Sep. 28 th 2006 KAIST Dependable Software LAB. Jun Sup Lee 이준섭 (20063449)

1. Path key establishment using multiple secured paths in wireless sensor networksCoNEXT’05Guanfeng Li  University of Pittsburgh, Pittsburgh, PA Hui Ling   University of Pittsburgh, Pittsburgh, PATaieb Znati   University of Pittsburgh, Pittsburgh, PA Sep.28th2006 KAISTDependableSoftwareLAB. Jun Sup Lee 이준섭(20063449) 1

2. Agenda • Motivation • RelatedWork • Contents • Multi-path key establishment with Proxy • ExperimentResult • SecurityAnalrysis • Conclusion • Q&A

3. Motivation Motivation: why security? Protecting confidentiality, integrity, and availability of the communications and computations S ensor networks are vulnerable to security attacks due to the broadcast nature of transmission Sensor nodes can be physically captured or destroyed Why not use existing security mechanisms? WSN features that affect security. Sensor Node Constraints Battery, CPU power, Memory. Networking Constraints and Features Wireless, Ad hoc, Unattended. This paper’s work: Improved key establish and maintain schemes within WSNs PathKeyEstablishmentSchemewhichleveragemultiplesecurepathswithonlyoneproxyforkeynegotiationandestablishment

4. Related Work | Security support in WSNs Existing schemes: Asymmetric cryptography Slow 2~4 times slower than symmetric encryption Hardware is complicated Energy consumption is high Trusted server schemes (e.g. Kerberos) Lacking of infrastructure Key pre-distribution schemes proposed by L. Eschenauer and V. Gligor (2002) • Battery Power Constraints • Computational Energy Consumption • Crypto algorithms • Public key vs. Symmetric key • Communications Energy Consumption • Exchange of keys, certificates, etc. • Per-message additions (padding, signatures, authentication tags) • Memory Constraints • Program Storage and Working Memory • Embedded OS, security functions (Flash) • Working memory (RAM)

5. Related Work | Key pre-distribution in WSNs Key pre-distribution in WSNs Loading Keys into sensor nodes prior to deployment Two nodes find a common key between them after deployment Challenges Memory/Energy efficiency Security: nodes can be compromised Scalability: new nodes might be added later Key Pool P Each node randomly selects R keys (Key Ring) N1 N2 N3 N4 … • When |P| = 1000, R=20 / 30 • p (two nodes have a common key) =0.335 / 0.605

6. Related Work | Key pre-distribution in WSNs N6 Key pool K1..K20 K7 N1 K11 N5 K5 N1 N3 N2 K1 N3 K13 N7 N2 K12 N4 N4 N5 N6 N7 Physical link Secure link

7. Related Work | Path-key establishment Path-key establishment Nodes not directly sharing a key will establish one through a secure path. Path key of node 4 and 6 (k4,6) is transmitted through node 7 and node 1. Node 4 -- Node 7 -- Node1-- Node6 K4,6 is revealed to node 7 and node 1 during the transmission. Generally, a path key is revealed to all intermediate nodes in the secure link path N6 K7 K11 N5 K5 N1 N3 K1 K13 N7 N2 K12 N4 K46 Path-key establishment problem!!

8. Contents| Multi-path key establishment Use multiple node disjoint secure paths to send the path-key. The path-key is divided into k pieces(nuggets) and one piece is sent through one path. Therefore, the attacker has to capture at least one node from all these k node-disjoint paths in order to capture the path-key. N2 K16=k1+ K2 k1+ K2=K16 k2 N1 N6 N4 k1 N3 N5 • It involves a high level of overhead to find nodes disjoint path. • Increasing the number of node disjoint paths does not necessarily improve the level of security.

9. Contents| Multi-path key establishment with Proxy Proxy To reduce the exposure of the key nugget along the path. Proxy ensures that no more than one node (Proxy) along a path knows the key nugget. The proxy shares a key with each end node respectively. it becomes feasible to relax the node disjoint requirement of the k paths without increasing the vulnerability of the path key. N6 K7 K11 N5 K5 N1 N3 K1 K13 N7 N2 K12 N4 Proxy Physical link Secure link

10. N6 K7 K11 N5 K5 N1 N3 K1 K13 N7 N2 K12 N4 Contents| End-to-EndKeyEstablishmentScheme • u sends out its key ID list to invite v to set up a pathkey. • v randomly construct a key and breaks it down to knuggets, K1;K2 : : :Kk • vthen selectskproxies • Upon receiving all k nuggets, node u reconstructs thekey K based on the sequence number carried by eachnugget v Proxy Proxy u Physical link Secure link

11. N6 K7 K11 N5 K5 N1 N3 K1 K13 N7 N2 K12 N4 Contents| FindingProxyAlgorithm#1 • v randomly selects k neighbors and sends out request-for-proxy packets containing key IDs from both u and v. • Each recipient examines the ID list to see if it shareskeys with both u and v. • If it does, it responds to v with key ID that ischosen to communicate with v, • If it does not, or it has received the same request from v, it forwards this request to a randomneighbor other than the sender. v u Physical link Secure link

12. N6 K7 K11 N5 K5 N1 N3 K1 K13 N7 N2 K12 N4 Contents| FindingProxyAlgorithm#2 • v creates a request packet and set its Time-To-Leave(TTL) field to t before locally flooding it into the network. • Nodes which receive a request packet respond withpositive acknowledgment only if they share a key withu and a key withvrespectively. • Upon receivingk positive acknowledgment, v selectsthe sender of these acknowledgments as k proxies. v u Physical link Secure link

13. Contents| ExperimentResultsofAlgorithm#1&#2 • The Algorithm #2 discovers proxies faster than Algorithm#1 • It is specially true in dense WSNs. • Requires more nodes than Algorithm #1 for localflooding. • The result shows if p is large, algorithm#1is preferred, while the second approach should be used if the network is dense.

14. Contents| SecurityAnalysis • The vulnerability of the system to nodecapture is measured by computing the likelihood that anattacker who captures x nodes may obtain all k key nuggets. • Assume that there are 2k distinct keysused to secure key nuggets by k proxies. • Consider a setof x collusive nodes. • Probabilityofcolludingxnodescoverall2kkeysis:

15. Contents| SecurityAnalysis • If either u or v is captured, the path key is revealed. • The probabilityofx nodes containing no end nodesbut covering all k proxies is: • The probability Pc of all key sharedbeing revealed after capture of x nodes is:

16. Contents| SecurityAnalysis • A satisfactory security level (0.00051%) can be achieved even when a large percentage ofnodes (5%) are captured and k is small (k = 4)

17. Conclusion The path-key establishment exposure problem commonly encountered in key pre-distributionschemes in WSNs. A Path Key Establishmentscheme, which uses multiple secured paths for the negotiation and exchange of symmetric keys between end nodes. Strength Furthermore this scheme assumes no specific routingprotocols thus it is not dependent on the physical topologyof the network. Itwillbeabletogreatlyimprovethesecurityofkeyestablishment Weakness Currently, the proposed scheme cannot defend against Denial of Service attacks, such as the case when an attacker lieson one or multiple paths from the proxies to the end nodesand drops packets.

18. Thankyou • Question? • Formorediscussion: • Rm4428,Jslee@dependable.kaist.ac.kr