Trust in the Virtual World

Trust in the Virtual World
By: Sadegh Dorri Nogoorani
http://ce.sharif.edu/~dorri
1390/8/2 – 2011/10/24
(ISC Monthly Seminar)
In the Name of Allah

Who Knows on the Net...?
A notion of trust similar to
real world trust is
needed in the virtual
world…
Coordinating Agent
Interactions without
Strict Control
Mechanisms
Fig. by Peter Steiner (The New Yorker, 5 July 1993)
1390/8/2 - 2011/10/24 2Trust in the Virtual World - Sadegh Dorri N. (http://ce.sharif.edu/~dorri)

Outline
 Concepts
Definitions and basic terminology
 Trust and Reputation in Action
Applications
Attacks
 Trust Engines
Probabilistic, logic, …
 Trust in CROWDS
A detailed example

Trust (‫)اعتماد‬
 Definition [CF10]
The expectation/belief that…
… trustee will perform actions designed to produce
positive results in the future for the trustor…
… in situations of consistent perceived risk.
 Properties
Subjective, context dependent, asymmetric, transitive,
dynamic
 Calculation
Structural: organizational, category-membership
Relational: history-based, using trust transitivity
Cognitive: dispositional, trustee attributes

Trust Scenario
Trustor
(‫اعتمادگر‬)
Trustee
(‫معتمد‬)
Direct Trust (‫مستقیم‬ ‫)اعتماد‬
Functional
(‫عملکردی‬)
Referential
(‫ارجاعی‬)
Functional
Functional
Indirect Trust (Inference)
‫مستقیم‬ ‫غیر‬ ‫اعتماد‬(‫استنتاج‬)
Recommenders (‫گران‬‫)توصیه‬

Reputation ( ‫شهرت‬/‫وجهه‬ )
 Definition (Concise Oxford Dictionary)
A widespread belief that someone or
something has a particular characteristic.
Common belief
 Relationship with Trust
Trust is subjective and has more weight
“I trust you because of your good reputation”
“I trust you despite your bad reputation”
1390/8/2 - 2011/10/24 Trust in the Virtual World - Sadegh Dorri N. (http://ce.sharif.edu/~dorri) 7

TRUST AND REPUTATION
IN ACTION
8

Aspects of a Trust System [HZN09]

Applications of Trust
 Soft Security Mechanism against
Low quality services
Misrepresentation of services
Incorrect information
Fraud
 Others
Recommender and filtering systems
 Targets
Content, services, people

Attacks on a Trust-Based System
 Self-Promotion
Falsely increase the trust on the attacker(s)
 Whitewashing (‫)الپوشانی‬
Restoring the broken trust
 Slandering (‫کردن‬ ‫)الغر‬
Falsely reduce the trust on other nodes
 Other
Hybrid of the above attacks, DoS, …

Example: Reputation in a P2P
System
● Nodes have no information about most others
● Fake or virus infected content
● Free riders
● Challenges
● Anonymity -> selfish users
● Highly distributed
● Unreliable network connections
● Partial information (in unstructured topologies)
● Untrustworthiness of storage peers

Example: Email Filtering
● Blind delivery of messages
● In Jan.of 2008, 75% of Internet email was spam.
● Detecting spam after delivery wastes a lot of
resources and is error-prone.
● KarmaNET [SXMW09]
● Messages are routed through social paths
● Trust is defined in three aspects:
– Routing (against free-riders)
– Forwarding (distinguish malicious nodes from careless
forwarders)
– Initiation
● Bad messages penalize all related peers, so they
(automatically) tune their behavior

Example: Social Routing
● Blind routing
● Has inherent security problems such as DDoS
and Spam
● No separation between routing addr. & identity
● Lack of msg. receiver control
● Solutions are not scalable and/or inefficient
● DSL [BYHW09]
● Messages are routed through social paths
between sender and receiver, and based on the
keywords describing the intention of the
message.

Example: Security and Privacy with
Trust
 Probabilistic Security
Security is not definite in many cases
Hard-to-break security: birthday attack
Trust can be used to tune the desired security
 Access control
User levels are determined using trust metrics
(Advogato, StackExchange)
Hybrid security policy: super computer example
 Privacy in Anonymity Networks

Other Applications
● News syndication
● Using trust in order to resolve contradictions
in information
● Discard the statements from the least trusted
sources
● Recommender systems
● To use trust in place of similarity
● Users are significantly more similar to their
trusted peers than to the population as a
whole

Simple Summation or Average of
Ratings
● More advanced: weighted average
● Trustworthiness/reputation
● Age of the rating
● Distance between rating and current
score

 

i
ii
w
wr


Probabilistic Engines
 Trust: Expected Probability of Success
 Bayesian Approach [JI02]
Use the Bayes rule to update p
 HMM Approach [ElS10]
Use a Hidden Markov Model to calculate p
},{ xxR 
),,|Pr( ,,
1
,, tetr
tn
tetr
t
tetr
t
tetr
t OOxOp 
][ ,, tetr
t
tetr
t pE
2
1



sr
r


Trust Inference
● Trust in an unknown peer can be
inferred according to paths in social
networks
● Strongest path
● Weighted paths
● BFS-like (TidalTrust)
● Probabilistic and Bayesian methods
● Subjective logic operators

Subjective Logic [JHP06]
),,,( audbA
B  B
C
A
B
BA
C  : B
C
A
B
BA
C  

Other Engines
 Fuzzy Inference Engines
Direct trust: multi-criteria decision making
Trust inference: fuzzy aggregation operators
 Game Theoretic Approaches
Try to defend strategic attacks
 Many Proposals:  or  ?
Evaluation: human-based vs. utility-based
Must be related to human notion of trust

A DETAILED EXAMPLE
Trust in CROWDS
23

The CROWDS Protocol [RR98]
 Provides Anonymous Web Transactions
A user is either completely honest or dishonest
The originator passes the message to a randomly
selected path of users to reach destination (the reverse
for reply).
 Probability of Forwarding
1-pf: forward to the end server
pf: forward to a random user
 Privacy (Anonymity) Level: Probable Innocence
… the sender appears no more likely to be the
originator than to not be.

CROWDS + Trust
 Extended Protocol [SEH10]
The users may switch between honest and
dishonest.
Trust (reputation) info + forwarding policy
 ti (in [0,1]): The Reputation of a User
Robustness of user i to becoming corrupt
(probability)
 {q1,…,qn}: The Forwarding Policy
Common to all users
qi: The probability of forwarding to user i

Anonymity in CROWDS + Trust
 Guaranteeing Probable Innocence
Idea: adjust the forwarding policy according to
reputation values
Solve the following system of linear inequalities
to find the desired forwarding policy(ies):
2
1

Anonymity in CROWDS + Trust (cont.)
 Example with Three Principles
The equations yield two solutions:
A possible choice:

Comparing with the Original CROWDS
 Forwarding Policy of the original protocol:
Does not satisfy the innocence inequalities
 Consequence:
If the users are partially honest, the CROWDS
may not provide probable innocence.
Trust information can be used to provide the
required anonymity.

Conclusions
● Trust in the VW
● Translating social concepts to computational
methods
● Many applications
● Sound mathematical basis
● Trust as a Soft Security Mechanism
● Access control
● Probabilistic security
● …
● A Long Way in Front!

THANKS!
More Info. on My Homepage:
http://ce.sharif.edu/~dorri
30

References
[BYHW09] L. Banks, S. Ye, Y. Huang, and S. F. Wu, “Davis social links: integrating
social networks with internet routing,” in Proceedings of the 2007 Workshop on
Large Scale Attack Defense (LSAD’07), New York, NY, USA, 2007, pp. 121–128.
[CF10] C. Castelfranchi and R. Falcone, Trust theory: a socio-cognitive and
computational model. Chichester, West Sussex, England: Wiley, 2010.
[ElS10] E. ElSalamouny, “HMM-based trust model,” Revised Selected Papers of the 6th
International Workshop on Formal Aspects in Security and Trust (FAST), Eindhoven,
The Netherlands, Nov. 2009, vol. 5983, pp. 21-35, 2010.
[Gol06] J. Golbeck, “Trust on the World Wide Web: A Survey”, Foundation and Trends
in Web Science, vol. 1, no. 2, pp. 131–197, 2006.
[HZN09] K. Hoffman, D. Zage, and C. Nita-Rotaru, “A survey of attack and defense
techniques for reputation systems,” ACM Computing Surveys, vol. 42, no. 1, pp. 1-
31, Dec. 2009.
[JHP06] A. Jøsang, R. Hayward, and S. Pope, “Trust network analysis with subjective
logic,” in Proceedings of the 29th Australasian Computer Science Conference -
Volume 48, Hobart, Australia, 2006, pp. 85-94.
[JI02] A. Jøsang and R. Ismail, “The Beta Reputation System,” in Proceedings of the
15th Bled Conference on Electronic Commerce, Bled, Slovenia, 2002.

References (cont’d)
[SEH10] V. Sassone, E. ElSalamouny, and S. Hamadou, “Trust in Crowds:
Probabilistic Behaviour in Anonymity Protocols,” in Trustworthly Global
Computing, vol. 6084, M. Wirsing, M. Hofmann, and A. Rauschmayer, Eds.
Berlin, Heidelberg: Springer Berlin Heidelberg, 2010, pp. 88-102.
[SXMW09] M. Spear, Xiaoming Lu, N. Matloff, and S. F. Wu, “KarmaNET:
Leveraging trusted social paths to create judicious forwarders,” in
Proceedings of the 1st International Conference on Future Information
Networks (ICFIN), Beinjin, China, 2009, pp. 218-223.
[RR98] M. K. Reiter and A. D. Rubin, “Crowds: anonymity for Web transactions,”
ACM Transactions on Information Systems Security, vol. 1, no. 1, pp. 66–92,
Nov. 1998.

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