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Transcript of Dimitri DeFigueiredo Earl Barr S. (Felix) Wu Adobe Systems Inc. UC Davis UC Davis International...
Beauty Is in the Eye of the Beholder
Dimitri DeFigueiredo Earl Barr S. (Felix) Wu
Adobe Systems Inc. UC Davis UC DavisInternational Conference on Privacy, Security, Risk and Trust
2009
1
OverviewMotivationConceptual PropertiesFormal ModelTheoremsKey Point
2PASSAT 2009
Can I trust him?What is it?What is it used for?
3PASSAT 2009
Trust
Trust is useful.
What is it?What is it used for?Ratings are not the same as trust. Why not?
4PASSAT 2009
How do we use Trust?
Trust helps us avoid risks that depend on othersWe don’t take risks with people we do not trustWe are vulnerable to the actions of people we
trust
Trust is under each person’s complete control
I trust you as much as I want5
PASSAT 2009
Basic PropertiesTwo-role ruleDomain specificComplete orderingHigher is better
6PASSAT 2009
Trust Values
Definition: Trust values are real numbers.
Think in dollar$
“How much would you bet C will be on time for dinner?”
7PASSAT 2009
Reputation Graph
From your experience, how much would you bet C is a good seller?
8PASSAT 2009
$100
BA
C
D
E
$100
$20
$50
$10
$60
$30
$10
World
G0 G1 Gk
A World is a sequence of reputation graphs
9PASSAT 2009
…
Reputation Function
World Trust Graph
f
…
10PASSAT 2009
Trust Graph
11PASSAT 2009
5010
A B
CD
20
Trust Graph
12PASSAT 2009
5010
A B
CD
?
??
??
?
?
??20
Trust Threshold
Trusted orUntrusted?
hA=30
hC=9
13PASSAT 2009
5010
A B
CD
20
Trust - Definition
Trust is the personal threshold determined by the trusting party that describes the
maximum utility the trusting party is willing to risk when dealing with the trusted party.
15PASSAT 2009
A
D C
B
Types of Reputation Function
Trivial
19PASSAT 2009
A B
CD
A
D C
B
0G
1G
Types of Reputation Function
TrivialConsensus-based
20PASSAT 2009
A
D C
BA B
CD
30
3030
= 30
Types of Reputation Function
TrivialConsensus-basedPersonalized
21PASSAT 2009
A
D C
BA B
CD
50
3040
A Collusion
22PASSAT 2009
5010
A B
CD
20
An Untrusted CollusionhA=30
23PASSAT 2009
50
5010
A B
CD
2010
WW
A Manipulated World
24PASSAT 2009
5010
A B
CD
2010
A
D C
B?
A B
CD
??
W
Manipulated W
f
f
hA=30
Non-exploitability
For any trust threshold h a vertex chooses, no untrusted collusion can fool it.
25PASSAT 2009
A
D C
B?
A B
CD
??
Manipulated W
f
Theorems
Impossibility
All non-exploitable consensus-based reputation functions are trivial
Possibility
There are non-trivial non-exploitable personalized reputation functions
26PASSAT 2009
Impossibility Proof SketchhA = 100
27PASSAT 2009
5010
A
C=20D=10
2010
B=50
What if hC = 30 ?!
Possibility Proof SketchConstructive proofFor a reputation threshold λ=0.5Define:
λ=0.5
28PASSAT 2009
otherwise,0
exists to frompath trusteda if ,1, ji
ji
vvvvf
0.6 0.7 1
1
1
0G
B
A
C
Possibility Proof Sketch
Non-trivial
λ=0.5
29PASSAT 2009
0.6 0.7 1
1
1
0
B
A
C
otherwise ,0
exists to frompath trusteda if ,1, ji
ji
vvvvf
Possibility Proof Sketch
Non-exploitableFor any trust threshold h set by any trusting
party:untrusted node is not reachableunreachable node node cannot
make other nodes reachable or unreachable
30PASSAT 2009
1
1
1
B
A
C
otherwise ,0
exists to frompath trusteda if ,1, ji
ji
vvvvf
Key Point
Trust is a personal value.Rating systems should allow different people to have different points of view
31PASSAT 2009
Questions?
Thank you!
Full version
http://www.cs.ucdavis.edu/~defigued/
32PASSAT 2009