CMSC 681 Project

1. CMSC 681 Project Secure Time Synchronization in Wireless Sensor Networks Carl J Ochs Jr

2. Agenda • Overview • Background • Security Threats • Time Synchronization Protocols • Countermeasures • Summary Carl J Ochs Jr

3. Overview • Scope • Time Synchronization in wireless sensor networks and security measures • Intent • Survey and evaluation of current time synchronization protocols and the effectiveness of their security measures Carl J Ochs Jr

4. Background • Wireless Sensor Networks • Network of multiple resource-constrained sensor nodes that monitor and forward information • Typically has a centralized node that collects the data and another node that will be used for processing the data. Carl J Ochs Jr

5. Background • Why is Time Synchronization Important • Location and proximity of siblings • Intranetwork coordination • Proper functioning of security measures • Maintain ordering of messages • Use of Time Division Multiplexing (TDMA)‏ • Energy efficiency Carl J Ochs Jr

6. Background • Why not NTP or GPS? • The majority of sensor nodes are designed with minimal resources available and consumed • NTP requires highly accurate clocks • GPS requires an additional receiver for the GPS transactions • Some networks with less restrictions may be able to afford a GPS sensor infrastructure. • Additional details are available in chapter 1 of the Handbook of Sensor Networks: Algorithms and Architectures: Time Synchronization and Calibration in Wireless Sensor Networks by Kay Römer, Philipp Blum, Lennart Meier available from http://www.vs.inf.ethz.ch/publ/papers/wsn-time-book.pdf Carl J Ochs Jr

7. Background • Clock Synchronization Basics • Clock Offset • Difference between clocks at 2 nodes • Clock Skew • Change in offset over time • One clock running faster than another • Drift Error • Random changes in clock frequency • Temperature differences • Aging of the hardware Carl J Ochs Jr

8. Background • Effects of Unsynchronized clocks • Invalid observations • Inefficient/ineffective coverage • Areas left uncovered during certain timeslots • Disabled communications architecture (worst case Carl J Ochs Jr

9. Threat Overview • Vectors of Attack • Malicious Outsider • Attacker with Jamming and Replay abilities • Compromised Node Carl J Ochs Jr

10. Threat Overview • Malicious Outsider • Wireless device inserted into the network • Can eavesdrop, but cannot contribute • No knowledge of security keys (at least initially)‏ • Similar in behavior to the “snoop”/Ethereal/Wireshark network capturing utility Carl J Ochs Jr

11. Threat Overview • Attacker with Jamming and Replay ability • Can initiate Pulse Delay Attacks • Jam a message, store, and replay later • Real message sent out of sequence (confusion)‏ • Jamming detection possible, but expensive Carl J Ochs Jr

12. Threat Overview • Compromised Node • Most difficult to defend against • Friendly node taken over by enemy (brainwashed)‏ • Primarily physical attack • In theory, it could be a pure software attack • Node has correct security keys and can contribute to the network communication Carl J Ochs Jr

13. Reference Broadcast Synchronization (RBS)‏ • External unit broadcasts a synchronization signal • Signal causes all nodes to record their current local time and then exchange this information amongst each other • Processing time and propagation delays can cause imprecise measurements • Solution: multiple reference signals broadcast, nodes exchange message arrival times, use least squares fit approximation • Attacks • Compromised node can exchange an incorrect time causing the other node to calculate an incorrect offset Carl J Ochs Jr

14. Reference Broadcast Synchronization (RBS) • Receiver-Receiver synchronization • Single node transmits a synchronization message to multiple nodes • Receiver nodes synchronize the arrival timestamp amongst themselves (transmitter does not interact). Carl J Ochs Jr

15. Time Synchronization Protocol Sensor Networks • Create a spanning tree with a designated “root” node • Each pair of nodes can be considered a root-child, where the child becomes the root for the next node • Child requests synchronization from the root, which responds with an acknowledgement message • The message data is the departure and arrival time of each, so the child, upon receipt of the last message can calculate its clock offset relative to the parent • Attacks • Compromised node can cause it's child node to calculate an incorrect offset, and this will trickle down the tree Carl J Ochs Jr

16. Time Synchronization Protocol Sensor Networks • Typical sender-receiver synchronization • Hierarchical • Root nodes at each level synchronize with child nodes Carl J Ochs Jr

17. Flooding Time Synchronization Protocol • Root node is elected (lowest ID wins) and is the node that sends out the synchronization message • If no synchronization message is received after a preset time period, a node can elect itself root and initiate the synchronization • Self healing tree structure in case of node failures • Attacks • Any compromised node could declare itself root since the lowest ID always wins • Node then sends incorrect time synchronization data that corrupts the entire tree Carl J Ochs Jr

18. Flooding Time Synchronization Protocol • Root node broadcasts time synchronization initiation message • Non-root sensors coordinate root message amongst neighbors • Non-root sensors use multiple neighbors timestamps to determine their own time.

19. Countermeasures • General • Cryptographic-based • Key sharing • Pre-shared or exchanged using Diffie-Hellman protocol • Random nonce's are inserted in the initial messages to prevent playback attacks • Proposals reviewed by article • Ganeriwal et al. • Song et al. • 2 by Sun et al. • Manzo et al. Carl J Ochs Jr

20. Ganeriwal et al. • Extended the Key-sharing with initial nonce by adding end-to-end delay calculations • Prevents “Pulse Delay Attacks” Carl J Ochs Jr

21. Song et al. • Statistical approach that intends to filter out the compromised nodes • Assumption is that compromised nodes will behave significantly different from uncompromised nodes – called outliers. • Implementations • Generalized Extreme Studentized Deviate (GESD)‏ • Outputs estimated number of outliers • Predetermined Delay thresholds • At startup, nodes determine the maximum delay tolerated and ignore any messages exceeding this threshold • No cryptographic techniques employed Carl J Ochs Jr

22. Sun et al.Original Approach • Statistical method with more resiliency in the face of compromised nodes • Synchronization can be level based – heirarchical – or diffusion based – synch with nearest neighbors • The synchronization method selects the median of 2t + 1 candidate measures • Tolerates up to t compromised nodes • Requires pairwise-key-based authentication to prevent Sybill attack (single node masquerading as multiple nodes)‏ Carl J Ochs Jr

23. Sun et al.Revised Approach • TinySeRSync • http://discovery.csc.ncsu.edu/software/TinySeRSync/ • Improved version of original approach proposed by Sun et al. • Still relies on the selection of a median from 2t + 1 candidates • Has 1+ source/root nodes that have highly precise time (i.e. GPS set)‏ Carl J Ochs Jr

24. TinySeRSync • Staggers the global synchronization into 2 asynchronous phases • First phase has pairs of nodes synchronizing to each other, independent of the source node • MAC layer timestamping is used for authenticating the source • Continuously done to maintain precision between pairs • Second phase is initiated by the source node broadcasting a synchronization message • This message is flooded to neighboring nodes who then forward it on to their neighbors • μTESLA is used as the authentication method • Does not require signature verification overhead, which can be a vector for Denial of Service attacks Carl J Ochs Jr

25. Summary • Majority of the proposed countermeasures provide protection against compromised nodes • Ganeriwal et al. Is the only method that does not • All apply some form of statistical filtering to weed out the attacks • Cryptography is not a necessary component to defend against attacks • Method proposed by Song et al. Protects against all 3 attacks, yet uses no cryptography for authentication Carl J Ochs Jr

26. References • A. Boukerche and D. Turget, “Secure Time Synchronization Protocols For Wireless Sensor Networks,” IEEE Wireless Communications, October 2007, pp. 64-69 • Kun Sun, Peng Ning, Cliff Wang, An Liu, Yuzheng Zhou, "TinySeRSync: Secure and Resilient Time Synchronization in Wireless Sensor Networks'', in Proceedings of the 13th ACM Conference on Computer and Communications Security (CCS'06), Alexandria, VA, November 2006. Carl J Ochs Jr

27. References • Roche, Michael. "Time Synchronization in Wireless Networks." CSE574S: Advanced Topics in Networking: Wireless and Mobile Networking (Spring 2006) Website.23 Apr. 2006. Washington University in St. Louis. 04 Dec. 2007 <http://www.cs.wustl.edu/~jain/cse574-06/ftp/time_sync/index.html>. • K. Römer, P. Blum, L. Meier: Time Synchronization and Calibration in Wireless Sensor Networks, In: I. Stojmenovic (Ed.), Handbook of Sensor Networks: Algorithms and Architectures, pp. 199-237, Wiley and Sons, October 2005, ISBN 0-471-68472-4. Carl J Ochs Jr