Secure Time Synchronization in Wireless Sensor Networks 11/8/2007Carl J Ochs Jr1

Secure Time Synchronization in Wireless Sensor Networks

11/8/2007 Carl J Ochs Jr 1

AgendaOverviewBackgroundSecurity ThreatsTime Synchronization ProtocolsCountermeasuresSummary


OverviewScope

Time Synchronization in wireless sensor networks and security measures

IntentSurvey and evaluation of current time

synchronization protocols and the effectiveness of their security measures


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.


BackgroundWhy is Time Synchronization Important

Location and proximity of siblingsIntranetwork coordinationProper functioning of security measuresMaintain ordering of messagesUse of Time Division Multiplexing (TDMA)Energy efficiency


BackgroundWhy 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


BackgroundClock 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


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BackgroundEffects of Unsynchronized clocks

Invalid observationsInefficient/ineffective coverage

Areas left uncovered during certain timeslotsDisabled communications architecture (worst

case


Threat OverviewVectors of Attack

Malicious OutsiderAttacker with Jamming and Replay abilitiesCompromised Node


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

Sensor

Sensor

Sensor

Sensor

Sensor

Collection Node

Processing Node (End User)

Area of O bservation

EnemySensor


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

Sensor

Sensor

Sensor

Sensor

Sensor

Collection Node



EnemySensor


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

Sensor

Sensor

CompromisedSensor

Sensor

Sensor

Collection Node




Reference Broadcast Synchronization (RBS)

External unit broadcasts a synchronization signalSignal 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 measurementsSolution: multiple reference signals broadcast, nodes

exchange message arrival times, use least squares fit approximation

AttacksCompromised node can exchange an incorrect time

causing the other node to calculate an incorrect offset


Reference Broadcast Synchronization (RBS)

• Receiver-Receiver synchronizationo Single node

transmits a synchronization message to multiple nodes

o Receiver nodes synchronize the arrival timestamp amongst themselves (transmitter does not interact).


Time Synchronization Protocol Sensor NetworksCreate a spanning tree with a designated “root”

nodeEach pair of nodes can be considered a root-child,

where the child becomes the root for the next nodeChild 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

AttacksCompromised node can cause it's child node to

calculate an incorrect offset, and this will trickle down the tree


Time Synchronization Protocol Sensor Networks

Typical sender-receiver synchronization

HierarchicalRoot nodes at

each level synchronize with child nodes


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


Flooding Time Synchronization Protocol

Root node broadcasts time synchronization initiation message

Non-root sensors coordinate root message amongst neighborsNon-root sensors use

multiple neighbors timestamps to determine their own time.

Sensor

Sensor

Sensor

Sensor

Sensor

RootNode

T ime Sync

T ime Sync

I nitiate T ime Sync

T ime Sync

I nitiate T ime Sync


T ime Sync

T ime Sync

I nitiate T ime SyncI nitiate T ime Sync

CountermeasuresGeneral

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 articleGaneriwal et al.Song et al.2 by Sun et al. Manzo et al.


Ganeriwal et al.Extended the Key-sharing with initial nonce

by adding end-to-end delay calculationsPrevents “Pulse Delay Attacks”


Song et al.Statistical approach that intends to filter out the

compromised nodesAssumption 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


Sun et al.Original ApproachStatistical method with more resiliency in the

face of compromised nodesSynchronization can be level based – heirarchical – or

diffusion based – synch with nearest neighborsThe 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)


Sun et al.Revised ApproachTinySeRSync

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)


TinySeRSyncStaggers 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


SummaryMajority of the proposed countermeasures

provide protection against compromised nodesGaneriwal 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 attacksMethod proposed by Song et al. Protects against all 3

attacks, yet uses no cryptography for authentication


ReferencesA. 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.


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.


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Secure Time Synchronization in Wireless Sensor Networks 11/8/2007Carl J Ochs Jr1