© 2007 Levente Buttyán and Jean-Pierre Hubaux Security and Cooperation in Wireless Networks Chapter 12: Secure protocols for

© 2007 Levente Buttyán and Jean-Pierre Hubaux

Security and Cooperation in Wireless Networks

http://secowinet.epfl.ch/

Chapter 12: Secure protocols for behavior enforcement

Security and Cooperation in Wireless NetworksChapter 12:Secure protocols for behavior enforcement 2/22

Motivation

Provide incentive to cooperate

within Routing and Forwarding protocols using a game theoretic approach

Packet forwarding consumes resources– Nodes are rational => Maximize their own payoff– Nodes avoid forwarding


Outline

• Introduction– Incentives– System Model

• Model– Dominant action/subaction– Cooperation optimal protocol

• Protocols– VCG payments with correct link cost establishment– Forwarding protocol with block confirmation

• Conclusion


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


Possible incentives

Incentive

Punish Reward

Internal External Internal External

Possible incentive strategies:– Punish: Reputation, Jamming, Isolation

– Reward: Virtual currency

Possible incentives:– Internally: With intrinsic mechanisms (e.g., deny

communication, jam)

– Externally: by dedicated protocols


System Model

Ad-hoc networks as non-cooperative strategic games • Called “Ad Hoc Games”

Channel model: • Packet successfully transmitted if Ptransmission >= Pmin

– Pmin = minimum power to reach receiver

• No errors (BER = 0)

Nodes can withhold, replace or send a message

Nodes can transmit at any power level

We define the payoff of a node as:– bi = benefit (reward, by micro-payment)– ci = cost of forwarding (energy, overhead,…)

iii cbu


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’s dominant action is F

iiiiii aauaau ,,

S P1 P2 D

p1\p2 F D

F (1-c,1-c) (-c,0)

D (0,0) (0,0)


Formal Model

Each node action is comprised of two parts: is node i’s subaction in the routing stage

(what it is supposed to do in the routing stage) is node i’s subaction in the forwarding stage (what it really does in the forwarding stage)

,r fi i ia a a

, ri iu u R a

fia

ria

• Routing decision R: determined by the routing subactions of all nodes • Prospective routing payoff:

ra


Routing stage

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

, ,R r r R r ri i i i i iu a a u a a


Forwarding stage

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

Node 2

Last node

forward

forward

forward

drop

drop

drop

p1\p2 F D

F (1-c,1-c) (-c,0)

D (0,0) (0,0)


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


VCG for routing protocols

VCG: Vickrey, Clarke, and Groves – second-best sealed auction

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


Example of VCG

Least cost path from S to D:LCP(S,D) = S, v2, v3, Dwith cost(LCP(S,D)) = 5 + 2 + 3 = 10 Least cost path without node v2:LCP(S,D;−v2) = S, v1, v4, Dwith 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:b2 = 14 − 10 + 2 = 6b3 = 12 − 10 + 3 = 5

These values represent the unit payment (the payment for one forwardeddata packet) to nodes v2 and v3, respectively.


Cheating about the power level

Assume mutual computation of link cost

Consider a node i and its neighbor j1. Node i cheats by making Pi,j larger:

– Node j is less likely to be on LCP– Node j’ s payment will decrease.

2. Node j can respond by cheating and making Pi,j smaller:– Node j more likely to be on LCP– Node j increases its payment

VCG is thus not truthful in this case

i jPi,j


Cryptographic protection

Assume private computation of link cost

(the details of the security mechanisms are in the book)

Protocol for 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– Complexity (computation at the destination): O(N^3)

i j[cost3]K¦HMAC D

[cost2]K¦HMAC

[cost1]K¦HMAC

[cost4]K¦HMAC

[cost3]K¦HMAC

[cost4]K¦HMAC


Conclusion on the routing stage

Theorem 12.1:

If the destination is able to collect all involved link costs as described above, then the described protocol is a routing dominant protocol to the routing stage.


r1

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 r5-2=r3

– Nodes verify

m1 m2 m3 m4 m5 m6 m7 m8 m9

b1 b2 b3 b4 b5

Hr0 H Hr2 r=r5

H

rrH 32


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 they do not get

confirmation

Eliminates incentive to cheat– Not respecting the protocol blocks the protocol


Theorems

Theorem 12.2:

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 12.3:

The complete protocol (routing protocol and packet forwarding protocol) is a cooperation-optimal protocol to AdHocGames.


Discussion

Modeling – Interference and mobility

• unreliable links make use of incentives more difficult

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


Summary

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


References 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. ACM Springer Wireless Networks (WINET), Special Issue of Selected Papers of Mobicom 2005

Punishement in Selfish Wireless Networks: A Game Theoretic AnalysisDave Levin. NetEcon 2006

On Selfish Behavior in CSMA/CA NetworksMario Cagalj, Saurabh Ganeriwal, Imad Aad and Jean-Pierre Hubaux. Infocom 2005

Ad hoc-VCG: A Truthful and Cost-Efficient Routing Protocol for Mobile Ad hoc Networks with Selfish AgentsLuzi Anderegg and Stephan Eidenbenz. Mobicom 2003

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© 2007 Levente Buttyán and Jean-Pierre Hubaux Security and Cooperation in Wireless Networks Chapter 12: Secure protocols for