1. Intrusion Tolerance for NEST
Bruno Dutertre, Steven Cheung
SRI International

2. Outline Objectives Proposed approach: Plan
Local authentication and initial key establishment
Leveraging local trust
Intrusion detection and response
Plan

3. Objective
Low-cost key management for large-scale networks of small wireless devices
Constraints:
Limited memory, processing power, and bandwidth
Networks too large and not accessible for manual administration/configuration
Devices can be compromised

4. Traditional Key Management
Decentralized approaches:
Public-key infrastructure
Diffie-Hellman-style key establishment
Approaches based on symmetric-key cryptography
Trusted authentication and key distribution server (e.g., Kerberos)
Too expensive
Limited scalability
High administrative overhead to set up long-term keys
Vulnerable to server failure
Server may be a bottleneck

5. Proposed Approach Goals: Approach:
Intrusion-tolerant architecture for key management in NEST
Use only inexpensive cryptographic algorithm (symmetric-key crypto)
Decentralized (no server) and self organizing
Approach:
Build initial secure local links
For nonlocal communication, rely on chains of intermediaries
Use secret sharing when intermediaries are not fully trusted
Develop complementary intrusion detection methods to locate nontrustworthy nodes

6. Bootstrapping
Establish secure local links between neighbor devices quickly after deployment
Weak authentication is enough (need only to recognize that your neighbor was deployed at the same time as you)
Exploit initial trust (it takes time for an adversary to capture/compromise devices)
Focusing on local links improves efficiency

7. Basic Bootstrapping Scheme
For a set S of devices to be deployed
Construct a symmetric key K
Distribute it to all devices in the set
K enables two neighbor devices A and B
To recognize that they both belong to S (weak authentication)
To generate and exchange a key for future communication
Possible drawback:
Every device from S in communication range of A and B can discover More robust variants are possible.

8. Leveraging Local TrustB C A D E
To establish keys between distant nodes:
use chains of trusted intermediaries
To tolerate compromised nodes:
disjoint chains and secret sharing

9. Tradeoffs Security increases with but these also increase cost
the number of disjoint paths
the number of shares
but these also increase cost
Challenges:
Implement cheap crypto and secret sharing techniques
Quantify the security achieved
Find the right tradeoff for an assumed fraction of compromised nodes

10. Intrusion Detection Goals:
Detect compromised nodes (to remove them from chains)
Detect other intrusions: denial-of-service attacks, attempt to drain power
Cryptography is ineffective against these

11. Intrusion Detection Approach
Develop models of attacks and relevant signature:
What must be monitored?
How to collect and distribute the data?
Develop diagnosis methods:
Identify the source of the attack if possible
Possible responses:
Avoid nodes that are considered compromised
Hibernation to counter DoS or power-draining attacks

12. Experimental Evaluation
Platform:
“motes” with TinyOS
up to 20% compromised nodes
Objective: show feasibility, measure overhead
Experiment scenario remains to be defined

13. Schedule