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Intrusion Tolerance for NEST

Bruno Dutertre, Steven Cheung

SRI International

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Outline

• Objectives

• Proposed approach:– Local authentication and initial key

establishment– Leveraging local trust– Intrusion detection and response

• Plan

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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

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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 administrativeoverhead to set up long-term keys

Vulnerable to serverfailure

Server may be a bottleneck

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Proposed Approach

• Goals:– 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

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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

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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.

abK

abK

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Leveraging Local Trust

• To establish keys between distant nodes:– use chains of trusted intermediaries

• To tolerate compromised nodes:– disjoint chains and secret sharing

A

B C

D

E

abKbcK

cdK

deKaeK

ceK

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Tradeoffs

• Security increases with– 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

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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

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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

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Experimental Evaluation

• Platform:– “motes” with TinyOS– up to 20% compromised nodes– Objective: show feasibility, measure

overhead

• Experiment scenario remains to be defined

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Schedule