Random Key Predistribution Schemes for Sensor Networks Authors: Haowen Chan, Adrian Perrig, Dawn Song Carnegie Mellon University Presented by: Johnny Flowers February 28, 2008

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

Outline Background The problem with sensor networks Related work Three schemes  q-composite keys scheme  Multipath-reinforcement scheme  Random pairwise keys scheme Future directions

The Bootstrapping Problem Initialization process Creating something from nothing

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

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

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

What Makes a Key Predistribution Scheme Good?

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

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

Evaluation Metrics Resilience against node capture Resistance against node replication Revocation of misbehaving nodes Scalability

The Basic Scheme

Three phases of operation:  Initialization  Key setup  Graph connection

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

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

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)) ]

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

Extensions of the Basic Scheme q-composite Random Key Predistribution Multipath Key Reinforcement Random Pairwise Keys

q-composite Random Key Predistribution Scheme

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

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

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

Evaluation Compromising a given # of nodes is more damaging Harder to compromise nodes, however

Evaluation Dangerous under large-scale attack Absolute # of compromised nodes vs. fraction of compromised communications

Multipath Key Reinforcement Scheme

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

Evaluation Better resistance against node capture Significantly higher maximum network size Comes at cost of greater communication overhead

Random Pairwise Keys Scheme

Key feature is node-to- node identity authentication Ability to verify node identities opens up several security features

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

Initialization n  # of unique node IDs m  keys on each node’s key ring p  Probability of two nodes connecting n = m/p

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

Key Setup Node IDs are broadcast to neighbors Verified through cryptographic handshake

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

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

Scheme Requirements Compromised nodes can’t revoke arbitrary nodes No vote spoofing Verifiable vote validity Votes have no replay value Not vulnerable to DoS

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

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

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

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

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

Evaluation, Cont. Maximum network size suffers slightly

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

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

Future Work How does the random pairwise scheme perform in small networks? Can the random pairwise scheme be modified to handle larger networks?