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Random Key Predistribution Schemes for Sensor Networks Authors: Haowen Chan, Adrian Perrig, Dawn Song Carnegie Mellon University Presented by: Johnny Flowers February 28, 2008
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The Big Idea Three key bootstrapping protocols for large sensor networks Alternatives to public key cryptosystems Each protocol trades a different drawback in exchange for the security it provides
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Outline Background The problem with sensor networks Related work Three schemes q-composite keys scheme Multipath-reinforcement scheme Random pairwise keys scheme Future directions
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The Bootstrapping Problem Initialization process Creating something from nothing
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Bootstrapping Security in Sensor Networks Especially challenging because of the limitations of sensor networks: Constrained resources Physical vulnerability Unpredictability of future configurations Temptation to rely on base stations
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Related Work Previously proposed solutions often depend on: Asymmetric cryptography Arbitration by base stations (e.g., SPINS) Some even require physical contact with a master device or assume that attackers do not arrive until after key exchange
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Finding a Solution Authors’ proposed schemes are based on the basic random key predistribution scheme Basic scheme is modified to meet the appropriate design goals
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What Makes a Key Predistribution Scheme Good?
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Key Predistribution Scheme Design Goals Secure node-to-node communication Must not rely on base stations for decision-making Adaptable to addition of nodes after initial network setup
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Key Predistribution Scheme Design Goals, Cont. Prevent unauthorized access No assumptions about which nodes will be within communication range of each other Resource-efficient and robust to DoS attacks
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Evaluation Metrics Resilience against node capture Resistance against node replication Revocation of misbehaving nodes Scalability
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The Basic Scheme
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Three phases of operation: Initialization Key setup Graph connection
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The Basic Scheme – Initialization Pick a random key pool, S For each node, randomly select m keys from S (this is the node’s key ring) The size of S is chosen so that two key rings will share at least one key with probability p
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The Basic Scheme – Key Setup Nodes search for neighbors that share a key Broadcast short IDs assigned to each key prior to deployment Keys verified through challenge-response
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The Basic Scheme – Graph Connection Nodes then set up path keys with any unconnected neighbors through existing secure paths # of secure links a node must establish during key setup (degree, d) to form a connected graph of size n with probability c is: d = [ (n-1)/n ][ ln(n) – ln(-ln(c)) ]
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The Basic Scheme – Graph Connection The probability, p, that two nodes successfully connect is p = d/n′ where n′ is the expected number of neighbor nodes within communication range of A ½
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Extensions of the Basic Scheme q-composite Random Key Predistribution Multipath Key Reinforcement Random Pairwise Keys
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q-composite Random Key Predistribution Scheme
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q-composite Scheme Instead of one key, a pair of nodes must share q keys to establish a secure link Key pool must be shrunk in order to maintain probability p of two nodes sharing enough keys
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Initialization and Key Setup Similar to basic scheme Each node has m keys on key ring Two nodes must discover at least q common keys in order to connect Before connecting, a new key is created as a hash of the q shared keys Broadcasting IDs is dangerous, however
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Evaluation Much harder for an attacker with a given key set to eavesdrop on a link Necessary reduction in key pool size makes large-scale attacks even more powerful
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Evaluation Compromising a given # of nodes is more damaging Harder to compromise nodes, however
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Evaluation Dangerous under large-scale attack Absolute # of compromised nodes vs. fraction of compromised communications
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Multipath Key Reinforcement Scheme
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Initialization and key setup as in basic scheme Key update over multiple independent paths between nodes Key update is damage control in the event that other nodes are captured
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Evaluation Better resistance against node capture Significantly higher maximum network size Comes at cost of greater communication overhead
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Random Pairwise Keys Scheme
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Key feature is node-to- node identity authentication Ability to verify node identities opens up several security features
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The Basics Sensor network of n nodes Pairwise scheme: Each node holds n-1 keys Each key is shared with exactly one other node Random pairwise scheme: Not all n-1 keys are needed for a connected graph Only np keys are needed to connect with probability p
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Initialization n # of unique node IDs m keys on each node’s key ring p Probability of two nodes connecting n = m/p
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Initialization Each node ID pairs with m other random & distinct node IDs Each pair is assigned a key Nodes store key-ID pairs on key rings
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Key Setup Node IDs are broadcast to neighbors Verified through cryptographic handshake
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Multi-hop Range Extension Node IDs are small Can be re-broadcast at low cost Neighbors forward IDs during key setup Increases communication radius Increases max. network size
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Distributed Node Revocation Faster than relying on base stations Public votes are broadcast against compromised nodes Offending node is cut off when votes reach threshold
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Scheme Requirements Compromised nodes can’t revoke arbitrary nodes No vote spoofing Verifiable vote validity Votes have no replay value Not vulnerable to DoS
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The Voting Process A node’s voting members are those that share a pairwise key with it All voting members are assigned a voting key Votes are verified through a Merkle tree Voting members keep track of votes received up to a threshold, t
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Voting Threshold If too high A node may not have enough voting members to be revoked If too low Easy for a group of compromised nodes to revoke many legitimate nodes
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Resisting Revocation Attacks Node B’s revocation key for node A must be activated before use Hashed with secret value known only by A A gives B its secret value only after the two establish communication Other DoS attacks are more practical
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Resistance to Node Replication and Node Generation Place a cap, d max, on the degree of a node d max is some small multiple of d Nodes keep track of degree and node IDs using same method as vote counting
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Evaluation Perfect resilience against node capture All pairwise keys are unique, so capturing one node reveals no information about communications outside of the compromised node’s
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Evaluation, Cont. Maximum network size suffers slightly
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Evaluation, Cont. Resistance to revocation attack Small number of compromised nodes only compromises a small portion of communications Compromising large number of nodes is not economical
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Summary Three efficient schemes for secure key bootstrapping Each scheme has trade-offs q-composite: good for small attacks, bad for large Multipath-reinforcement: improved security, more communication overhead Random pairwise: max. network size is smaller
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Future Work How does the random pairwise scheme perform in small networks? Can the random pairwise scheme be modified to handle larger networks?
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