Delay Aware, Reconfigurable Security for Embedded Systems Philip Brisk 2 Tammara Massey 1 Foad Dabiri 1 Majid Sarrafzadeh 1 2 1

Outline ECG Application Security in BANs Related Work Dynamic Security System (DYNASEC) Conclusion 1/15

Body-Area Network Application Electrocardiogram (ECG) Sensor Large bandwidth requirement Parallel transmission of many waveforms Environmental interferences Patient movement Respond to anomalies Activate drug delivery mechanism Based on SQRS Algorithm [Pino et al., ’05] ECG waveforms May misclassify features due to noise Addressed via moving threshold with independent values Extra noise classification removed to reduce code size 2/15

Security in Body-Area Networks Health Insurance Portability and Accountability Act (USA) “… ensure integrity and confidentiality of information” “… protect against reasonably anticipated threats or hazards to the security of integrity of the information” Security-Processing Gap [Ravi et al., ’03] Exacerbated for body area networks Limited Memory Battery lifetime Bandwidth 3/15

Related Work TinyPK [Watro et al., ’04] Authentication and key exchange via RSA encryption TinySec [Karlof et al., ’04] Link-layer security for wireless sensor networks Authentication, Skipjack/RC5 encryption Elliptic curve cryptography on motes [Malan et al., ’04] DYNASEC: adds reconfigurable element to security 4/15

Dynamic Security System (DYNASEC) Normal operation High security all around Anomaly detected Hard real-time requirements Reconfiguration to meet the deadline Dynamically change security levels Throttle packet size 5/15

DYNASEC System Design Sensor Operating System (SOS) [Han et al., ’05] Message Integrity Code (MIC) Encryption (is optional) Skipjack, RC5 implemented in SOS kernel Memory requirements Original – 17% of Mica2 mote memory Modified – 39% of Mica2 mote memory Encryption algorithms have large data segments 6/15

Four Security Levels L0 – No Security L1 – MIC Authentication + No Encryption L2 – MIC Authentication + Skipjack Encryption L3 – MIC Authentication + RC5 Encryption RC5 is faster and stronger than Skipjack Pre-computed key schedule consumes 2.6% of Mica2 mote memory 7/15

Security Cost 8/15 Packet size (bytes)

Dynamic Security Allocation Network organized as a directed ayclic graph (DAG) Sink is a centralized node with more processing power than other nodes In response to anomaly redistribute security levels Integer Budgeting Problem on a DAG Model as Linear Program Solve LP (CPLEX) on centralized node Relax to nearest integer solution If no solution is available Reduce the packet size and try again 9/15

Budgeting Problem Formulation Given: Hard-timing constraint for communication Maximize: Aggregate security of all motes in the DAG Subject to the following constraints Exactly one security algorithm assigned to each mote Each source-to-sink path satisfies the timing constraint For each link: The flow of packets sent into each link does not exceed the link capacity For each internal mote: The flow of outgoing packets is equal to the flow of incoming packets 10/15

Pragmatic Issues Propagation time from centralized node to other nodes is non-negligible Only run the algorithm if there is at least a 15% change in link quality Can re-use the old solution for repeated anomalies If there is a change… Nodes make temporary local decisions for the best routing path… Link quality used to choose next hop Modifies security level Lower if it cannot achieve desired throughput Higher if it exceeds desired throughput Until the centralized node propagates its solution 11/15

Experimental Setup Randomly generated network Simulation on Avrora [Titzer et al., ’05] 10 nodes or less (for now) Runtime of LP solver doesn’t scale 3 Types of Links Well-connected 20% of packets dropped Lossy 50% of packets dropped Very lossy 80% of packets dropped 12/15

Evaluation Small Network (10 nodes or less) Reasonable for today’s body area networks LP converges in approximately 1.5 ms, on average Does not include time to propagate solution to the nodes in the network Establishes feasibility of the approach Will not scale for larger network Must solve budgeting problem in distributed fashion 13/15

Conclusion (1/2) Security-Processing Gap Exacerbated for BANs Limited memory New cryptographic algorithms are needed Smaller code/data segments Approximately 11% of mote memory, per algorithm, could be improved Configurability Allow the user to select a tradeoff between runtime and cipher quality 14/15

Conclusion (2/2) Anomalies  Hard real-time constraints DYNASEC reconfigures security levels in response Security allocation is a budgeting problem Solved using LP-relaxation on centralized node Future work: solve the problem in distributed fashion on the motes Increase emphasis on throttling packet size 15/15