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Secure Protocols for Behavior Enforcement

Security and Cooperation in Wireless Networks. Secure Protocols for Behavior Enforcement. Slides elaborated by Julien Freudiger and adapted by Jean-Pierre Hubaux http://secowinet.epfl.ch

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Secure Protocols for Behavior Enforcement

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  1. Security and Cooperationin Wireless Networks Secure Protocols for Behavior Enforcement Slides elaborated by Julien Freudiger and adapted by Jean-Pierre Hubaux http://secowinet.epfl.ch Note: this chapter (and therefore this slide show) is derived from the paper by S. Zhong, L. Erran Li, Y. Liu, and Y. R. Yang, “On Designing Incentive-Compatible Routing and Forwarding Protocols in Wireless Ad Hoc Networks”, Mobicom 2005

  2. Motivation • Packet forwarding consumes resources • Nodes are rational => Maximize their payoff • Nodes avoid forwarding Provide incentive to cooperate within Routing and Forwarding protocols using a Game Theoretic approach

  3. Outline • Introduction • Incentives • System Model • Formal Model • Dominant action/subaction • Cooperation optimal protocol • The Corsac Protocol • VCG payments with correct link cost establishment • Forwarding protocol with block confirmation • Evaluation • Conclusion

  4. 1. Introduction • Routing protocol • Discover efficient routing paths:global welfare • Deal with selfish nodes: local welfare • Packet forwarding protocol • address the fair exchange problem => Joint Incentive

  5. Incentives • Incentive strategy: • Punish:Reputation, Jamming, Isolation • Reward: Virtual currency • Incentive is achieved: • Internally:With 802.11 primitives • Externally: Dedicated protocols Incentive Punish Reward Internal External Internal External

  6. System Model • Ad-hoc networks as uncooperative strategic games • Called Ad Hoc Games • Channel model: • Packet successfully transmitted if Ptransmission >= Pmin • Pmin = minimum power to reach destination • No errors (BER = 0) • Nodes can withhold, replace or send a message • Node can transmit at any power level • We define the payoff of a node as: • bi = benefice (reward) • ci = cost of forwarding

  7. 2. Formal Model • Dominant Action: • A dominant action is one that maximizes player i payoff no matter what actions other players choose Example: Joint packet forwarding game • Imperfect information • Message from S to D • Two players: p1 and p2 • P1 has no dominant action • P2 dominant action is F S P1 P2 D

  8. Forwarding Dominant • A forwarding protocol is said forwarding dominant protocol if following the protocol is a dominant action • We need incentives to enforce cooperation • Theorem 1: • There does not exist a forwarding-dominant protocol for ad-hoc games.

  9. Formal Model for Divided Solution • Each node actions is divided into two parts: • Routing subaction: A routing decision specifies what node is supposed to do in the forwarding stage • Forwarding subaction: Specifies what the node actually does • The total payoff comprises both subactions

  10. Routing stage • Routing payoff of a node is the payoff that it will achieve under the routing decision • Dominant subaction: • In a routing stage, a dominant subaction is one that maximizes its routing payoff no matter what subactions other players choose. • A routing protocol is a routing-dominant protocol to the routing stage if following the protocol is a dominant subaction of each potential forwarding node in the routing stage

  11. Forwarding stage • Consider an extensive game model with imperfect information • A forwarding protocol is a forwarding-optimal protocol to the forwarding stage under routing decision R if • All packets are forwarded to their destinations • Following the protocol is a subgame perfect equilibrium • A path is said to be a subgame perfect equilibrium if it is a Nash equilibrium for every subgame Node 1 drop forward Node 2 drop forward Last node drop forward

  12. Cooperation-Optimal Protocol • A protocol is a cooperation-optimal protocol to an ad-hoc game if • Its routing protocol is a routing-dominant protocol to the routing stage • For a routing decision R, its forwarding protocol is a forwarding optimal protocol to the forwarding stage

  13. 3. The Corsac Protocol • Corsac is a cooperation optimal protocol • Routing: • VCG • Forwarding: • Reverse Hash chains

  14. VCG for routing protocols • Nodes independently compute and declare their packet transmission cost to destination • Destination computes Lowest Cost Path (LCP) • Source rewards the nodes • declared cost + added value • The added value is the difference between LCP with the node and without it • Incentive to declare the true price => Truthful

  15. Example of VCG Least cost path from S to D: LCP(S,D) = S, v2, v3,D with cost(LCP(S,D)) = 5 + 2 + 3 = 10 Least cost path without node v2: LCP(S,D;−v2) = S, v1, v4,D with cost(LCP(S,D);−v2) = 7 + 3 + 4 = 14 Least cost path without node v3: LCP(S,D;−v3) = S, v2, v4,D with cost(LCP(S,D);−v3) = 5 + 3 + 4 = 12. VCG payments: p2 = 14 − 10 + 2 = 6 p3 = 12 − 10 + 3 = 5 These values represent the unit payment (the payment for one forwarded data packet) to nodes v2 and v3, respectively.

  16. VCG flaw • Assume mutual computation of link cost • Consider a node i and its neighbor j • Node i cheats by making Pi,jgreater: • Node j is less likely to be on LCP • Node jpayment will decrease. • Node j responds by cheating and making Pi,jsmaller: • Node j more likely to be on LCP • Node j increases its payment • VCG is not truthful in this case • Possible to cheat in determining link cost Pi,j i j

  17. Truthful VCG • Assume private computation of link cost • Protocol for VCG link cost establishment: • Nodes share a symmetric key with D • Nodes send an encrypted and signed test signal at increasing power levels containing cost information • Messages are protected from forging with HMAC • O(N^3) [cost4]K¦HMAC [cost4]K¦HMAC [cost3]K¦HMAC [cost3]K¦HMAC i j D [cost2]K¦HMAC [cost1]K¦HMAC

  18. VCG conclusion • Theorem 2: • If the destination is able to collect all involved link costs as described above, then the VCG protocol is a routing dominant protocol to the routing stage.

  19. Forwarding Protocol • Messages bundled in blocks • Block confirmation with a Reverse Hash Chain • r is made public by source in an authenticated way • Confirmation of block 2 is done by sending r(5-2)=r3 • Nodes verify m1 m2 m3 m4 m5 m6 m7 m8 m9 b1 b2 b3 b4 b5 r1 r2 r=r5 H H H H r0

  20. Fair Exchange Problem • Source and intermediate nodes can disagree about successful transmission of a block • Mutual decision = contract between source an intermediate nodes • Confirmation is sent with the last packet of each block to destination • Destination forwards confirmation to intermediate nodes if block correctly received • Intermediate nodes stop forwarding if do not get confirmation • Eliminates incentive to cheat • Disregarding the protocol blocks the protocol

  21. Cooperation Optimal • Theorem 3: • Given a routing decision R, assuming that the computed payment is greater than the cost, the reverse hash chain based forwarding protocol is a forwarding optimal protocol. • Theorem 4: • The Corsac protocol is a cooperation-optimal protocol to ad-hoc games.

  22. 4. Evaluation (1) • Nodes that accumulate more credits spend more energy in forwarding others’ traffic => The protocol is fair

  23. Evaluation (2) Consider the following topology:

  24. Evaluation (3) Node 19 as session source: Reach destination directly + = payment X = cost

  25. Evaluation (4) Node 28 as session source: Node 3 is critical point + = payment X = cost Mainly the topology that determines payment

  26. Future challenges • Modeling • Interference and mobility • unreliable link harden use of incentive • Game theoretic model assumes • Tamper proof Hardware to compute best path at destination • Payment center to resolve payment issues • Performance vs. incentive compatibility • Control channel overhead • Throughput • Complexity

  27. 5. Conclusions • Cooperation optimal protocol • Routing dominant + Forwarding optimal • Routing based on VCG • Forwarding based on Reverse Hash Chain • Corsac provides incentives for cooperation • Protocol is fair • The topology determines payment • The incentive protocol reduces the network traffic

  28. References [1] « On Designing Incentive-Compatible Routing and Forwarding Protocols in Wireless Ad-Hoc Networks ».Sheng Zhong, Li Erran Li, Yanbin Grace Liu and Yang Richard Yang. Mobicom 2005 [2] « Security and Cooperation in Wireless Networks». Levente Buttyan and Jean-Pierre Hubaux. Book Cambridge University Press, Chapter 12 [3] « Punishement in Selfish Wireless Networks: A Game Theoretic Analysis». Dave Levin. NetEcon 2006 [4] « On Selfish Behavior in CSMA/CA Networks ». Mario Cagalj, Saurabh Ganeriwal, Imad Aad and Jean-Pierre Hubaux. Infocom 2005 [5] « Ad hoc-VCG: A Truthful and Cost-Efficient Routing Protocol for Mobile Ad hoc Networks with Selfish Agents ». Luzi Anderegg and Stephan Eidenbenz. Mobicom 2003

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