Efficient Secure Aggregation in VANETs Maxim Raya, Adel Aziz, and Jean-Pierre Hubaux Laboratory for computer Communications and Applications (LCA) EPFL

2 Outline Motivation Attacker model Secure group formation Secure aggregation mechanisms Simulation results Conclusion

3 Why efficient secure aggregation? VANET security is indispensable but expensive De facto security: limited flooding of signed messages Since many vehicles broadcast the same event, why not try aggregation? 1.Can we make it work in VANETs? 2.And can we make it secure? The answer is in this presentation and it is: YES

4 How to make aggregation efficient and secure? Requirements: Channel efficiency Low delay Data correctness Non-repudiation We propose 3 solutions: Combined signatures Overlapping groups Dynamic group key creation

5 Who is the attacker? Major threat: false information dissemination Assumption: Any group of more than 2 vehicles should contain a majority of honest nodes under normal density conditions

6 The secret of efficient aggregation: groups Information is relayed between groups, not individual vehicles

7 How to make a group? Preset groups: efficient but not flexible On-the-fly groups: flexible but not efficient Location-based groups: efficient and flexible The keyword is where and not who a vehicles neighbors are Group formation step-by-step: 1.Dissect the map into small area cells, each defining a group 2.Load map dissection function/dissected maps into vehicles 3.Cells (groups) overlap to ensure handover 4.One option for leader election: group leader = vehicle closest to center (with lowest ID if many), elected for a given duration 5.A vehicle checks its GPS position to determine its cell (group)

8 Group formation

9 Cell Overlap TX range = 300 m Cell size = 400 m Leader Not to scale

10 Group formation I am in cell X

11 SVGP (Secure VANET Group Protocol) Goal: establishment of a symmetric group key Secure groups protect the network from outsiders only Concept: group leader transports group key to members Subsequent messages include only a HMAC On leave, nothing needs to be done Vehicles at boundaries receive messages from 2 groups

12 Aggregation mechanism 1: Combined signatures Concept: a group of vehicles reporting the same event combine their signatures Advantages: Overhead is grouped in one message => better channel efficiency A groups combined message => the group agrees on the content Three types of combined signatures: m = message, S = Signature, C = Certificate

13 Aggregation mechanism 2: Overlapping groups Concept: vehicles in the intersections of groups make a bridge for data Group keys and messages are distributed using SVGP The good: Cheap symmetric crypto The bad: Need for position verification Need for honest majority Lack of non-repudiation

14 Aggregation mechanism 3: Dynamic Group Key Creation Conciliates low overhead (symmetric crypto) with non-repudiation (digital signatures) Dynamic group scenarios (e.g., platoon) Step-by-step: 1.The leader sends a key request to the CA (Certificate Authority) 2.The CA generates an asymmetric group key pair and unique IDs for members (for non-repudiation) 3.Vehicles sign messages with the new group key and include their ID

15 Simulation results ns-2 simulator Rice scenario generator EPFL VANET patch (available at Cell size: 400 meters ECC with key size of 256 bits 100 simulations Simulated mechanism: concatenated signatures Correctness level of messages: number of supporting signatures to consider a message correct. It is 4 in our simulations 2400 m Scenario Source Destination

16 Effect of density on channel usage

17 Effect of density on message delay

18 Effect of speed on channel usage

19 Effect of speed on message delay

20 Efficiency vs. Security (correctness level) Destination aggregation Source aggregation

21 Conclusion Objective: the tradeoff between efficiency and security Efficient secure aggregation is a feasible answer: Combined signatures Overlapping groups Dynamic group key creation The advantages: Better channel usage Lower message delivery delay Better data correctness and hence security Visit and

22 SEVECOM (SEcure VEhicular COMmunication) Objectives: Identification of threats and Specification of a security architecture

23 CALL FOR PAPERS IEEE Journal on Selected Areas in Communications Vehicular Networks Architecture of Vehicular networks Vehicle-to-Vehicle Vehicle-to-Roadside Security and privacy Cross-layer optimization techniques Mobility and traffic models Protocol design (low-power, multi-channel, etc.) PHY, MAC, Network Layer (Routing protocols) Channel Modeling Cooperative aspects of vehicular communication Scalability and Availability issues in Vehicular networks Safety and commercial applications Manuscript SubmissionFebruary 1, 2007 Acceptance Notification May 15, 2007 Final Manuscript Due to PublisherJuly 1, 2007 Publication Date3rd Quarter