1. Copyright © 2007 VDG, Sept 27, 2007 1 Handling New Adversaries in Secure MANETs Virgil D. Gligor Electrical and Computer Engineering University of Maryland College Park, MD. 20742 gligor@umd.edu ZISC Wireless Security Workshop Zurich September 27-28, 2007 * based on joint work with S. F. Bahari

2. Copyright © 2007 VDG, Sept 27, 2007 2 Overview 1.New Adversary: Different from DY and Byzantine Models - capabilities: node capture, replication 2. An Approach for Handling Node Capture - example of emergent property 3. Ongoing and Future Research

3. Copyright © 2007 VDG, Sept 27, 2007 3 Approaches for Handling New Adversary 1.Detection and Recovery - Ex. Detection of node-replica attacks [Parno et al 2005] -Cost ? Traditional vs. Emergent Protocols -Advantage: always possible, good enough detection -Disadvantage: “when you’ve been had, you’ve been had by a professional” [S. Lipner, cca. 1985] 2.Avoidance: early detection of node capture - Ex. Periodic monitoring (depending on node protection) - Cost vs. timely detection ? False positives ? Missed detection? - Advantage: avoids damage done by new adversary - Disadvantage: cannot always be used (e.g., disconnected nodes – are these really networked ?) 3. Future: “prevent” attacks - questionable proposition

4. Copyright © 2007 VDG, Sept 27, 2007 4 Avoidance: Periodic Monitoring of Target Nodes Observation: Access to Node State (e.g., keys, memory content) requires the node to be taken “off-line” for time X - X is a random variable depending on - node security; e.g., quality of content obfuscation, physical protection - node overload; e.g., on-line attempts to access Node State - node failure; e.g., tampering with node while on-line leads to failure Idea: Node Status (on-, off-line) Monitoring by Neighbors in time T - T < X, capture (i.e., node offline) is always detected - T >= X, capture is never detected Key Design Parameters - cost (i.e., no. and frequency of messages) - false alarm rate - missed detection rate

5. Copyright © 2007 VDG, Sept 27, 2007 5 3 2 1 8 9 10 4 5 6 7 Communication Neighborhood monitoring target Approach: Periodic Monitoring of Target Nodes 11 12 13 14 propagate status Keying Neighborhood

6. Copyright © 2007 VDG, Sept 27, 2007 6 Pair-wise Monitoring Scheme Continuous network self monitoring in each neighborhood - really bad idea ? Ping message in time Response message in time Interval assignment for pinging based on node’s ID, j d-1 d i 2 1 K >> node degree

7. Copyright © 2007 VDG, Sept 27, 2007 7 Pair-wise Monitoring Scheme Failure to respond appropriately to ping message in next T p interval suggests node capture For example: –delayed response past next T p –inappropriate message content –packet loss, collision, or congestion –physical damage or battery depletion of the node Detection interval T= MxT e helps distinguish node capture from response failures for other reasons Successful capture requires access to node’s internal states within T No response within T (i.e., after M retries) => alarm Larger T (or M) => increased vulnerability to capture Smaller T (or M) => increased false-alarm rate

8. Copyright © 2007 VDG, Sept 27, 2007 8 Design Objectives – normal mode Missed Detection Capture time X (pdf f X (x)) is smaller than detection interval T Minimize the probability of a missed detection P m False Alarms: device did not respond properly in interval T but device is not captured Exchange messages are lost with probability p l Reach end of a T=MxT e interval without monitoring message (“pinging”) Maximize expected residual time-to-false-alarm of nodes L f Cost: neighbor “pinging” rate; p r = probability of sending a pinging message in T e Minimize p r

9. Copyright © 2007 VDG, Sept 27, 2007 9 Markov Chain Model Detection (steady) state S n (0  S n  M) of neighbor i w.r.t neighbor node j at epoch n: no. of successive T e epochs s (1  s  M) in which node i does not ping node j (probability 1-p r ) no. of successive epochs T e in which node i has not received any response »communication errors with probability p l »node j is captured and unable to respond probability of receiving a “ping” response P e = p r (1-p l )

10. Copyright © 2007 VDG, Sept 27, 2007 10 Steady State Analysis Steady state probability of being at each state s (no capture in progress)

11. Copyright © 2007 VDG, Sept 27, 2007 11 Probability of being at each state Increasing p r (and p e ) leads to longer time to false alarm more concentration of mass in higher states, i.e. around the regenerative points but incurs higher energy and communication costs Note: where p l is constant

12. Copyright © 2007 VDG, Sept 27, 2007 12 Missed Detection Probability of missed detection Given a witness node is in state s, the capture time for an adversary’s success on a target node should be X < T= sT e Therefore,

13. Copyright © 2007 VDG, Sept 27, 2007 13 Missed Detection Increasing detection interval T (or M) increases P m longer detection interval => more time to complete node capture for a given detection interval T (or M), higher p r => higher P m - in the limit, the entire detection interval T is available to adversary

14. Copyright © 2007 VDG, Sept 27, 2007 14 False Alarms Expected Residual time-to-false-alarm, L f T s = residual time-to-false-alarm at current state; i.e., time for transition to state 0, given in state s and no capture in progress False alarm rate = Inverse of expected residual time-to-false- alarm

15. Copyright © 2007 VDG, Sept 27, 2007 15 Increasing p r increases L f higher p r maintains nodes in higher states(i.e., longer time for non- captured nodes to reach false alarm state 0) Increasing M increases L f Higher M (or T) => higher chance to go back to regenerative state M False Alarms

16. Copyright © 2007 VDG, Sept 27, 2007 16 Sensitivity of L f to p r  higher p r leads to more concentration of states around higher values with correspondingly larger T s False Alarms

17. Copyright © 2007 VDG, Sept 27, 2007 17 Design Trade-offs Minimizing requires reducing and Maximizing requires increasing and Cost (e.g., energy) efficiency requires reducing Application is more sensitive to than Tradeoffs for Determining and Cost analysis Communication: message RX and TX per node per epoch Computation: MAC verifications and generations, and counter inc. Memory: registers per node (each associated with a neighbor) Probability of collision is upper-bounded by Increase K s.t K>> d

18. Copyright © 2007 VDG, Sept 27, 2007 18 Two Simplistic Examples Case 1: Weak node protection (e.g., obfuscation and physical security) Case 2: Strong node protection (e.g., obfuscation and physical security)

19. Copyright © 2007 VDG, Sept 27, 2007 19 An Emergent Protocol Goals Robustness of capture-detection scheme against faulty/malicious neighbors judgments about a common node Reducing the required energy (e.g., communication) costs for given node security Optimal parameters for given node security measures; e.g., p r, M, p r q-node Probabilistic Pinging Scheme

20. Copyright © 2007 VDG, Sept 27, 2007 20 q-node Probabilistic Pinging Scheme

21. Copyright © 2007 VDG, Sept 27, 2007 21 q-node Probabilistic Pinging Scheme each neighbor runs pair-wise probabilistic pinging protocol with a (target) node independently each received alert flag increments the counter corresponding to the target node kept in all its neighbors counter= q => set revocation flag by q parties (consensus among q neighbors about the target node) commit revocation flag and broadcast it by all q parties to the entire network each revocation flag expires after time T and corresponding Markov chain is reset back to its initial state M

22. Copyright © 2007 VDG, Sept 27, 2007 22 q-node Missed Detection missed detection: - at least d-q+1 witness neighbors do not flag “node capture” or equivalently, at most q-1 neighbors flag “node capture”

23. Copyright © 2007 VDG, Sept 27, 2007 23 q-node Missed Detection - no. of parties, q < d (=20) - lower P m than in pair-wise case below threshold q (e.g., q<=14); higher above Pair-wise case

24. Copyright © 2007 VDG, Sept 27, 2007 24 Expected Residual Time to False Alarm False alarm:  at least q neighbors inaccurately flag a target node as a “captured” Residual time-to-false-alarm  the average time it takes for at least q neighbors to reach false alarm Lower bound on the expected residual time-to-false- alarm  first q alarm flags arrive within time interval T given

25. Copyright © 2007 VDG, Sept 27, 2007 25 Residual time-to-false-alarm T s vs s in q-level consensus note limited number of possibilities for having q-level consensus within time interval T

26. Copyright © 2007 VDG, Sept 27, 2007 26 Probability of False Alarm Probability of False-Alarm = Pr(q alerts come within T)  depends on q almost exponentially; i.e. exp(-q)  threshold values above which the prob. of false alarms is min. e.g., q>= 4

27. Copyright © 2007 VDG, Sept 27, 2007 27 Rule of Thumb for Setting q Set the consensus level q as about 25% to 30% of the node degree in to minimize  probability of a missed-detection  probability of a false-alarm How robust is this “design rule” ? Overall cost ?

28. Copyright © 2007 VDG, Sept 27, 2007 28 Ongoing and Future Research 1. Explore the design space for “pinging” protocol - vary model parameters within all practical values - derive design rules 2. Find semi-synchronous protocols - viz., revocation approach of H. Chan et al IEEE-TDSC 2005 3. Find other tell-tale signs of node capture and compose them with current approach. - other emergent properties 4. Extend approach to other networks; e.g., mesh nets