1. A Survey of Trust Management for Mobile Ad Hoc Networks
Jin-Hee Cho & Ananthram Swami , Army Research Laboratory
Ing-Ray Chen, Virginia Tech

2. Outline Background Motivation Multidisciplinary Trust Concept
Trust, Trustworthiness, and Risk Assessment
Trust Properties in MANETs
Survey on Trust Management in MANETs
Future Research Directions
Survey on trust management in MANETs: attacks, metrics, trust properties
Case study: based on a recently submitted paper as our ongoing research, we briefly show our current on based on future research directions.
Could skip this slide as needed

3. Background Design Challenges in Mobile Ad Hoc Networks:
Resource constraints
energy, bandwidth, memory, computational power
High security vulnerability
open medium derived from inherent nature of wireless networks
rapidly changing network topology due to node mobility or failure, RF channel conditions
decentralized decision making and cooperation (no centralized authority)
no clear line of defense
Trust: the degree of subjective belief about the behavior of a particular entity.
In general, trust in social science is defined as….

4. Motivation
Trust management is needed in MANETs with the goal of establishing a network with an acceptable level of trust relationships among participating nodes:
During network bootstrapping
To support coalition operation without predefined trust
For authentication for certificates generated by other parties when links are down
To ensuring safety when entering a new zone
Diverse applicability as a decision making mechanism for:
Intrusion detection
Key management
Access control
Authentication
Secure routing
Many others
These are “application”

5. Trust in Communications & Networking
A set of relations among entities participating in a protocol based on evidence generated by the previous interactions of entities within a protocol
If the interactions have been faithful to the protocol, then trust will accumulate between these entities
Context-aware trust: trust is the quantified belief of a trustor node regarding competence, honesty, security, and dependability of a trustee node in a specific context
Multidisciplinary
Concept of Trust
Sociology
Risking betrayal
Subjectivity
Economics
Incentive-based selfishness
Autonomic computing
automation reliability
Organizational management
risk assessment
Psychology
Cognitive process
Philosophy
Context-dependent moral relationship
Communications & Networking
more...
security
reconfigurability
scalability
reliability
dependability
System designers in communications and networking want to derive all properties including security, reconfigurability, scalability, reliability, dependability, and more from the multidisciplinary concept of trust.

6. Trust, Trustworthiness, and Risk Assessment1
Trustworthiness
Trust
b. misplaced trust
a. misplaced distrust
Trust =Trustworthiness
0.5
Definition (Trustworthiness): Trustworthiness is objective probability by which the trustee performs a given action on which the welfare of the trustor depends
Definition (Trust): Trust is the subjective probability by which the trustor expects that the trustee performs a given action on which the welfare of the trustor depends
Definition (Risk): risk is defined by the probability and the consequence of an incident. The risk value is given by the function r : P x C -> RV, where P is a set of trust values in [0,1], C is the set of consequence values and RV is the set of risk values.
Some scholars distinguished subjective trust, the so called trust, and objective trust, the so called trustworthiness.
Misplaced distrust happens when objective trust is higher than subjective trust. Trustworthiness > trust, losing possible helpful collaborations with trustable parties.
Misplaced trust happens when subjective trust is higher than objective trust, leading to high possible risky situations, experiencing betrayal.
Fire exercise: reliability trust
Real fire situation: decision trust
If we know sometimes a particular node does not have a sufficient capability to perform a mission given, if there is no other resource to use, we choose to employ the node with lack of capability. Particularly when not using it is directly related to the definite system failure.
Trust Level [Solhaug et al., 2007]

7. Trust vs. Risk
Trust-based decision making: a trust threshold is used to say yes/no
yes when t > trust threshold (t2 in
the graph)
Risk-based decision making: a risk threshold is used to say yes/no
yes when r < high risk threshold (high risk zone in the graph)
In general when trust is high, risk is low but it really depends on the stake (consequence of failure). It is not enough to consider trust only and then say that trust is risk acceptance, trust is inverse to risk, or the like. 1
Stake
Trust
0.5 S2 S1 t1 t2
High risk
Medium risk
Low risk
Trust vs. Risk
[Solhaug et al. 2006,
Josang & LoPresti, 2004]

8. Trust Properties in MANETs
Subjectivity
Dynamicity
Asymmetry
Context-dependency
incomplete transitivity
Dynamic, not static
Trust in MANETs should be established based on local, short-lived, fast changing over time, online only and incomplete information available due to node mobility or failure, RF channel conditions
Expressed as a continuous value ranging from positive and negative degree
Subjective
Different experiences derived from dynamically changing network topology
Not necessarily transitive
Asymmetric, not necessarily reciprocal
Heterogeneous entities
Context-dependent
Sensing/Reporting vs. forwarding
Trust properties in MANETs.
Transitivity: mention about what trust chain is.
Trust properties in existing trust management in MANETs.

9. Classification of Trust Management
Risk Management
Risk Assessment
Risk Mitigation
Trust Management
Trust Update
Trust Revocation
Trust Establishment
Risk Control
trust evidence collection, trust generation, trust distribution, trust discovery, and trust evaluation
The terms trust management and trust establishment are also interchangeably used in the literature. To clarify
these two terms, according to Aivaloglou et al. [5], trust establishment is a process to deal with representing,
evaluating, maintaining, and distributing trust among nodes. Trust management deals with the problems such as
formulating evaluation rules and policies, representing trust evidence, and evaluating and managing trust
relationships among nodes. As Figure 6 explains, trust establishment belongs to one of trust management tasks.
[Solhaug et al., 2006]

10. Attacks considered in existing trust management in MANETs.
Attacks in MANETs
By the nature of attack and the types of attackers [Liu et al., 2004]
Passive Attacks: when an unauthorized party gains access to an asset but does not modify its content, (e.g., eavesdropping or traffic analysis)
Active Attacks : masquerading (impersonation attack), replay (retransmitting messages), message modification, DoS (e.g., excessive energy consumption)
By the legitimacy of attackers [Liu et al., 2004]
Insider attacks: authorized member
Outsider attacks: illegal user
Trust management in MANETs has studied in the presence of attackers.
Attacks considered in existing trust management in MANETs.

11. Metrics for Measuring Network Trust in MANETs
Network trust has been evaluated by general performance metrics, e.g., detection accuracy, goodput (useful information bits/sec), throughput (data bits/sec), overhead, delay, network utility, route usage (for secure routing), packet dropping rate, etc.
Recently, trust level as a metric has been used, e.g., trust level of a network path or session
Metrics used for evaluating network trust

12. Composite Trust Metrics
Quality-of-Service (QoS) Trust
Competence, dependability, reliability, successful experience, and reputation or recommendation representing capability to complete an assigned “task”
Examples are the node’s energy lifetime, computational power level, and capability to complete packet delivery
Social Trust
Use of the concept of social networks
Friendship, similarity, common interest, social connectivity, honesty, and social reputation or recommendation derived from direct or indirect interactions

13. Trust Management in MANETs based on Design Purpose
Summary of existing trust management schemes in MANETs based on specific design purposes

14. Trust-based Applications in MANETs
Secure Routing
Detect and isolate misbehaving nodes (selfish or malicious)
Reputation management
Extension of the existing routing protocols (e.g., DSR, AODV) using trust concept
Incentive mechanism to induce cooperation
Revocation + redemption possible
Authentication
Use trust to authenticate nodes or routing paths
Use direct evidence (certificates or observations of packet forwarding behavior) plus second hand information (e.g., recommendation)
Extension of the existing routing protocols (e.g., DSR, Zone Routing Protocol)
Key Management
Establish keys between nodes based on their trust relationships
Trust-based PKI
Distributed - each node maintains its public/private keys
Hierarchical – a CA is elected based on trust

15. Trust-based Applications in MANETs (Cont.)
Intrusion Detection
Trust as a basis for developing an intrusion detection system (IDS)
Trust-based IDS provides audit and monitoring capabilities to enhance security
Evaluating trust and identifying intrusions can be integrated together to build a trustworthy environment
Access Control
Use trust for decision making of access control to MANET resources
Trust-based admission control (role-based)
A node can use resources if it is trusted by k trusted nodes
Can integrate with policy-based access control (with a proof of identity or certificate)

16. Issues for Future Trust Management in MANETs
How should we select a trust metric that can reflect the unique properties of trust in MANETs?
What constitutes trust? Is it multi-dimensional with multiple trust components? Should we have a different set of trust components reflecting the application characteristics and node behavior (including selfish/malicious behavior)?
How can trust contribute to scalability, reconfigurability, security, and reliability of the network?
How should a trust protocol be designed to achieve adaptability to rapidly changing MANET environments?
How do we design a trust system to reflect adequate tradeoffs, e.g., altruism vs. selfishness, and effectiveness vs. efficiency?
Can we identify optimal trust protocol settings under various network and environmental conditions?

17. Questions? Contact us at:
Jin-Hee Cho Army Research Laboratory
Ananthram Swami , Army Research Laboratory
Ing-Ray Chen Virginia Tech