a prime example of a RTM which is getting to be a commonplace in many other domains as well.
Despite the proliferation of RTMs in the trust-reliant domains, ensuring trusted collaborations among participants remains a challenge. Several unaddressed threats (discussed in the later part of this chapter) still limit the effectiveness of reputation
systems.
Furthermore, some of the existing RTMs in literature focus on historical and recent information in determining the reputation of domain members. However, the dynamic nature of reputation and trust requires an equally dynamic approach to computing and resolving trust related issues in any domain.
2.2
Reputation and Trust
The reputation of a node in the survey of Djenouri et al. [DKB05] is the amount of trust that a trustor grants a trustee regarding the trustee’s cooperation and participation in the domain. In this thesis, an entity is said to be trusted when it has obtained positive feedbacks from its interactions with others coupled with high ratings in a domain.
Reputation and trust have been used interchangeably and synonymously in literature. However, some studies [JIB07, BAS09] indicate that whilst there is some correlation between them, there is a clear difference between the concepts. An example is given by Josang et al. in [JIB07] detailing the differences between the concepts as:
1. Trust systems produce a score that reflects the relying party’s subjective view of an entity’s trustworthiness, whereas reputation systems produce an entity’s reputation score as seen by all.
2. Transitivity is an explicit component in trust systems, whereas reputation systems usually only take transitivity implicitly into account.
3. Trust systems usually take subjective and general measures of trust as input, whereas information or ratings about specific events, such as transactions, are used as input in reputation systems.
Furthermore, Momani and Challa in [Mom10] defines trust as a derivation of the reputation of an entity. In the work, the authors state that based on the reputation, a level of trust is bestowed upon an entity. By motivating from a social perspective, reputation is built over time based on a person’s history of behaviour, and determines the level of trust that is bestowed on the person in a society. Reputation is said to be the amount of trust inspired by a particular member of a community in a specific domain. These definitions show the interlink between both concepts (trust and reputation); which is also the position of this thesis. For simplicity, this thesis describes reputation as the overall status of an entity within a particular domain and context while an entity is said to be trusted if it obtains high ratings from its interactions (reputation). Also, we focus on reputation rather than the subjectivity of trust, the time-dependency of reputation and direct trust relationships are highlighted in this thesis.
2.2.1
Properties of Reputation and Trust
The general properties of reputation and trust in the work of Liu and Issarny [LI04], and Adam and Davies in [AD05] are discussed below:
• Subjective: The subjective property of trust implies that an entity in a domain may have different RVs depending on the individual perception of other entities.
2.2 Reputation and Trust 21
That is, node A might have a very good reputation as perceived by B, but just an average reputation with C.
• Context Dependent: Reputation is context dependent because it is possible for an individual to exhibit contrasting behaviours in different situations. An example is a domain member that may be a good seller but not a good buyer on an online auctioning system.
• Time Dependent and Dynamic: From the social perspective, a person’s be- haviour in a society can fluctuate with time and the person can be classified as reputable at a certain time but become less reputable at another time. This implies that trust (and reputation) change over time and are therefore, dynamic and time dependent.
• Transitive: Transitivity can be described using the example: if a node A trusts B and B trusts C then A trusts C. There is a conflict about the transitivity of trust in literature. This is due to the fact that trust can either be treated as subjective or global. When trust is regarded as subjective, then it is not transitive whereas the reverse is the case when it is global [BAS09]. In this research, trust is treated as a global property and is therefore transitive.
2.2.2
Trust Relationships
Trust relationships, which are a major component of reputation management, shows the connections between participants. They help in capturing the behaviour of domain members through monitoring, whether cooperative or otherwise as evidence to compute
their level of trustworthiness and of course, reputation.
According to Srinivasan et al [STW+08], trust relationships can be broadly classified
into:
1. First-hand (direct trust): This is a direct trust relationship that represents a trust assertion of a member (trustor ) about another (trustee). It is a one-to-one trust relation between both parties. The trust relation is maintained locally by trustors and represents the trustors’ personal opinion about the trustee [SRIT11]. The system therefore uses direct observation or its own experience to update reputation. Typical examples of this are a trust relationship among friends in a social context such as on Facebook.
2. Second-hand (Recommendation-based trust): This is a third-party’s opinion about another entity. It can either be a one-to-many or many-to-one relationship. In this relationship, the system uses information provided by domain members about a certain participant.
Most systems proposed so far use both first-hand and second-hand information to update reputation. This allows the system to make use of the experience of its neighbours to form its opinions. Some systems choose not to use both types of information. In systems that use only first hand information, a node’s reputation value of another node is not influenced by others.