In  researchers propose a trust
computation method in connectivity layer with
respect to MANNET. Additionally they implement
a cross layer protocol based on trust to improve
the security of packet exchanging and delivery
ratio of the network. Moreover,  suggests
trust calculation method based QoS while 
presents a method to predict the trust based on
QoS parameters particularly considering the
service providers side.
Considering sensor layer, establishing trust
for IoT devices is a challenging task due to
heterogeneous relations. To extract trust
information in sensor layer, several mechanisms
like trusted computing  and computational
trust mechanisms proposed in  are required.
Nevertheless it is mandatory to provide
necessary trust information to every entity
that matters and hence ontology based
mechanisms need to be deployed as described
above. Also authors in  provide an algorithm
based on partial correlation to achieve data
trust when computing trustworthiness of an
entity and in decision making process.
Trust based on
Trust based on
Trust based on
Trust Informat ion Trust Information Trust Informat ion
Figure 4.Trust Computation Steps in IoT
IV. Trust Computation Models
There are two conventional ways of trust
models are policy-based approach (or rule-based
approach) and reputation-based approach. These
two trust models have been investigated under
the context of different network environment
including IoT with different purposes and goals.
Traditionally, policy mechanisms manage the
decision of a system by describing pre-defined
set of conditions (rules) and specific set of
actions in accordance with each condition. In
this manner, policy can assist in making
decision for trust computation when a certain
ambiguity level occurs while assessing trust.
As a result, policy-based trust models normally
involve the exchange or verification of trust-
related credentials called trust negotiation
A reputation-based trust model is basically
used in trust computation for assessing trust
score or trust level based on the history of
interactions of related entities. The
reputation information in this scenario could
be either directly with the evaluator (direct
reputation) or as recommendation by other
entities (indirect reputation, recommendation
or third party information). The trust model
based on a certain levels of reputation
information is obviously since it happens in
the process when people analyze and examine
In recent years, most of researchers have
accepted that reputation is one important
factor of trust resulting in the dominance of
reputation-based trust models compared to
policy-based models. Some have tried to
integrate both approaches in their trust models
in order to leverage the advantages of them.
Nevertheless, both credentials and reputations
are the important information involving in the
trust transitivity among entities; and each of
them has its own pros and cons that have
motivated researchers to work on.
A. Policy-based Trust Computation Models
This approach has been intensively
investigated in the previous decade (mostly
from 2000 to 2005) in which policies or rules
are used in the trust computation. To establish
and calculate trust, a trust management need to
integrate trust negotiation protocols for
creating, exchanging and managing credentials
of network entities. The policy-based trust
methods generally assume that a trustor after
several processes of credential creation and
exchange, it will obtain a sufficient amount of
credentials from trustee and from other
entities for trust establishment and trust
calculation. There is an issues called
“recursive problem” which is related to the
trust of the credentials in this approach. This
problem can be solved by introducing a trusted
authority (a third party entity) for issuing
and verifying these credentials.
The policy-based trust mechanism are usually
used in the context of distributed network
system as a solution for access control and
authorization ,,,. The goal is
simple by judging whether a user is trustful or
not based on a set of credentials and predefine
rules before granting rights to access network
resources. The focus in this situation is how
to apply policy languages, entities ontology
and reasoning engines for specifying and
producing additional rules and trust knowledge
for trust computation procedures.
For the summary research related to policy-
based mechanism, we organize the research work
into sub-categories of trust computation
procedures: trust credentials establishment,
trust negotiation process, and policy/rules
Trust Credentials Establishment:
Conventionally, credential is information
about an entity and context of the environment
needed to evaluate trust. Although the word
“credential” is frequently used in many
research works, there is no common definition
or standard to specify and determine it.
Policies should rely on credential
information and other context properties in
order to judge trust. An obvious example of
credentials in trust is the use of username
and password to gain access control when
logging to a computer. According to the system
policy, having a correct username in
accordance with an appropriate password
proves that the user is trusted by that
computer system. In a more complicated example,
credentials are also automatically generated
during a negotiation process by leveraging
security certificates with digital signatures
or using public key infrastructure (PKI). Note
that only certificates that includes trust-
related information of an entity or context
can be used as credentials. For example,
TrustBuilder  dealt with trust by
establishing trust credentials using
traditional security techniques such as
authentication and encryption which is called
“hard security” trust.
There is a well-known research work related
to credential exchange is Kerberos
protocol. The protocol considers a user
as the trustee and a computer as the trustor
and enables them to securely exchange their
own verifiable credentials. To do this,
Kerberos system needs to use a third party,
in this case is another computer, to
facilitate the credentials exchange process.
However, this approach is no longer used since
the current network systems like IoT are much
more complex and are facing many intelligent
Recently, many researchers consider
“credentials” in a broader perspective and
have used the term “trust metrics” and
“technical attributes” instead of
“credentials”. This approach allows us to
develop trust more flexible, scalable and
Trust Negotiation Process
An important issue when exchanging and
generating credentials is the undesirable
reveal of information to malicious entities,
resulting in loss of security and privacy.
The question raised is: To what extend an
entity trusts other entities to see its own
credential information in exchange of earning
their credentials. There are many research
works dealing with this trade-off between
gaining trust and sacrificing privacy such as
in ,,. These researchers
considered several particular context in
accordance with types of credentials and
number of credentials. They analyzed the loss
of privacy once any credentials are revealed
to other entities. This trade-off approach has
motivated some researchers to develop a trust
platform by developing architecture systems
based on that trade-off principles.
TrustBuilder is a typical example in which
a mechanism is implemented for analyzing and
choosing the reasonable solution for the
trade-off in the context of web services.
The trustor needs to understand the risk of
losing privacy information when revealing
credentials in exchange of earning trust.
Based on this mechanism, trust is gained when
a successful trade-off is made: sufficient
credentials are revealed while sacrifice
privacy is still maintained in some level.
The concept of trust transitivity property is
also characterized in TrustBuilder in the form
of “credentials chain”. For example, if
entity A trusts B’s credentials, and B trusts
C’s credentials, then A trust in the
credentials of C in some degree.
Based on the credentials chain concept, some
research works designed and developed trust
frameworks that perform credential chaining
and credential exchange such as in
PeerTrust, PROTUNE, RT10.
Table 4. A Comparison on Research Work related to Policy and Trust Languages
Policy/Trust Language Features
Access Control for KAoS
KAoS Policy language with ability of dynamic policy
For Security and Privacy
Use semantic representation and model for dynamic policy
Allow each entity to set their own policy,
To replace key-based
Include observation of trustee, recommendation from
others and reference to other sources of the trustee.
Use a formal policy language. Trust can be proved
Specification and OASIS
extensions to WS-
Security Assertion Markup Language (SAML).
Trust is gained through proofs of identity,
authorization, and performance.
To validate the security token.
For trust-based security
Policy language that use lattices of relative trust
Allows fine-tuned control over trust decisions
control and Context-
based system for
Use a policy specification language based on Datalog with
constrains with five special predicates.
Trust is obtained after credentials exchanged.
control for web
Use ontology for representing trust negotiation policies.
Rules are used to negotiate trust.
Policies are more flexible than standard policy set,
allowing simplify policy specification
Provide “proof of compliance” for request, credentials
Allow individual system to have different trust policies.
PolicyMaker assertions can be written in any programming
Same principles with PolicyMaker: directly authorize
actions (in accordance with credentials) instead of
processing both authentication and access control.
Require credentials and policies be written in a specific
assertion language to work with KeyNote compliance
Ontologies and Context-aware mechanisms are
also soon introduced when developing
credentials on the context of client-server
system  and Semantic Web.
Policy Languages and Trust Languages
It is needed to design formalism for trust-
related information, e.g credentials and
trust metrics in order to develop a trust
system. This objective can be achieved by
incorporating findings from logic to automate
various kinds of reasoning, such as the
application of rules and policies or the
relations of sets and subsets for the Trust
Computation process. Most of researchers have
used the Semantic Webs techniques such as
semantic representation, policy languages,
ontologies and reasoning mechanisms to the
trust computation. The issue is how to
represent and express trust information and
trust knowledge. Some efforts have been made
to create policy languages for trust as
described in Table 4.
B. Reputation-based Trust Computation Models
This approach uses history of interactions and
behaviors among trustor, trustee and related
entities, combines them in accordance with a
reputation model in order to make a trust
decision about the trustee. The history of
interactions between trustor and trustee is
sometimes called personal experience or direct
reputation. The history of interactions between
other entities and trustor is also called
indirect reputation, referral reputation or
There are much parallel research works on both
reputation-based trust model and reputation
model. The confusion between a reputation
system and a trust system should be clarified.
Trust and reputation are sometimes in the same
across multiple contexts or are treated as the
same mechanism to support services. Basically,
a reputation system collects feedbacks from
entities after an interaction incurs. These
feedbacks will be combined and calculated using
several mathematical models to get a reputed
score. This reputed score is sometimes
misunderstood as trust level. Several
reputation systems have been developed in the
context of e-commerce systems and web services
such as eBay  and Keynote . These
systems use a centralized authority to get
ratings and feedbacks from users after each
transaction and then update the overall reputed
score by using several mathematical models as
mentioned above. There are also some
distributed approaches for reputation system in
which each entity establishes and maintains
reputed scores to its neighbors by updating once
any related interaction occurs by using several
heuristic algorithms. It is required to
integrate these score due to the use of
deterministic numbers for representing
Reputation-based trust system can be
considered as a step forward compared to
reputation system in which trust computation
mechanism combines not only ratings or
feedbacks from entities but also trustor and
trustee properties and preferences; and context
information to calculate trust level. In this
sense, reputation system is a part of trust
system. There have been a large amount of effort
to investigate the reputation-based trust model
and to develop reputation-based trust systems
in many type of network environment such as in
distributed systems, P2P networks, sensor
networks, and grids. There are also some
research works to build a network of trust in
which trust is established and maintained
between any two entities over time, resulting
in creating a “web of trust”.
Reputation-based Trust in Distributed
System and P2P Networks
The trust models in this part try to create
a trust system that entities are able to
establish, calculate trust level, and make
trust decisions rather than rely on a
centralized authority. The contribution in
this approach is how to create appropriate
credentials, TMs and TAs that provided to each
entity to produce trust. Depending on
different purposes of applications in each
network environment, reputation-based trust
systems are utilized accordingly. For example,
in distributed system, many research works
focus on the detection of malicious entities
and prevention of network attacks while trust
system in P2P networks is to guarantee the
quality of data transfer.
Reputation-based Web of Trust
Almost effort in this idea uses the concept
of credentials chain. The majority of trust
computation transitivity has been focus on
using reputation. Reputation, in this
scenario, is defined as a TM, and each entity
maintains reputation information on other
entities, thus creating a “trust network”
or “web of trust”.
There are two approach for trust systems in
the web of trust. The first approach assumes
that trust credentials and TMs are already
existed, and the trust systems are trying to
propagate trust among entities which may not
have been evaluated for trust. The later
supposes that a web of trust is given in which
a link between two entities mean the trust
decision with a trust value. There is no matter
how these links are made as long as the trust
can be quantified. If there is no link between
two entities, it means no trust decision has
been made, and trust transitivity should be
applied in this scenario. The summary and
comparisons of reputation-based trust
computation in the above discussed perspectives
are described in detailed in Table 5.
Table 5. Features comparisons among reputation-based trust models
Define Agent, Trust Relationships, Trust Value and Trust Categories.
Define first-hand knowledge as direct reputation and second-hand
knowledge as recommendation.
Propose Recommendation protocol for trust propagation.
Reputation information is obtained from external sources.
Allow entities actively determine trust using reputation information
obtained from other entities.
Avoid hard security by distributing reputation information allowing
individuals to make trust decisions instead of a centralized trust
Weight the reputation information by the reputation of those sources
for providing good information.
Analyze the reputation information by characterizing the indirect and
Considering the social relation in calculating reputation score.
Put the context information into account.
Provides methods for computing degrees of trust in the presence of
Propose PageRank algorithm for ranking websites by authority.
EigenTrust algorithm using PageRank to calculate global reputation
value for each entity.
Credentials for reputation in this work is the quality of a peer’s
uploads (e.g., did the file successfully upload?) within a peer-to-
Propose XRep protocol which allows for an automatic vote using user’s
feedback for the best host for a given resource.
Web of Trust
Use ontologies to express trust and reputation information, which then
allows a quantification of trust for use in algorithms to make a trust
decision about any two entities.
Trust transitivity is considered as credentials chain.
Local reputation and Global reputation is also taken into account.
Web of Trust
s in Open
Define controversial users who are both trusted and distrusted in
Globally computed trust value (in a web of trust) for a controversial
user may not be as accurate as a locally computed value due to the
global disagreement on trust for that user.
Propose a method that performs a global computation on reputation
values but considers the individual’s input to the evaluation as the
V. Hybrid Trust Model and Trust
Several research works have tried to combine
both reputation and policy-based models as a
hybrid trust model in order to take advantages
of both approaches while may get rid of their
drawbacks. This idea has recently become more
popular in the context of IoT where trust is
more complex because many factors contributed
to the trust establishment and to the trust
computation. In such IoT environment, history
of interactions and behaviors of entities are
not only for reputation information but also
for trust-related knowledge extraction. The
combination of reputation information,
knowledge and relationships among entities in
IoT draws a very complicated picture of trust
Table 6. Summary of Trust Aggregation Techniques
Importance Technique Features
Entities with a higher reputation or transaction relevance have a higher weight.
Entities with strong relationships to trustor have higher weight.
Use credibility as weight associated with indirect trust (recommendation or
Use similarity as weight for indirect trust aggregation.
Fuzzy Logic deals with reasoning that is approximate rather than fixed and exact.
Fuzzy logic variables may have a truth value that ranges in degree between 0 and 1
and produce a partial trust where the truth value may range between completely true
and completely false as trust levels.
Linguistic variables are used as trust levels and managed by specific membership
functions. Then trust is represented as a fuzzy measure with membership functions
describing the degrees of trust (trust level).
Belief theory (evidence theory or Dempster-Shafer theory (DST)) deals with reasoning
with uncertainty, with connections to other techniques such as probability,
possibility and imprecise probability theories.
Trust can leverage the subjective logic by operating on subjective beliefs about the
network environment, and used opinion metric to denote the representation of a
Used in trust computational model to compute trust of agents in autonomous systems
by modeling the trust by belief, disbelief and uncertainty of an entity to other
entities. It makes use of a base rate probability in the absence of evidence. The
average trust then can be calculated as the probability expectation value between
trustor and trustee.
Subjective logic operators such as the discount and consensus operators can be used
to combine opinions (self-observations or recommendations).
Trust can be considered as Bayesian interference: a random variable following a
probability distribution with its model parameters being updated upon new
Can be used as a trust computational model because of its simplicity and sound
Trust value can be modeled as a random variable in the range of [0, 1] following
Beta distribution in which Belief discounting can be applied to defend against
malicious entities such as bad-mouthing attacks ballot-stuffing attacks.
In the hybrid model, reputation is considered
as one of several TMs. The reputation TM can be
obtained by using the reputation mechanisms and
reputation systems that have already been
developed and mentioned above. That is the
content of Trust Aggregation procedure in which
trust evidences (TAs, TMs) are collected
through several techniques, such as self-
observation or reputed information in the form
of feedbacks and recommendations.
TMs can be gained from sufficient TAs by using
trust aggregation techniques, for example, TMs
can be computed by using Weighted Sum ,,
Fuzzy-based algorithms ,, Belief Theory
,, Bayesian mechanisms ,.
To calculate the overall trust score or trust
level, a policy-based mechanism with one of a
trust aggregation method mentioned above or
with a reasoning method is needed to combine
It is needed to note that the trust
aggregation is a dynamic process which heavily
depends on context-aware information, service
requirements and trustor's preferences. Each
trustor needs appropriate trust data, context
data and aggregation methods for producing
desired overall trust score which reflects the
trustor’s perspective and context awareness.
Specific trustors might use and define
different trust aggregation techniques for
dealing with their associated trust data. There
is currently no complete trust aggregation
mechanism can deal with the personalized trust
in dynamic context-awareness environment,
however, several researchers have proposed some
solutions for particular contexts and services.
The summary is described in Table 6. The trust
aggregation techniques and reasoning mechanism
are the crucial parts needed to investigate and
develop in order to build a completed trust
platform in the IoT.
VI. Discussion and Future Research
In our study, extensive range of trust
computation mechanisms has been discussed.
However the current research methods are only
focused only on specific context and hence
lacking completeness. Therefore a single unique
solution is not presented for the trust
computation and acquisition. Thus issues are
still open for investigation and some of the
ideas are discussed here.
A. Research Gaps and Discussion
Based on many papers that have been analyzed
above, there are many gaps that needed to be
filled in order to have a complete trust
understanding and development.
One of the most important gap that we intend
to discuss and go for doing research is the lack
of using environment information to trust
computation. The network system here is the IoT
in which physical devices are owned by human-
related factors and inherently socially
connected by physical-cyber-social system.
Moreover, trust computation methods also lack
concerns on trustor’s subjective properties,
in other words, the trust results are not
reflected of personalized expectation. The
solutions for this gap could be two-fold
approaches: The first one is to develop the
trust relationships among entities in the IoT,
thus creating a reliability and readiness of
the trust network, based on the existing social
models in the network systems. The second one
is to explore other social TMs such as
trustor’s similarity and friendship behaviors,
centrality, community of interest, and more
appropriate reputation TM.
Along with the two approaches, trustor
preferences should be taken into account in
order to reflect the personalized trust and to
enhance the intelligence of trust. There are
possibly large number of TMs depending on each
context of IoT and services requirements such
as honesty, cooperativeness, QoS, community of
interest, and etc. In order to explorer more
TMs, it is needed to investigate the network
environment ontologies and trust ontologies in
which relationships among entities and the
relationships’ properties are represented and
clarified. Consequently, by using a reasoning
mechanism or a machine learning technique, new
trust information and trust knowledge could be
extracted and help enhancing the effectiveness
of trust computation.
Another big gap in the area of trust
computation is the trust aggregation methods
and trust reasoning that have been stated in
the previous section. This gap incurs in both
situation in the trust computation procedure:
when there are several distinct TAs needed to
combine into one overall TM; and when there are
several TMs needed to combine into the overall
trust score or trust level. There are limited
literatures in this area as mentioned in Section
IV. The most popular and simple method to deal
with the trust aggregation and trust reasoning
currently is to apply the use of static weighted
sum for trust formation. However, this solution
is not smart enough due to the complicated IoT
environment. Thus, there is an urgent need for
a novel research on the use of more effective
trust formation methods including dynamic
weighted sum, belief theory, fuzzy logic and
regression analysis. For example, an
intelligent weighted sum method can dynamically
adjust the weights associated with TA and TMs
based on context awareness and user preferences.
The weighted sum method can also use a
regression analysis that links context
information with TA and TM and user preference
so as to determine the best weight assignment.
B. Other Research Directions
As compared to network security, it is
essential to investigate on trust validation
methods to effectively combat and defend with
all sort of attacks including self-promoting,
good mouthing/bad mouthing attacks and other
possible attacks. While defending from attacks,
it is also important to investigate resilient
self-healing approaches to enhance trust
recovery after a positive attack. Further
effectiveness of trust management when it comes
to billions of devices and applications should
be studied carefully. One possible direction
is to investigate trust management with
concepts like Big Data and Data-mining.
Essentially employing trust capabilities should
minimally compromise performance and process of
IoT as many devices have limited resources. A
possible research direction is the
investigation of intelligent trust-based
routing protocols which are more reliable while
consuming minimum energy and traffic overhead.
Static methods for dealing with trust
discussed above will not be enough to implement
context-aware scheme. Thus, an autonomous or
dynamic trust computation mechanism should be
considered for the process involved with TMs
acquisition, calculation and finally for
decision making process.
This research was supported by the ICT R&D
program of MSIP/IITP [R0190-15-2027,
Development of TII (Trusted Information
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Nguyen B.Truong received
Master and Bachelor
degrees in Computer
Engineering from Pohang
University of Science and
Technology, South Korea
(POSTECH) and Hanoi
University of Science and
(HUST) in 2008 and 2013,
respectively. He is
currently doing PhD at Liverpool John Moores
University, United Kingdom. He was a Software
Engineer at DASAN Networks, a leading company
on Networking Products and Services in South
Korea from 2012 to 2015. His research interest
is including, but not limited to, Trust in the
Internet of Things, Vehicular Network, Software
Defined Networking, Wireless Networks, Sensor
Networks, Fog and Cloud Computing. His work also
involved in problems of Load Balancing, Channel
Utilization and Energy Efficiency Protocols.
received the B.Sc.
degree in electronics
honours) from University
of Moratuwa, Sri Lanka
in 2010 and the M.Sc.
from Asian Institute of
Technology, Thailand in
2013. Mr. Jayasinghe, is the recipient of The
A.B. Sharma Memorial Prize in recognition
having the best thesis from the fields of
Information and Communication Technologies and
Telecommunications, Asian Institute of
Technology in 2013. Currently He is doing PhD
at Liverpool John Moores University United
Kingdom. He has worked as a researcher in Centre
for Wireless Communication, University of Oulu,
Finland and Computer Communications and
Applications Laboratory, EPFL, Switzerland. His
research interests include IoT, Networking and
Security, Wireless Communication and Mobile
Tai-Won Um received the BS
degree in electronic and
from Hong Ik University,
Seoul, Korea, in 1999, and
the MS and PhD degrees
from the School of
University (ICU), Daejeon, Korea, in 2000 and
2006, respectively. He is currently a senior
researcher with ETRI, Daejeon, Korea.
Dr. Um has been actively participating in
standardization meetings including ITU-T SG 13
(Future Networks including mobile, cloud
computing and NGN), and currently serves as an
editor of Q16/13.
Gyu Myoung Lee
his BS degree in
electronic and electrical
engineering from Hong Ik
University, Seoul, Rep.
of Korea, in 1999 and MS,
and PhD. degree from
the Korea Advanced
Institute of Science and Technology (KAIST),
Daejeon, Rep. of Korea, in 2000 and 2007,
respectively. He joined School of Computing &
Mathematical Sciences at the Liverpool John
Moores University, Liverpool (LJMU), UK, as a
Senior Lecturer in 2014. He is also with KAIST
Institute for IT convergence, Daejeon, Rep. of
Korea, as an adjunct professor from 2012.
His research interests include future networks,
Internet of Things, multimedia services, energy
saving networks including Smart Grid. He has
actively contributed for standardization in
ITU-T as a Rapporteur (currently Q11/13 and
Q16/13) and IETF. He was an editor of draft
recommendations for SUN under developing in the
ITU-T. He is an IEEE senior member.