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TrustDavis: A Non-Exploitable Online Reputation System
Dimitri DeFigueiredo and Earl T. BarrDept. of Computer Science, University of California at Davis
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TrustDavis: A Non-Exploitable Online Reputation SystemD. DeFigueiredo and E. T. Barr Computer Science, UCDavis
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Motivation
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TrustDavis: A Non-Exploitable Online Reputation SystemD. DeFigueiredo and E. T. Barr Computer Science, UCDavis
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Motivation
• Should we buy?• How do we decide?
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Motivation
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Motivation
• Should we buy?• How do we decide?
• What we want:– accurately estimate risk of default– minimize the risk of default– minimize losses due to pseudonym change– avoid trusting a centralized authority
• How do we achieve these goals?
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Motivation
• TrustDavis is a reputation system that realizes these goals.
• It recasts these goals as the following properties:
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Motivation
1. Agents can accurately estimate risk– Third parties provide accurate ratings
2. Honest buyer/seller avoids risk (if possible)– Insure transactions
3. No advantage in obtaining multiple identities – Agents can cope with pseudonym change
4. No need to trust a centralized authority– No centralized services needed
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Motivation
Incentive Compatibility:
Each player should have incentives to perform the actions that enable the system to achieve a desired global outcome.
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Motivation
1. Agents can accurately estimate risk– Third parties provide accurate ratings
2. Honest buyer/seller avoids risk (if possible)– Insure transactions
3. No advantage in obtaining multiple identities – Agents can cope with pseudonym change
4. No need to trust a centralized authority– No centralized services needed
Incentive Compatibility!
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Motivation
A Reference is:Acceptance of Limited Liability.
$100BAC
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Motivation
1. Agents can accurately estimate risk– Third parties provide accurate ratings– Parties are liable for the references they provide
2. Honest buyer/seller avoids risk (if possible)– Insure transactions– Buyers/sellers pay for references to insure their transactions
3. No advantage in obtaining multiple identities – Agents can cope with pseudonym change– References are issued only to trusted identities
4. No need to trust a centralized authority– No centralized services needed– Anyone can issue a reference
Use References!
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Motivation
Related Work:• Z. Abrams et al. – Workshop P2P Econ 2004• S. Buchegger et al. – Workshop P2P Econ 2004• C. Dellarocas – Ideabook 2004• S. Lee et al. – IEEE Infocom 2003• S. Kamvar et al. – “EigenTrust” 2003• J. Golbeck et al. – Sematic Web 2003• T. Riggs et al. – ACM/IEEE-CS CDL 2001
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Outline
• Motivation• The Model
– Buying references– Selling references
• A Non-Exploitable Strategy• Future Work• Conclusion
– Key ideas
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Outline
• TrustDavis leverages social networks
• For now, examples assume No False Claims (NFC)
• The use of TrustDavis does NOT preclude trade outside the system.
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Paying for References
150
150100
50
50
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Outline
• Motivation• The Model
– Buying references– Selling references
• A Non-Exploitable Strategy• Future Work• Conclusion
– Key ideas
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• vb wants to buy three shirts.• Shirts cost $100 each from a
trustworthy seller• Unknown seller offers shirts for
$50 each (but maybe they are only worth $25).
• vb would risk 3 x $50 = $150 in the transaction
• vb can borrow and lend money at rate r=1.25 through the period of the transaction
For $30, vb can insure herself!
Paying for References
How much is vb willing to pay to insure the transaction? (No riskless profitable arbitrage criterion)
Example:
$100 eachTrust-me.com
Blowout SALE!
$50 each!$150!
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Paying for References
To insure herself vb buys the shirts and a hedging portfolio as follows:
1. Instead of buying 3 shirts for $50 each she buys only 2, saving $50.
2. The buyer, vb , adds $30 of her own money and lends the resulting $80 at rate r = 1.25.
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Paying for References
On Success:– vb obtains $100 from the loan and buys
the 3rd shirt
On failure:– vb sells the two shirts for $25 each – gets $100 from the loan. – She obtains a total of $150
Thus, vb can insure herself for $30.
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TrustDavis: A Non-Exploitable Online Reputation SystemD. DeFigueiredo and E. T. Barr Computer Science, UCDavis
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Outline
• Motivation• The Model
– Buying references– Selling references
• A Non-Exploitable Strategy• Future Work• Conclusion
– Key ideas
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Selling References
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Selling References
Seen as an investment…
On Success the ROI is:
On failure the ROI is:
If repeated many times the insurer may go bankrupt. Assume the insurer has W dollars available to insure this transaction.
KC
KCK
1
KC
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Selling References
Insurer maximizes the expected value of the growth rate of capital (Kelly Criterion).
For given:– probability of failure p,– a desired growth rate of capital R; and,– fraction of the total funds W being risked in a transaction.
The insurer can obtain a lower bound on the premium C.
nn
WWER
1
0
log
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Selling References
Insured Value as a fraction of total funds – f
Cos
t/Ins
ured
Val
ue –
C/K
Minimum Return/Risk Ration for Different Failure Probabilities
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Outline
• Motivation• The Model
– Buying references– Selling references
• A Non-Exploitable Strategy• Future Work• Conclusion
– Key ideas
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A Non-Exploitable Strategy
Two Scenarios:• No False Claims - NFC• With False Claims - FC
False claims only change the probability p.We can incorporate the cost of verification.
Key Idea:
Save part of the money obtained in successful transactions in excess of the opportunity cost.
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A Non-Exploitable Strategy
Example.
The buyer, vb, has $190 to spend on 1 of 3 options:
1. Buying 3 shirts from an unknown seller for $50 each and insuring the transaction for $40. She values each shirt at $100.
2. Buying 2 pairs of shoes from a reliable retailer for $70 each. She thinks each pair is worth $90.
3. Buying 1 game console for $150, from a reliable online shop. She values the console at $240.
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A Non-Exploitable Strategy
vb’s valuation for each of the 3 options is:
1. Shirts: 100 x 3 + 0 (no cash leftover) = $300
2. Pairs of Shoes: 90 x 2 + 50 (cash) = $230
3. Console: 240 x 1 + 40 (cash) = $280
Gains in excess of the opportunity cost are:300-280=$20.
Part of these $20 should be saved to insure future transactions.
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A Non-Exploitable Strategy
The Strategy:
1. Initially only provide references to known agents or those that leave a security deposit.
2. Insure all trade through references provided by trusted agents.
3. Do not provide more insurance than you can recover. Charge at least the lower bound for providing a reference.
4. Save part of the money received “in excess of the opportunity cost”.
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A Non-Exploitable Strategy
150
150100
50
50
50
OK!$10 saved to
provide future insurance 10
Failed!Payment made
automatically by v1
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Outline
• Motivation• The Model
– Buying references– Selling references
• A Non-Exploitable Strategy• Future Work• Conclusion
– Key ideas
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Future Work
• Simulation– sensitivity to estimates of p– growth rate of capital– dynamic behavior
• Price Negotiation– should avoid “double spending” problem– fair distribution among insurers of the premium
paid
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Outline
• Motivation• The Model
– Buying references– Selling references
• A Non-Exploitable Strategy• Future Work• Conclusion
– Key ideas
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Conclusion
TrustDavis provides:• Accurate Ratings• Non-exploitable strategy for honest agents• Pseudonym change tolerance• Decentralized infrastructure
Through the use of References.
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Conclusion
Key Ideas:
• Incentive Compatibility– Incentive to accurately rate– Incentive to insure– No incentive to change pseudonym
• Saving gains in excess of the opportunity cost to insure future transactions.
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The End
Questions?
Thank you!
{defigueiredo,etbarr}@ucdavis.edu