Random Key Predistribution Schemes For Sensor Networks Haowan Chen, Adrian Perigg, Dawn Song

2 Index Introduction Basic Scheme Q-composite Scheme Multi path Key Reinforcement Scheme Random Pair wise Scheme Conclusion

3 Sensor Networks What are Sensors ? –A device that responds to physical stimulus (as heat, light, motion etc) and transmits a resulting measurement impulse Revolutionizes information gathering and processing Networking sensors: ability to coordinate among themselves on a larger sensing task

4 Applications Real time traffic monitoring Real time pollution and temperature monitoring Building safety monitoring systems Wild Life Monitoring and Tracking Military sensing and tracking Monitoring complex machinery and processes Video surveillance

5 Sensor Network Limitations Impracticality of public key cryptosystems Vulnerability of nodes to physical capture Nodes not tamper resistant (neighbor distrust) Lack of a-priori knowledge of post deployment configuration Limited memory resources Limited bandwidth and transmission power Over-reliance on base stations exposes vulnerabilities

6 Bootstrapping Security Requirements Deployed nodes must be able to establish secure node to node communication Scheme should be functional without involving the base station as arbiter or verifier Additional legitimate nodes deployed at a later time can form secure connections with already-deployed nodes Unauthorized nodes should not be able to establish communications with network nodes and thus gain entry into the network The scheme must work without prior knowledge of which nodes will come into communication range of each other after deployment. The computational and storage requirement of the scheme must be low, and the scheme should be robust to DoS attacks from out-of-network sources.

7 Evaluation Metrics In Key Setup Schemes Resilience against node capture Resistance against node replication Revocation Scale

8 Review Of “Basic Scheme” Proposed by Eschenauer and Gligor 4 phases - Initialization - Node Deployment - Key Setup - Path Key Generation

9 Initialization Phase Pick a random set of keys S out of the total possible key space “Key Ring” : for each node, randomly select m keys from S and store in node memory Criteria : two random subsets of size m in S will share at least one key with probability P

10 Deployment And Key Setup Phases Sensor nodes are deployed Key Setup Phase –key discovery: a short identifier is assigned to each key before deployment each node broadcasts its set of identifiers –verification: nodes containing shared keys in their “key rings” verify that neighbor actually holds key by challenge response protocol

11 Path Key Generation A connected graph of secure links is formed Nodes setup path keys with nodes in their vicinity whose share keys are not present in their key rings Path can be found from source node to its neighbor from connected graph Source node generates path key and sends it securely via the path to target node

12 Parameter choices for connected graph (Erdös-Rényis Formula) For high graph connectivity during key-setup phase right parameters need to be picked D -> degree for the vertices in graph such that graph is connected with a high probability c =0.999 D = ((n-1)/n) (ln(n) – ln(-ln(c))) where n is network size Probability of successful key setup with some neighbor, p = (d/n’) where n’ is expected no. of neighbors

13 Q-composite scheme : An improved “Basic Scheme” Initialization same as Basic Scheme but with different size of selected key pool S In Key Setup Phase, key discovery is more secure, using Merkle Puzzles In Key Discovery every node identifies every neighbor node with which it shares at least ‘q’ keys Link Key K is generated as a hash of all shared q’ keys, where q’ >= q eg : K = hash( k1 ll k2 ll k3 ll….ll kq’ ) Key Setup is not performed between nodes that share fewer than q keys

14 Key Pool Size Computation- A Tradeoff amount of key overlap required for key setup is q (increased from 1 in Basic) Hence exponentially harder for adversary with a given key set to break a link But to preserve probability of two nodes sharing sufficient keys to establish a secure link, size of key pool S to be reduced Reduced pool size allows attacker to gain larger sample of S by breaking fewer nodes Optimum overlap – best security !!

15 Observation : For Optimal Choice of key overlap, expected no. of nodes to be captured for eavesdropping (0.1 probability) is high M = 200 keys P = 0.5 Evaluation: Pool Size Computation

16 Pool Size Computation P(i) -> no. of ways to choose two key ring with i common keys Pconnect -> probability of any two nodes sharing sufficient keys to form a secure connection Then p(i) is given as : Pconnect = 1 – (p(0) + p(1) +…..+p(q-1)) For minimum key overlap q and min. connection probability p, choose largest ISI such that pconnect >= p

17 Evaluation Metric : resilience against node capture by calculating the fraction of links in the network that an attacker is able to eavesdrop on indirectly as a result of recovering keys from captured nodes

18 Evaluation Metric : estimation of max. supported size of network given certain security properties hold

19 Multipath Key Reinforcement – An Add On to “Basic Scheme” Initial Key Setup using Basic Scheme Now, consider the secure link between nodes A and B after key-setup This link is secured using a single key k from pool S

20 Problem Problem - k may be present in key ring memory of some other nodes If any of these nodes are captured, security of A->B is in jeopardy Solution : update communication key to a random value after key – setup Coordinate key update over multiple independent paths

21 Multipath Key Update Assumption : j be the no. of disjoint paths between A and B created during key setup Node A generates j random values v1,v2…vj of same length as shared key Each value is routed along a different path to B and when B receives all j keys, new link key is computed as: k’ = k + v1 + v2 + ….+ vj Long paths are not suitable 2-hop multipath key reinforcement is optimal –Discovery overhead is minimized

22 Evaluation Observation : reinforced basic scheme works best Metric : Resistance against node capture

23 Evaluation Observation : Multipath Key Reinforcement gives boost when implemented with basic scheme Metric : Maximum Supportable Network Sizes

24 In all schemes so far, no node can authenticate the identity of a neighbor it is communicating with Ex. A shares some set of keys with B –It is possible that C could also posses this key –Hence, A does not know if is communicating with B for sure Random-pairwise keys scheme

25 Node to node authentication Possible if a node can ascertain the identity of the nodes that it is communicating with Useful in many cases: -Detecting node misbehavior -Resisting node replication attack -Shift security functions away from the base station

26 Random pairwise scheme: properties Perfect resilience against node capture Node to node identity authentication Distributed node revocation Resistance to node replication Comparable scalability

27 Random pairwise scheme: description To achieve the probability p described by ER formula, in a network of n nodes: –Each node need only store a random set of np pairwise keys (instead of n-1) –Thus, if node can store m keys, network size n=m/p –“n should increase with increasing m and decreasing p”

28 Phase 1: Initialization n=m/p unique node identities generated Each node identity matched with m other randomly selected distinct node IDs –Pairwise key generated for each pair of nodes –Along with ID of other node that also knows the key, key is stored at both nodes

29 Phase 2: Key Setup Each node broadcasts node ID to immediate neighbors By searching in each others key rings, neighboring nodes can tell if they share a common pairwise key Cryptographic handshake performed between neighbors to accept the fact that they both have knowledge of key

30 Multihop range extension Key discovery involves much less traffic than random key predistribution Hence can have nodes rebroadcast node ID for certain number of hops

31 Multihop range extension Has impact on maximum supportable network size n  n=mn’/d (as seen earlier, p=d/n’, n=m/p )  Since n’ increases, maximum network size n also increases  Should be used with caution: since message rebroadcast is performed without authentication/verification: can lead to potential DoS attacks  To prevent, can remove multihop range extension, as is not required for random pairwise scheme

32 Support for Distributed Node Revocation Node revocation in random pairwise possible via base stations (but is slow) Assumption: mechanism present in each sensor to detect if neighboring nodes have been compromised Nodes broadcast public votes against a detected misbehaving node. If any B observes more than threshold number t of public votes against A, then B breaks off all communication with A Voting scheme, voting members

33 Support for Distributed Node Revocation Scheme 1: Consider any node A in the network; there are m nodes matched with it –These are voting members for A –Each assigned a random voting key K i –Each also knows hashes of other nodes’ keys –Nodes compute hash of K i to verify vote –Increases memory requirement to O(m 2 )

34 Support for Distributed Node Revocation Scheme 2: Merkle tree mechanism: O(log m) computation per output (fractal traversal) –Only a single verifying hash value (root) needs to be stored –Drawback: necessary to remember which nodes already traversed, to avoid replay votes

35 Threshold issues t should be –Low enough that unlikely that any node has degree < t –High enough that compromised nodes cannot revoke legitimate nodes

36 Broadcast Mechanism Voting scheme uses naïve broadcast, vulnerable to DoS attack Network of voting members form random graph with almost same (high) probability of being connected as original network (mn’/n)

37 Resisting revocation attack To prevent widespread release of revocation keys by compromised nodes, only nodes that have established direct communication with a node B have ability to revoke B Done by distributing revocation keys to voting members in deactivated form, source node knows secret S Bi, which voting members request during key discovery and setup

38 Resistance against node replication/node generation To be resistant to addition of infiltrator nodes derived form captured nodes, in case of capture being undetected by the network Degree of a node limited to counter replication Method for degree counting implemented with public vote counting, thus a node able to track nodes which share pairwise keys with it

39 Conclusion Efficient bootstrapping of secure keys important for secure sensor networks Tradeoffs exist in each scheme, choice depends on which tradeoff is most appealing (scenario dependent) q-composite scheme: good security for small scale attacks/vulnerable to large scale 2-hop multipath: improved security/network traffic overhead Random pairwise: resilient, good security/does not support as large networks as other schemes