Key Management in Mobile and Sensor Networks Class 17

Outline  Challenges in key distribution, trust bootstrapping  Pre-setup keys (point-to-point, public)  Resurrected ducking  PGP trust graph  Trusted third party (TTP) Kerberos, SPINS PKI  Key infection  Random-key predistribution

Key Management  Goal: set up and maintain secure keys Public keys for signature verification or node-to- node key setup Shared keys for confidentiality or authenticity Group keys for secure group communication  Challenges Trust establishment (Class example?) Node compromise Dynamic node addition/removal

Network Architectures  Closed networks, centralized deployment (trusted authority controls and deploys nodes) All-pairs shared keys, or all public keys PKI, TTP (Kerberos, SPINS) Zhou & Haas threshold key management Randomkey predistribution  Open networks, autonomous deployment Resurrected duckling PGP web of trust Key infection

Full Key Deployment  Symmetric case All-pairs shared keys (need O(n 2 ) keys) Challenge: node addition  Asymmetric case Distribute every node’s public key (n keys) Nodes can easily set up secure shared keys

Trusted Key Management Center  Symmetric case Trusted third party (TTP) shares key with each node (n keys) Set up key between two nodes through TTP Kerberos, SPINS key agreement protocol  Asymmetric case Public-key infrastructure (PKI) Certification authority (CA) signs public keys of nodes All nodes know CA’s public key

Zhou & Haas Key Management  PKI drawbacks Revocation requires on-line PKI Single point of failure, CA replication increases vulnerability to node compromise  Distributed CA Model, tolerates t faulty nodes  Threshold signatures Signing needs coalition of t+1 correct nodes Secret sharing prevents t malicious nodes from reconstructing CA private key  Proactive security Defend against mobile adversary

Discussion  How can share refreshing tolerate faulty nodes?  How can we tolerate compromised combiner? Who decides to be a combiner?  How can we bootstrap this system? How can we introduce a new node?  Why should node sign a message? How does node authenticate message?  Is signature combination expensive if we have t faulty nodes?  How efficient are these mechanisms?

Randomkey Predistribution  Scenario: deploy 10 4 mote sensor from airplane  Goal: set up secure node-to-node keys  Simple approaches impractical Network-wide secret key Pairwise shared key with every other node Pairwise shared key with neighbors Public key infrastructure

Basic Random Key Scheme  Eschenauer and Gligor, ACM CCS 2002  Observation: no need for all pairs of nodes to be able to communicate to get a connected network  For any 2 nodes, if they can communicate with some probability p, then the network is a random graph that is connected with high probability (e.g )  p is a given parameter, dictated by communication range and density of deployment of the nodes

Basic Random Key Scheme Total Key Space Key Pool P Randomly choose |P| keys Randomly choose m keys Key ring of node A Key ring of node B Pick |P| s.t probability of any 2 nodes sharing at least 1 key = p

Key capture  Security of the basic scheme is dependent on the adversary not knowing the key pool P  Suppose adversary can compromise sensor nodes and read the keys off their key rings  E.g., adversary captures node X and discovers key k. If node A and B were communicating using key k, the adversary can now eavesdrop although neither A or B was compromised.  How can we improve resilience to node capture?

q-Composite Keys scheme  Require any 2 nodes to share at least q keys to communicate  Adversary must discover all q keys to eavesdrop  To maintain probability of communication between any 2 nodes = p, must reduce size of key pool (samples from a smaller pool are more likely to overlap)  Smaller key pool  keys are more likely to be reused

Resilience vs node capture

Duckling Key Establishment  Anderson and Stajano, IWSP ‘99  Problem: how can we set up keys in a ubiquitous computing environment? Devices use wireless communication How to set up a key between household devices and PDA?  Solution: set up keys using trusted communication channel Physical contact establishes a secure channel

Duckling Security Model 1  Assumes wireless communication  Goals Availability – Guard against jamming and battery exhaustion – “Sleep deprivation torture attack” Secure transient association with device – Even in absence of a trusted server – Security assiciations keep changing, as devices change owners, or owner changes controller

Duckling Security Model 2  Life cycle “similarities” Life cycle of a device – Buy device in store – Unpack it at home – Device breaks or gets a new owner Life cycle of a duckling – Duckling is in egg – When duckling hatches, first object is viewed as mother: imprinting – Duckling dies Device ownership similar to duck’s soul

Duckling Security Model 3  Device life cycle Imprinting: device meets master when it wakes up Reverse metempsychosis: device dies and gets new owner Escrowed seppuku: manufacturer can kill device to enable renewed imprinting  Physical contact establishes secure key during imprinting phase

PGP Web of Trust  Problem: how can we establish shared keys in ad hoc network without trusted PKI?  Approach: use PGP web of trust approach  Jean-Pierre Hubaux, Srđan Čapkun and Levente Buttyán: The Quest for Security in Mobile Ad Hoc Networks, MobiHoc 2001

Distributed storage of local certificates  Nodes issue certificates (sign others’ keys), as in PGP  Each node stores the certificates that it issued (out- bound certificates) and the certificates that other nodes issued for it (in-bound certificates) u v

Creating the subgraphs  Each node builds up its own out-bound and in- bound subgraphs  To establish secure communication, u and v merge their subgraphs and see if they intersect u v

Key Infection  Ross Anderson and Adrian Perrig, 2001  Goal: Light-weight key setup among neighbors  Assumptions: Attacker nodes have same capability as good nodes Attacker nodes less dense than good nodes Attacker compromises small fraction of good nodes  Basic key agreement protocol A  * : A, K A B  A : { A, B, K B } K A K AB = H( A | B | K A | K B )

Key Infection AB M4 M2 M3 M1  Broadcast keys with maximum signal strength

Key Whispering Extension AB M4 M2 M3 M1  Broadcast keys with minimum signal strength to reach neighbor

Secrecy Amplification A B C D E  A & B share K AB, A & C share K AC,, etc.  Strengthen secrecy of K’ AB A  C : { B, A, N A } K AC C  B : { B, A, N A } K CB B  D : { A, B, N B } K BD D  E : { A, B, N B } K DE E  A : { A, B, N B } K AE K’ AB = H( K AB | N A | N B )

Key Infection Summary  Highly efficient  Detailed analysis in progress  Preliminary simulation results: Nodes uniformly distributed over a plane D (density): average # of nodes within radio range # of attacker nodes = 1% of good nodes Table shows fraction of compromised links DBasicWhisperSASA-W 21.1%0.4%1.0%0.3% 31.8%0.6%1.4%0.5% 52.9%1.0%2.4%0.8%

Discussion  Tradeoff Trust perimeter and security? Security and management?